U.S. patent number RE43,699 [Application Number 11/879,725] was granted by the patent office on 2012-10-02 for reconfigurable scanner and rfid system using the scanner.
This patent grant is currently assigned to Theodore R. Anderson. Invention is credited to Igor Alexeff, Theodore R. Anderson.
United States Patent |
RE43,699 |
Anderson , et al. |
October 2, 2012 |
Reconfigurable scanner and RFID system using the scanner
Abstract
A scanner has plasma loop or plasma window antennas for
selectively scanning for ID tags along distinct radials of the
scanner. Scanner elements are made electromagnetically invisible to
adjacent elements by removing power or lowering plasma densities so
that the scanner elements do not interfere with its own operation.
Activatable ID tags and a shipping container suitable for scanning
with electromagnetic energy are also disclosed.
Inventors: |
Anderson; Theodore R.
(Brookfield, MA), Alexeff; Igor (Oak Ridge, TN) |
Assignee: |
Anderson; Theodore R.
(Brookfield, MA)
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Family
ID: |
46924920 |
Appl.
No.: |
11/879,725 |
Filed: |
July 18, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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10648878 |
Aug 27, 2003 |
6870517 |
|
|
|
10067715 |
Feb 5, 2002 |
6700544 |
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Reissue of: |
10693477 |
Oct 24, 2003 |
6922173 |
Jul 26, 2005 |
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Current U.S.
Class: |
343/701 |
Current CPC
Class: |
H01Q
1/366 (20130101); H01Q 15/0013 (20130101); H01Q
19/32 (20130101); H01Q 3/46 (20130101); G06K
7/10356 (20130101) |
Current International
Class: |
H01Q
1/26 (20060101) |
Field of
Search: |
;343/701,870,908,741-743,866-868 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mancuso; Huedung
Attorney, Agent or Firm: Knox Patents Kulaga; Thomas A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. Pat. No.
6,700,544 application Ser. No. 10/067,715 filed Feb. 5, 2002, the
entirety of which is hereby incorporated by reference. This
application is also a continuation-in-part of U.S. Pat. No.
6,870,517 application Ser. No. 10/648,878 filed Aug. 27, 2003, the
entirety of which is hereby incorporated by reference.
Claims
What is claimed is:
1. A reconfigurable scanner for scanning for ID tags containing
scannable antennas oriented in multiple directions relative to the
scanner, without need for physical movement of the scanner, the
reconfigurable scanner comprising: a scanning element broadcasting
a signal in a selected direction, the scanning element having a
plurality of variable conductive elements; control means for
electrically controlling and changing the selected direction in
which the scanning element broadcasts the signal by powering and
unpowering the plurality of variable conductive elements; and
transceiver means for generating an electromagnetic wave and
receiving a responsive electromagnetic wave signal from a sensed ID
tag within an effective range of the scanner, whereby unpowered
variable conductive elements do not cause any interference with the
scanning signal.
2. A reconfigurable scanner according to claim 1, wherein the
plurality of variable conductive elements are a plurality of plasma
loop sensors.
3. A reconfigurable scanner according to claim 2, wherein the
plasma loop sensors each comprise a loop antenna having at least a
portion of which is an arcuate tube section containing an ionizable
gas, such that the loop antenna is only conductive when the
ionizable gas is ionized.
4. A reconfigurable scanner according to claim 1, wherein the
scanning element comprises an antenna and an electromagnetic shield
formed by the plurality of variable conductive elements, the
electromagnetic shield intersecting transmission lobes of the
antenna in at least the multiple directions being scanned.
5. A reconfigurable scanner according to claim 4, wherein the
plurality of variable conductive elements are mounted in an array
on a substrate forming the shield.
6. A reconfigurable scanner according to claim 5, wherein the
substrate is a conductive metal.
7. A reconfigurable scanner according to claim 4, wherein the
electromagnetic shield is formed by stacked layers of arrays of the
variable conductive elements.
8. A scanner system comprising: a plurality of electromagnetically
scannable ID tags; and a reconfigurable scanner having a scanning
element with a plurality of variable conductive elements switchable
between electromagnetically active and electromagnetically
invisible, control means for switching the variable conductive
elements between electromagnetically active and electromagnetically
invisible, and a transceiver means for generating and receiving an
electromagnetic scanning signal in a direction determined by the
control means, the scanning signal interacting with the scannable
ID tags located in the direction of the scanning signal.
9. A scanner system according to claim 8, wherein the variable
conductive elements are plasma loop sensors.
10. A scanner system according to claim 8, wherein the scanning
element comprises an antenna and an electromagnetic shield formed
by the plurality of variable conductive elements, the
electromagnetic shield intersecting transmission lobes of the
antenna in at least the multiple directions being scanned.
11. A scanner system according to claim 10, wherein the plurality
of variable conductive elements are mounted in an array on a
substrate forming the shield.
12. A scanner system according to claim 11, wherein the substrate
is a conductive metal.
13. A scanner system according to claim 10, wherein the
electromagnetic shield is formed by stacked layers of arrays of the
variable conductive elements.
14. A scanner system according to claim 8, wherein at least one of
the plurality of ID tags comprise an antenna and a code connected
with the antenna for detection and reading by the reconfigurable
scanner.
15. A scanner system according to claim 14, wherein the at least
one ID tag further comprises a power source for powering the
antenna into an active state.
16. A scanner system according to claim 15, wherein the power
source is external of the ID tag.
17. A scanner system according to claim 14, wherein the antenna of
the at least one ID tag is a plasma loop, and the at least one ID
tag further comprises a power source for weakly or partially
ionizing a plasma in the plasma loop, whereby the plasma loop
remains electromagnetically invisible until external energy is
received by the plasma.
18. A scanner system according to claim 17, wherein the external
energy is provided by the scanning signal.
19. A scanner system for detecting the contents of a shipping
container, the system comprising: a plurality of slots formed in
the shipping container for permitting a selected bandwidth of
electromagnetic wave to penetrate the shipping container; at least
one electromagnetically scannable ID tag associated with the
contents of the shipping container; and a reconfigurable scanner
having a scanning element with a plurality of variable conductive
elements switchable between electromagnetically active and
electromagnetically invisible, control means for switching the
variable conductive elements between electromagnetically active and
electromagnetically invisible, and a transceiver means for
generating and receiving an electromagnetic scanning signal, the
scanning signal having a frequency within the selected bandwidth
for penetrating the shipping container to detect the at least one
ID tag.
20. A scanner system according to claim 19, wherein the shipping
container comprises dielectrics on the interior of the container
for damping resonant signals.
21. A scanner system according to claim 19, wherein the slots are
formed by dielectric materials.
22. A scanner system according to claim 19, wherein the slots are
formed by one of variable dielectric materials surrounded by
conductive material and fixed dielectric materials surrounded by
variable conductive material.
.Iadd.23. A reconfigurable scanner for scanning for ID tags
containing scannable antennas oriented in multiple directions
relative to the scanner, without need for physical movement of the
scanner, the reconfigurable scanner comprising: a scanning element
broadcasting a signal in a selected direction, the scanning element
having a plurality of variable conductive elements; a switch that
electrically controls and changes the selected direction in which
the scanning element broadcasts the signal by powering and
unpowering the plurality of variable conductive elements; and a
transceiver that generates an electromagnetic wave and receives a
responsive electromagnetic wave signal from a sensed ID tag within
an effective range of the scanner, whereby unpowered variable
conductive elements do not cause any interference with the scanning
element..Iaddend.
.Iadd.24. A reconfigurable scanner according to claim 23 wherein
the plurality of variable conductive elements are a plurality of
plasma loop sensors..Iaddend.
.Iadd.25. A reconfigurable scanner according to claim 24 wherein
the plasma loop sensors each comprise a loop antenna having at
least a portion of which is an arcuate tube section containing an
ionizable gas, such that the loop antenna is only conductive when
the ionizable gas is ionized..Iaddend.
.Iadd.26. A reconfigurable scanner according to claim 23 wherein
the scanning element comprises an antenna and an electromagnetic
shield formed by the plurality of variable conductive elements, the
electromagnetic shield intersecting transmission lobes of the
antenna in at least the multiple directions being
scanned..Iaddend.
.Iadd.27. A reconfigurable scanner according to claim 26 wherein
the plurality of variable conductive elements are mounted in an
array on a substrate forming the shield..Iaddend.
.Iadd.28. A reconfigurable scanner according to claim 26 wherein
the electromagnetic shield is formed by stacked layers of arrays of
the variable conductive elements..Iaddend.
.Iadd.29. A reconfigurable scanner according to claim 23 wherein
said plurality of variable conductive elements are of at least two
dimensional configurations..Iaddend.
.Iadd.30. A steerable antenna for directing the sensitivity of a
scanner that scans for ID tags, said steerable antenna comprising:
an omnidirectional antenna having a first axis, an annular shield
having a longitudinal axis parallel to said first axis, said
annular shield positioned a selected distance from said first axis,
said annular shield having a plurality of elements, each one of
said plurality of elements including a plasma element that is
variably conductive between a conducting state and a non-conducting
state, said annular shield allowing passage of a signal in a
selected radial direction relative to said first axis; and a switch
configured to control each one of said plurality of elements
between said conducting state and said non-conducting
state..Iaddend.
.Iadd.31. The steerable antenna of claim 30 wherein each said
plasma tube has a plasma density sufficient to reflect a signal at
a selected frequency, said omnidirectional antenna operable at said
selected frequency..Iaddend.
.Iadd.32. The steerable antenna of claim 30 wherein each said
plasma tube has a plasma density less than that necessary to
reflect a signal at a selected frequency, said omnidirectional
antenna operable at said selected frequency..Iaddend.
.Iadd.33. The steerable antenna of claim 30 wherein each said
plasma tube has a plasma density less than that necessary to
reflect a signal at a selected frequency and said switch operates
at a speed wherein said selected radial direction is changeable
between a first and a second radial direction in less than a
millisecond..Iaddend.
.Iadd.34. The steerable antenna of claim 30 wherein said selected
distance is a multiple of one wavelength of a signal to which said
omnidirectional antenna is responsive..Iaddend.
.Iadd.35. The steerable antenna of claim 30 wherein said selected
distance is greater than one wavelength of a signal to which said
omnidirectional antenna is responsive..Iaddend.
.Iadd.36. The steerable antenna of claim 30 wherein said plurality
of elements are individually made to conduct at a plurality of
frequencies..Iaddend.
.Iadd.37. The steerable antenna of claim 30 wherein each one of
said plurality of elements is substantially parallel to said first
axis..Iaddend.
.Iadd.38. The steerable antenna of claim 30 wherein each one of
said plurality of elements is a ring that encircles said first
axis, said rings being stacked to form a cylindrical
shape..Iaddend.
.Iadd.39. The steerable antenna of claim 30 wherein said annular
shield has a substantially cylindrical shape..Iaddend.
.Iadd.40. The steerable antenna of claim 30 wherein said annular
shield has a substantially spherical shape..Iaddend.
.Iadd.41. The steerable antenna of claim 30 wherein said plurality
of elements includes a first group of elements and a second group
of elements, said first group of elements having a substantially
cylindrical shape, said second group of elements having a
substantially cylindrical shape, said first group of elements
positioned inside said second group of elements..Iaddend.
.Iadd.42. A pulsed scanner for scanning for ID tags containing
scannable antennas, said pulsed scanner comprising: a scanning
element responsive to a signal having a selected bearing, said
scanning element having a plurality of variable conductive
elements; a switch configured to electrically control and change a
direction to which said scanning element is responsive by powering
and unpowering selected ones of said plurality of variable
conductive elements, each one of said plurality of variable
conductive elements including a plasma element; and a transceiver
connected to said scanning element, said transceiver configured to
generate an electromagnetic wave signal and receive a responsive
electromagnetic wave signal, said responsive electromagnetic wave
signal received by one of said plurality of variable conductive
elements when said plasma element is in an afterglow
state..Iaddend.
.Iadd.43. The pulsed scanner of claim 42 wherein said plasma
element includes an energized state and said afterglow state, said
energized state resulting from an ac bipolar pulse applied to said
plasma element..Iaddend.
.Iadd.44. The pulsed scanner of claim 43 wherein said ac bipolar
pulse has a frequency greater than an ion acoustic wave
frequency..Iaddend.
.Iadd.45. A pulsed scanner for scanning for ID tags containing
scannable antennas, said pulsed scanner comprising: a scanning
element receiving a signal from a selected direction, said scanning
element having a plurality of variable conductive elements; a
switch that electrically controls and changes a direction to which
said scanning element is responsive by powering and unpowering
selected ones of said plurality of variable conductive elements,
each one of said plurality of variable conductive elements
including a plasma element, each said plasma element being powered
by a pulse, said pulse being an ac bipolar pulse; and a transceiver
connected to said scanning element, said transceiver configured to
generate an electromagnetic wave signal and receive a responsive
electromagnetic wave signal..Iaddend.
.Iadd.46. The pulsed scanner of claim 45 wherein said ac bipolar
pulse has a frequency greater than an ion acoustic wave
frequency..Iaddend.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to the field of RFID (radio
frequency identification) and in particular to a new and useful
plasma-based sensor array used to detect the presence of an
interactive element resulting from interaction of antennas having
variable conductive sections by magnetic induction and/or
electromagnetic waves.
RFID systems have gained much popularity recently as a means for
wireless tracking of individual objects for a variety of purposes.
For example, some retailers have proposed using unique RFID tags
attached to products they sell to be able to track each piece from
the distribution warehouse to the store shelves, and potentially,
to customer's home. RFID systems have applications in anti-theft,
product marketing, intelligence gathering, and security systems,
among others.
Near-field readers incorporating sensors and identification tags
are generally known for use in scanning systems. As used herein,
near fields exist at distances ranging from a fraction of a
millimeter to a few miles, depending on frequency. The near field
is defined as when the wavenumber times the distance of the range
of the antenna is less than one. The far field is defined as when
the wavenumber times the distance of the range of the antenna is
greater than one. The wavenumber is 2.pi./.lamda..
Near-field reader systems take advantage of magnetic field
interference between a powered transceiver and a powered or passive
object to detect the presence of the object by receiving a return
signal from the object with the transceiver.
Presently, card and label near-field readers are formed by metal
loops which read data in the near electromagnetic field. In the
near-field situation, for a loop antenna, the electric field is
effectively zero and only the magnetic field is present. Thus, near
field loop antennas use mutual inductance between active and
passive loop antennas to cause the active loop antenna to receive
data from the passive loop antenna. That is, the magnetic flux from
one loop antenna induces a current in a second loop antenna having
properties dependent on the current and voltage in the first loop.
The magnetic flux interaction and induced current can be used to
transmit information between the loop antennas because of the
dependency. The near-field loop antennas can be more correctly
considered loop sensors or loop readers, since there is no electric
field interaction between the active source and a passive loop.
RFID systems, in contrast, can be both near and far field devices.
RFID systems generally have a longer range than most near-field
systems, because they use radio frequencies, such as 900 MHz, 2.4
GHz, and, more recently, 5.8 GHz to transmit and receive
information between sensor units and passive ID tags.
A problem with all metal antennas used in a sensing array is that
even when they are not active, several antennas arranged in a
multiple orientation array still create unavoidable mutual
inductance and electromagnetic wave interferences between antennas.
That is, even if the metal antenna sensors in an array are
sequentially activated, they still cause mutual interference with
other ones of the antennas. The interferences result in detuning of
the antennas in the array, so that special considerations must be
made when forming arrays of metal antennas.
In the case of inductive loop antennas, to optimize the strength of
the mutual inductance field between an active loop sensor and a
passive loop antenna, the antennas must be parallel to each other.
If the antennas are perpendicular, the magnetic field is zero at
the passive loop and there is no mutual induction. The strength of
the magnetic field at the passive loop increases as the loops move
from a perpendicular to a parallel orientation. For a device to
effectively scan a region for a passive loop, a single loop must
move through a variety of orientations. The range of effectiveness
of an antenna is based on the orientation of the passive and active
loops to each other and the diameter of the loop of the active
sensor.
Patents describing scanning antenna systems using interaction
between active and passive antennas include U.S. Pat. No.
3,707,711, which discloses an electronic surveillance system. The
patent generally describes a type of electronic interrogation
system having a transmitter for sending energy to a passive label,
which processes the energy and retransmits the modified energy as a
reply signal to a receiver. The system includes a passive antenna
label attached to goods that interacts with transmitters, such as
at a security gate, when it is in close proximity to the
transmitters. The label has a circuit which processes the two
distinct transmitted signals from two separate transmitters to
produce a third distinct reply signal. A receiver picks up the
reply signal and indicates that the label has passed the
transmitters, such as by sounding an alarm.
U.S. Pat. No. 3,852,755 teaches a transponder which can be used as
an identification tag in an interrogation system. An identification
tag can be encoded using a diode circuit in which some diodes are
disabled to produce a unique code. When the identification tag is
interrogated by a transponder, energy from the transponder signal
activates the electronic circuit in the tag and the code in the
diode circuit is transmitted from the tag using dipole antennas.
The transponder uses a range of frequencies to send a sufficiently
strong signal to activate a nearby identification tag.
A vehicle identification transponder using high and low frequency
transmissions is disclosed by U.S. Pat. No. 4,873,531. A
transmitting antenna broadcasts both high and low frequency signals
that are received through longitudinal slots in a transponder
waveguide. Transverse pairs in the waveguide adjacent the
longitudinal slots indicate a digital "1", while the absence of
transverse pairs produces a digital "0". The high and low
frequencies are radiated from the transverse pairs to high and low
frequency receiving antennas. The transmitting and receiving
antennas are fixed relative to each other and move with respect to
the transponder.
U.S. Pat. No. 5,465,099 teaches a passive loop antenna used in a
detection system. The antenna has a dipole for receiving signals, a
diode for changing the frequency of the received signal and a loop
antenna for transmitting the frequency-altered signal. The original
transmission frequency is changed to a harmonic frequency by the
diode.
As discussed above, near-field loop sensors or readers differ from
far field loop antennas by the basic difference that in the
near-field, the electric field is usually very small and the
magnetic field of an electromagnetic radiant source is controlling,
while in the far field, the interaction is via electromagnetic
waves. As will be appreciated, the relationships between sources
and receivers are different as well due to the different distances
and fields which affect communication between them.
Plasma antennas are a type of antenna known for use in far field
applications. Plasma antennas generally comprise a chamber in which
a gas is ionized to form plasma. The plasma radiates at a frequency
dictated by characteristics of the chamber and excitation energy,
among other elements.
Plasma antennas and their far field applications are disclosed in
patents like U.S. Pat. Nos. 5,963,169, 6,118,407 and 6,087,992
among others. Known applications using plasma antennas rely upon
the characteristics of electromagnetic waves generated by the
plasma antenna in far field situations, rather than magnetic fields
in near-field conditions.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a scanning
sensor array which eliminates interference between adjacent sensors
in the array in both near-field and far-field application
environments.
It is a further object of the invention to provide a scanning
reader array which can be arranged to scan in multiple directions
without concern for interference between array components.
Yet another object of the invention is to provide a scanning array
composed of variable conductive elements.
A still further object of the invention is to provide an apparatus
and method for scanning a volume for an interactive component
containing a data using a reader with variable conductive
elements.
Accordingly, an scanner using antennas is provided which
effectively scans for items having readable data sources in
controllable directions without interference between scanner
components. The scanner transceives signals by magnetic induction
or electromagnetic wave interaction along sequentially selected
radials using antennas formed using variable conductive
elements.
The scanner is provided in two embodiments. In a first embodiment,
an array of plasma loop sensors are sequentially made active to
scan a space to identify an interactive object comprising a data
source based on mutual inductance or electromagnetic wave
interaction of the scanning plasma reader with the data source. The
data source can be an active or passive antenna of any type,
including loop antennas. The plasma loop sensors are variable
conductive elements, in that they are conducting only when
powered.
The array of plasma loop sensors are connected to a power source,
which may include a frequency switching circuit, and to a sensor
circuit. The power source provides power to each of the plasma loop
sensors as determined by a sequential switch circuit to make the
loop sensors active in turn. The sensor circuit is used to
interpret signals received from the data source by each plasma loop
sensor while it is active.
One or more plasma loop readers can be arranged in arrays in
different orientations to form a sensor and then sequentially
activated to simulate a change in orientation of the sensor without
any physical movement of the plasma loops in the array. Since the
inactive plasma loop sensors are effectively electromagnetically
invisible to the active plasma loop reader, there is no
interference created between them. That is, so long as at least a
section of the loop is formed by a plasma tube, the loop will be
electromagnetically invisible to other sensor loops. When the loop
has at least a section is plasma, the remainder may be another
conductor, such as metal.
The plasma loops can be activated and deactivated in microseconds,
so that very rapid switching among several plasma loops is
possible. The plasma loop readers in the sensor can be arranged in
a variety of configurations, including a sphere, a cylinder or
other geometric shape. The terminals of each plasma loop reader in
the configuration are connected to the power source via a switching
circuit and to the sensor circuit.
In a further embodiment of the plasma loop readers, they may have
several loops of different diameter joined at a common side. That
is, there is a common area at the terminals where a portion of the
circumference of each loop is the same. When a frequency switch is
used in connection with the power source, the power frequency used
to activate the plasma loops can be varied to change the frequency
at which the plasma loop reader is active. The particular diameter
loop in which the plasma is active in the plasma loop sensor is
also changed by changing the active transmission frequency.
In yet another alternative of the plasma reader, the plasma loops
are replaced by metal loops with sections of plasma loop which can
be turned on and off. The plasma loop sections, or plasma switches,
are sufficiently large so that when they are turned off, or made
inactive, the metal loop is opened enough that it rendered
electromagnetically invisible and no longer interferes with any
surrounding active loop readers. The plasma loop sections are
connected to the power source in the same manner as the full loops
and can be switched in the same way.
In a still further alternative, plasma loop sections may be
combined with metal loop sections and mechanical switches, such as
relays and solid state devices. The metal loop sections may form up
to a length of the loop which is effectively electromagnetically
invisible when the switch is used to deactivate the loop.
It is intended that the sensor circuit connected to the antennas in
the array will be capable of interpreting data received from
existing types of passive loops commonly used in security devices
and the like. The plasma loop sensor interacts with existing
passive loops in the same manner as metal loop sensors, but does
not suffer from detuning or interference from surrounding loop
sensors.
In a second embodiment of the scanner, a steerable antenna is
provided combining a transceiving antenna with one or more arrays
of variable conductive elements for filtering, phase shifting,
steering, polarizing, propagating and deflecting an incident signal
at non-backscattering angles.
One embodiment of the steerable antenna comprises an antenna having
a switchable electromagnetic shield of variably conductive elements
for controllably opening a transmission window at selected radial
angles positioned at an effective distance to intersect at least
the transmission radials for the antenna. Preferably, the antenna
is omnidirectional and the shield is concentric around the antenna
to intersect all transmission radials for the antenna. The shield
may also include switchable variable conductive elements for
controlling an elevation angle of the transmission lobe passing
through the window, so that the antenna is steerable on two
axes.
The electromagnetic shield is formed by a cylindrical annular ring
of switched variable conductive elements. In one embodiment, the
shield is a ring of plasma tubes extending parallel with the
omnidirectional antenna. Alternately, when transceiving in
appropriate frequency ranges, the shield is a ring of photonic
bandgap crystal elements or semiconductor elements. When the
variable conductive elements are non-conducting or at low density
in the case of plasma, so that the plasma frequency is lower than
the incident transceived frequencies, the variable conductive
elements are off and form a transmission window. The
omnidirectional antenna can be a conventional metal dipole or other
configuration antenna, a plasma antenna or an optical wavelength
transmitter. Plasma antennas include nested plasma antennas and
even stacked plasma arrays of the same type used to form the
shield.
The transmission window is formed by either turning off power to
the appropriate electromagnetic shield elements, or otherwise
making the desired shield elements transparent to the transmitting
antenna, such as by reducing plasma density below the threshhold
needed to block transmission of an incident signal frequency. The
shield elements are preferably rapidly switchable, so that the
radial transmission direction of the antenna can be changed within
microseconds, or faster by Perot-Etalon effects. The shield
elements are selected for use with antennas broadcasting on a broad
range of frequencies including microwave to millimeter range (kHz
to GHz), TeraHertz, infrared and optical ranges.
An alternate embodiment of the shield utilizes a cylindrical array
of switchable variable conductive elements to provide more
selective control over where openings in the shield are formed. The
cylindrical annular shield with the array surrounds an antenna. The
elements forming the array are arranged in multiple rows and
columns on a substrate. The substrate can be a planar sheet rolled
into a cylinder shape. The variable conductive elements can be
either switchable regions surrounding air or other dielectrics in
fixed gaps or slots, so that the effective size of the fixed slots
can be changed rapidly, or the elements can be formed as linear
conductors, rectangles, stars, crosses or other geometric shapes of
plasma tubes, photonic bandgap crystals or solid state
semiconductors on the substrate. The substrate is preferably a
dielectric, but may also be made from a conductive metal.
A more complex shield for the antenna has one or more stacked
layers, with each layer being a switchable array of variable
conductive elements. The layers are spaced within one wavelength of
adjacent layers to ensure proper function. Each switchable array in
the stack can be a filter, a polarizer or a phase shifter, a
deflector, or a propagating antenna. The layers are combined to
produce a particular effect, such as producing a steerable antenna
transmitting only polarized signals in specific frequency
bands.
Layers of annular rings, for example, can be stacked at distances
corresponding to wavenumber times distance from the central antenna
which correspond to transmission peaks for particular frequencies.
By stacking several frequency-selective layers, a multi-frequency
antenna is produced which is controllable to selectively transmit
and/or receive each frequency along a particular radial of the
antenna.
In a further embodiment of the invention, the scanner can be used
to track a particular ID tag when one or both are moving, without
physical re-orientation of the scanner. A central unit can be
stationary or mobile and has a scanner with one of the two antenna
configurations described which is controllable to scan along a
specified radial from the scanner. The central unit includes
circuits for determining when a connection is made between the
scanner and ID tag and maintaining the connection while they move
relative to each other. Once a connection is made, the
electromagnetic shield of the satellite unit steerable antenna is
activated to produce only a transmission window and radiation lobe
along the radial axis needed to maintain the connection with the
central unit. The steerable antenna shield on the central and each
connected satellite unit is adjusted to compensate for their
relative movement while maintaining the connections.
Conventional ID tags made of metal which are either passive or
actively transmit can be used with the scanner of the invention. An
ID tag having a variable conductive element forming the tag antenna
is provided as well.
The ID tag with variable conductive element antenna can be an
active transmitting or a passive transmitting antenna. Further, the
ID tag can have an active variable conductive element or a passive
variable conductive element. That is, the antenna is a plasma
element which is either connected to an active transmitter, or does
not transmit any information and is only sensed by electromagnetic
interference. And, the plasma element can be normally powered and
active and capable of being sensed by a scanner, or inactive and
thus, electromagnetically invisible. The antenna can be normally
inactive, but weakly or partially ionized and made active by
exciting the plasma element to an active energy state is provided
as well.
The inactive plasma element is excitable to an active state by an
incident received signal. The plasma is energized and permits the
ID tag to generate a detectable return signal with date or
interference in response to the incident signal. The incident
signal may be a scanning signal or other energizing signal. The
plasma in the plasma element may be maintained in a weakly or
weakly partially ionized state by a power source, such as a
battery, laser, voltage source, a radiation source or radioactive
source in a known manner, so that the plasma is more easily fully
energized by the incident signal.
The various features of novelty which characterize the invention
are pointed out with particularity in the claims annexed to and
forming a part of this disclosure. For a better understanding of
the invention, its operating advantages and specific objects
attained by its uses, reference is made to the accompanying
drawings and descriptive matter in which a preferred embodiment of
the invention is illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1A is a front elevation view of a plasma loop antenna of the
invention;
FIG. 1B is a front elevation view of an alternative plasma loop
sensor according to the invention;
FIG. 2 is a side elevation view diagram of the magnetic field
interaction between a plasma loop sensor of FIG. 1 and a passive
loop;
FIG. 3 is a diagram of an array of plasma loop readers at different
orientations;
FIG. 4 is a schematic diagram of a transceiver circuit for use with
a plasma sensor system;
FIG. 5A is a front elevation view of a metal loop sensor with a
plasma section;
FIG. 5B is a front elevation view of an alternative embodiment of
the metal loop sensor and plasma section of FIG. 5A;
FIG. 5C is a front elevation view of a second alternative
embodiment of the metal loop sensor and plasma section of FIG.
5A;
FIG. 5D is a front elevation view of a third alternate embodiment
of a loop having metal and plasma sections and a switch;
FIG. 6 is a front perspective view of an array of plasma loop
readers mounted in a spherical substrate;
FIG. 7 is a sectional top plan view of an alternative embodiment of
the array of FIG. 6 taken across an equator of the spherical
substrate;
FIG. 8 is a front perspective view of a cylindrical substrate
holding an array of plasma loop sensors;
FIG. 9 is a top plan view diagram of a grocery or department store
checkout using a plasma loop sensor array of the invention;
FIG. 10 is a side elevation view of a diagram of a toll collection
system using plasma loop arrays according to the invention;
FIG: 11 is a front perspective view diagram of a security gate
system using a plasma loop scanning array according to the
invention;
FIG. 12 is a top, left, front perspective view of a cube having a
sensor loop on each of the three faces adjacent a vertex;
FIG. 13A is a schematic representation of a planar array of
variable conductive elements on a dielectric surface in a
non-conducting state;
FIG. 13B is a schematic representation of a planar array of slot
elements on a dielectric surface in a non-conducting state;
FIG. 13C is a schematic representation of a polarizer in the form
of a planar array of spoked variable conductive elements on a
dielectric surface in a non-conducting state;
FIG. 13D is a schematic representation of a planar array of
progressively sized, variable conductive elements on a dielectric
surface in a non-conducting state;
FIG. 14A is a schematic representation of an omnidirectional
antenna surrounded by an annular plasma ring;
FIG. 14B is a diagram of an omnidirectional antenna surrounded by
eight plasma tubes with seven energized;
FIG. 14C is a diagram of an omnidirectional antenna surrounded by
sixteen plasma tubes with fifteen energized;
FIG. 15A is a top plan view of a omnidirectional antenna used with
layered arrays of the invention;
FIG. 15B is a side elevation view of the antenna configuration of
FIG. 6B;
FIG. 16 is a diagram illustrating the radiation pattern of a
steerable antenna of the invention;
FIG. 17 is a diagram illustrating the radiation pattern for a
differently configured steerable antenna of the invention;
FIG. 18 is a diagram displaying electromagnetic wave interaction
between a scanning antenna and passive and active ID tags;
FIG. 19 is a diagram illustrating a scanner of the invention used
to determine the contents of a ship containing goods marked with ID
tags.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As used herein, plasma loop sensor and plasma loop reader are
intended to both mean an active loop device having at least a
section of plasma tube, as will be described further herein, when
used in the near-field, and composed of only plasma tubes in
far-field applications. The active loop device is an
electro-magnetic transducer having a conductive plasma section.
That is, the plasma loop reader or sensor can both generate a
magnetic field or electromagnetic wave, depending on whether it is
for near or far-field applications, and sense a corresponding
interfering induction current or electromagnetic wave caused by a
passive or active loop within range of the reader or sensor.
The terms plasma tube or plasma loops referring to plasma elements
should not be taken as limiting on the geometric shape generally
defined by the stated shape, except when the shape is essential to
the function of the plasma element. Any linear dipole, traveling
wave antenna, Yagi antenna, log periodic antenna, horn antenna, or
aperture antenna can be used for the plasma loop antenna herein.
Thus, the plasma element may be formed as a circular loop, a helix,
a coil, an ellipse, a rectangle, a spiral or another shape suitable
for emitting or receiving a signal.
Further, variable conductive element as used herein includes a
plasma element, a photonic bandgap crystal, or a semiconductor,
unless otherwise specified.
Referring now to the drawings, in which like reference numerals are
used to refer to the same or similar elements, FIG. 1A shows a
plasma loop sensor 10 primarily comprising a tube 12 having
electrodes 25, 27 at each end. The tube 12 is bent into a circular
loop. A pair of leads 20, 22 are attached to the electrodes 25, 27
for connecting the tube 12 to a power source (not shown in FIG.
1A).
The tube 12 of the plasma loop sensor 10 contains a gas 15 inside
the plasma loop sensor 10. The gas 15 may be neon, xenon, argon or
other noble gases, as well as mercury or sodium vapors, or other
materials found to produce a suitable plasma. The gas 15 can be
ionized to form a plasma in the tube 12 by applying energy to the
gas 15 using any of several devices including electrodes 25, 27,
inductive couplers, capacitive sleeves, lasers or RF heating.
When the gas 15 is ionized, a current I begins to flow between the
electrodes 25, 27, which in turn generates a magnetic field having
a magnetic flux B. The magnetic field is generated in a direction
perpendicular to the plane of the loop antenna 10. The magnetic
field is characteristic of the current I and voltage used to power
the plasma in the tube 12.
The plasma loop sensor 10 optimal magnetic induction range is equal
to the radius r of the loop. The plasma loop sensors 10 may be made
any size as is practical and required by a particular application.
For purposes of the invention herein, however, the preferred radius
for the plasma loop antennas is between 0.5 cm and 100 cm. Further,
it should be noted that although the optimal range of the plasma
loop sensors 10 is limited by the radius of the loop, the sensors
10 are still effective across a wider range of distances. The
plasma loop sensors 10 may be switched on and off in a matter of
1-10 microseconds, with rapid rise and decay times, so that very
rapid switching of the plasma loop readers 10 is possible.
The frequency of the ionization energy source also affects the
plasma magnetic field radiation frequency. It is possible for the
sensors 10 to radiate at frequencies ranging from 0.1 MHz into the
Terahertz range.
The plasma loop reader of FIG. 1B is a multiple loop plasma reader
71 having three different diameter tubes 72, 73, 74 with a common
tangential side 75 and electrodes 25, 27. A gas inside the tubes
can be ionized to different excitation levels depending on the
energy applied at the electrodes 25, 27. The different ionization
levels correspond to different radiant frequencies for the
electro-magnetic fields generated by the plasma reader 71. Thus,
the multiple loop plasma reader 71 can be used to generate multiple
transmission frequencies or to receive on different frequencies
from transmission by changing the energy supplied to the plasma
loop reader 71.
FIG. 2 illustrates the interaction of a magnetic field 40 of a
plasma loop sensor 10 with a passive metal loop 35. Plasma loop
sensor 10 has a plasma current of which generates magnetic field 40
around the loop 10. The magnetic field 40 is sufficiently strong to
at least effectively extend a distance of about twice the radius r
of the loop 10 to passive loop 35. Magnetic field 40 induces a
current I.sub.i in the passive loop 35.
Passive loop 35 includes a frequency changing circuit 36, which
operates on induced current I.sub.i to alter the frequency of the
received magnetic field and produce a frequency-changed response
magnetic field. The frequency changing circuit 36 causes the
induced current I.sub.i to have the altered frequency. The circuit
36 may be connected to the terminals of the passive loop 35 in a
known manner. Passive loop 35 and frequency changing circuits 36
known in the prior art disclosed herein, for example, may be used
for these components.
The induced current I.sub.I, with a different frequency from the
plasma current I.sub.A, generates a response magnetic field 45
emanating from the passive loop 35. The response magnetic field 45
is also sufficiently strong so as to interact with the plasma loop
sensor 10. As described further below, the plasma loop sensor 10
can also operate in a receive mode to detect response magnetic
field 45. In the receive mode, the plasma loop sensor 10 has a
second induced current that is different from plasma current
I.sub.A, with characteristics corresponding to the response
magnetic field 45.
It should be noted that if the response magnetic field 45 is varied
in response to a changing induced current I.sub.i controlled by the
frequency changing circuit 36, that more complex communication is
possible, such as transmission of an identifying code in addition
to simply indicating the presence of the passive loop 35.
When the plasma loop sensor 10 and passive loop 35 too far apart to
take advantage of the near-field situation and magnetic induction
is insufficient to generate a response, the plasma loop sensor 10
can be used in a far-field type application instead. The plasma
loop sensor 10 can be configured to transmit an electromagnetic
wave, which generates a corresponding response similar to the
magnetic induction response in the passive loop 35.
Thus, regardless of whether the interaction is only through
magnetic induction or by electromagnetic wave, a single plasma loop
sensor 10 can be used to detect the presence of a passive loop 35
and receive communications therefrom. However, the ability of the
plasma loop sensor 10 to generate the induced current I.sub.i so
that a response magnetic field is subsequently generated and
received is dependent in part on the relative orientation of the
plasma loop sensor 10 and passive loop 35 to each other. The loops
10, 35 must be oriented parallel to each other, as shown in FIG. 2,
so that the interaction between the generated magnetic fields 40,
45 is maximum. As the relative orientation between the antennas 10,
35 changes from parallel to perpendicular, the field interaction
with the antennas 10, 35 goes from maximum to zero.
To solve this problem, there are two primary solutions. One is to
physically move the loops 10, 35 relative to each other to cover
different orientations. The other is to create an array of several
differently oriented plasma loop sensors 10 that can be
sequentially activated to send and receive magnetic fields 40,
45.
In the latter case, plasma loop sensors 10 provide the benefit that
they can be easily switched on and off rapidly in sequence.
Further, plasma loop sensors 10 can be arranged in any type of
sequentially-fired array without affecting adjacent ones of the
plasma loop sensors 10 because when the gas 15 is not being ionized
to form plasma, the inactive sensor 10 is electromagnetically
invisible to another, active plasma loop sensor 10.
An example of an array 100 is shown in FIG. 3, in which seven
plasma loop sensors 10 are arranged co-planar directed to different
angles at 30.degree. intervals. Although the plasma loop sensors 10
are shown arranged in an arc, this is only for purposes of
illustrating the rotation to different angles and is not required.
The plasma loop sensors 10 may be arranged co-linear as well, with
each loop sensor 10 being rotated 30.degree. from the facing of the
previous loop sensor 10. Further, the angular rotation from one
antenna to the next may be more or less than 30.degree., depending
on the number of plasma loop sensors 10 in the array 100 and the
desired effective range of each plasma loop sensor 10 based on both
the expected distance and angular orientation offset from a passive
loop 35.
Each plasma loop sensor 10 has its electrodes connected to a
transmitting and receiving circuit (not shown in FIG. 3) with
switching between modes and loop sensors 10, such as will be
described in more detail below.
FIG. 4 diagrams one possible transceiver circuit 200 for use with
an array 100 of plasma loop antennas 10 mounted in substrates 5 for
protection during use. A DC power supply 205 is connected to a
mixer 210 and an analog to digital converter 230. The power supply
205 is preferably one which provides standard digital and other
voltages needed for operating the circuit components.
The transmit segment 215 of the circuit 200 includes RF CW
oscillator 210 having its output connected to an RF amplifier 220.
The RF amplifier 220 combines a CW signal from the oscillator 210
with a modulated signal from a connected RF modulator 225 and
generates an amplified pulse modulated (PCM) signal having
information for transmitting with the plasma loop sensors 10. The
PCM signal is sent to the plasma loop sensor array 100 for
energizing an active one of the plasma loop sensors 10 and creating
a magnetic field and electromagnetic wave.
The PCM signal may be varied using a digital code generator 230
connected to the RF modulator to produce different RF modulated
signals. The varying PCM signal in turn provides a time-varying
signal to the active plasma loop sensor 10 and results in a
time-varying magnetic field and electromagnetic wave being produced
by the plasma in the active plasma loop sensor 10. The digital code
generator 230 provides a code word from a look-up table stored in
ROM 240. Changing the code word causes the RF modulator to produce
different RF modulated signals.
The RF amplifier 220 outputs the PCM signal to sensor switch 270
connected to plasma loop sensor array 100. Sensor switch 270
controls switching between the transmit 215 and receive 235 circuit
segments. Preferably, the sensor switch 270 cyclically alternates
between transmit and receive modes.
A switch 105 within array 100 is used to sequentially switch power
to the several plasma loop sensors 10 in array 100. Only one plasma
loop sensor 10 is made active at one time; the remaining plasma
loop sensors 10 do not receive any power so that they are
effectively rendered electromagnetically invisible to the active
sensor 10 and do not detune the active sensor 10. While a plasma
loop sensor 10 is active, the sensor switch 270 provides at least
one transmit/receive cycle for the active plasma loop sensor
10.
After the sensor switch 270 permits a transmit phase in which the
active plasma loop sensor 10 generates a magnetic field and
electromagnetic wave, the sensor switch 270 changes to connect the
active plasma loop sensor 10 to a receive segment 235 of the
transceiver circuit 200.
The receive segment 235 includes a limiter circuit 260 for ensuring
the received signal from the array is scaled within the operating
range of a receiver 265. The limiter circuit 260 protects the
receiver 265 from over-voltage instances in the received signals.
The receiver then demodulates a coded reply RF PCM signal, which
can be generated by interaction of the active plasma loop sensor 10
with a passive loop within the near-field range. If necessary, the
receiver can also amplify the received RF PCM signal to ensure
proper decoding.
The transceiver circuit 200 includes components for interpreting
the received signal. The demodulated coded reply signal is sent
from the receiver 265 to a signal processor 255. The signal
processor 255 conditions the coded reply signal for input into a
code comparator 250. When the conditioned reply signal is input at
the code comparator 250, the coded reply is compared to known or
expected replies stored in a look-up table stored in ROM.
The result obtained by the code comparator 250 is sent to an output
232. The result may be information received from the passive loop
or it may be a null if no passive loop was detected during the
transmit/receive cycle.
The output 232 can be connected to any device capable of using the
digital signal from the A/D converter. For example, in grocery
scanning system, the output 232 may be connected to a cash register
to provide price and item information received from a scanned
object in a grocery bag.
While loop sensors wholly composed of plasma tubes are preferred
for use, FIGS. 5A-5C illustrate metal loop sensors 30 having plasma
sections 31 which are electromagnetically equivalent to the plasma
loop sensors 10 described above. The metal loop sensors 30 with
plasma sections 31 are also magnetically invisible to adjacent
loops when the plasma sections 31 are deactivated. That is, the
plasma sections 310 are sufficiently long that when the ionizing
energy is removed from the electrode terminals 25, 27, the loop
circuit is broken so that a magnetic field will not generate a
current in the metal loop 30. Since current cannot flow through the
loop 30 except when the gas 15 is ionized to form plasma, the metal
loop sensor 30 also appears electromagnetically invisible and does
not cause detuning of surrounding sensors 10, 30 when it is
inactive.
The plasma sections 31 act like switches for the metal loop sensors
30 to activate and deactivate them in the same manner as the plasma
loop sensors 10 are activated and deactivated. When power is
supplied to the plasma section 31 through leads 20, 22 and
electrodes 25, 27, the metal loop sensor 30 is activated and
transmits a magnetic field which can interact with other adjacent
loop sensors. The metal loop sensors 30 can be connected to a
circuit such as that shown in FIG. 4 in the same manner as the
plasma loop sensors 10. Arrays of the metal loop sensors 30 can be
connected, oriented and sequentially switched using the plasma
sections 31 in the same manner as the plasma loop sensors 10
described herein as well.
The plasma section 31 can be as short as a 1.degree. arc segment of
the metal loop sensor 30, up to the entire circumference, less a
gap for electrodes, so that it is the same as plasma loop sensor
10. However, when the metal loop sensor 30 embodiment of the loop
sensors 10 is used, it is preferred that the plasma section 31 is
an arcuate segment between about 1.degree. and 10.degree. long.
In FIG. 5D, a further alternative loop 10 structure is provided in
which a plasma loop 10 has a switch 80 in series. The switch 80 may
be an electromechanical relay switch, a solid state switch or other
similar switch that is electrically changeable between conducting
and non-conducting positions.
FIGS. 6-8 illustrate scanning arrays 100 of plasma loop readers 10
supported in rigid substrates 290, 295.
In FIG. 6, a spherical non-magnetic substrate 295 supports an array
100 of plasma loop readers 10 on its surface. The substrate 295 is
selected so that it does not interfere with the magnetic fields and
electrical properties of the plasma loop sensors 10. Although
non-magnetic substrates are preferred, it should be understood that
ferrite materials may be used for the substrate as well.
The terminal leads 20, 22 of each plasma loop sensor 10 are
connected to a switching transceiver (not shown in FIG. 6), such as
one like that illustrated in FIG. 4, so that each plasma loop
sensor 10 may be sequentially activated.
The plasma loop sensors 10 are arranged around the surface of the
sphere oriented along many different radii of the sphere. The
orientation of the plasma loop sensors 10 allows sequential
scanning of a broad range of angles for corresponding passive loops
35 within the effective range of the plasma loop sensors 10. Since
the orientations of the plasma loop sensors 10 varies across the
surface of the spherical substrate 295, the substrate itself does
not need to rotate. The sequential activation of the plasma loop
sensors 10 virtually rotates the scanning angle without moving the
substrate 295. Clearly, when the substrate 295 is spherical, a wide
range of angles can be scanned for corresponding receiving loops in
objects carrying the receiving loops.
FIG. 7 illustrates another embodiment of the spherical substrate
295 having an array 100 of plasma loop readers 10 embedded within
the thickness of the substrate 295. The substrate 295 is shown with
the top half of the sphere removed. As can be seen, the plasma loop
readers 10 are oriented at different angles along each of several
axes of the sphere. The orientations of the plasma loop readers 10
are selected to maximize the scanning coverage of the array 100. As
in FIG. 6, the plasma loop readers 10 are each connected to a
switch and transceiver circuit (not shown in FIG. 7) for sequential
activation to ensure there is no electromagnetic interference
between plasma loop readers 10 in the array 100.
In FIG. 8, a cylindrical substrate 290 has an array of plasma loop
sensors 10 arranged around the surface of the substrate 290. The
substrate is selected to have the same properties as the spherical
substrate 295. The cylindrical substrate 290 scans for
corresponding receiving passive loops located around the axis of
the cylinder within the effective range of the plasma loop sensors
10. The cylindrical substrate 290 with the plasma loop sensors 10
mounted only on the surface is limited compared to the spherical
substrate 295 in that only two axes of receiving passive loop
orientations can be fully scanned versus three.
However, if the plasma loop sensors 10 are embedded in a
cylindrical substrate 290 around the surface and oriented rotated
about the cylinder radial axis to different angles, then all three
axes can be scanned with a sensor array using the cylindrical
substrate 290. That is, passive loops oriented perpendicular to the
longitudinal axis of the cylindrical substrate 290 could be
detected as well.
Arrays 100 of the plasma loop readers 10 can be used in a variety
of scanning applications to detect a receiving passive loop, such
as the one shown in FIG. 2.
FIGS. 9-11 depict different scanning applications for arrays of the
plasma loop sensors which take advantage of the fact that the array
itself does not need to move physically to scan a wide range of
angles, as discussed above.
In FIG. 9, a checkout lane 54 of a grocery or department store is
shown having a cart 53 containing packages or bags 33a containing
goods. Depending on the circumstances, either the packages or the
goods are each encoded with a unique receiving passive loop (not
shown). The lane 54 has two counters 55, 55a each having a plasma
loop scanner 56, 56a located vertically at about the level of the
bags 33a in the cart 53. Each plasma loop scanner includes an array
of plasma loop sensors and a switching and transceiver circuit for
sequentially activating each sensor in the array to query the goods
in the bags 33a. The outputs of the transceiver circuits are
connected to a cash register 58 for ringing up each unique goods
detected in the cart 53 and completing the sale.
The scanners 56, 56a use an array such as the spherical or
cylindrical arrays of FIGS. 6-8, or a semi-sphere array which scans
the 180.degree. in the lane 54. The semi-sphere array can be
created by cutting the spherical substrate 295 in half and using
only one half. The arrays are connected to a transceiver circuit
like that of FIG. 4, or another circuit having a similar
function.
When the transceiver of FIG. 4 is used, the ROM 240 provides a
look-up table for identifying each uniquely coded object having a
receiving passive loop that is detected by the scanners 56, 56a.
Either of the cash register 58 or the scanners 56, 56a includes a
logic circuit or computer for determining when the same receiving
passive loop is detected by a subsequently activated plasma loop
sensor in the array. The logic circuit or computer ignores the
duplicate detection, while passing newly detected goods to the cash
register 58 for pricing and totaling the purchase.
The scanner system of FIG. 9 provides a checkout line in which it
is unnecessary for a customer to unload the cart 53 for a clerk to
individually scan items in the bags 33a. The contents of the bags
33a can be determined solely by using the scanners 56, 56a.
Further, depending on the effective range of the arrays in the
scanners 56, 56a, only one of the scanners may be needed. Where the
distance across the lane 54 is too great for a scanner 56 from one
side to effectively detect receiving sensors on the far side of the
lane, the second scanner 56a can be used as well.
Used in combination with a known debit and credit card terminal 58a
connected to the cash register 58, a single clerk can effectively
manage several checkout lanes 54 at once, since the checkout is
fully automated except when cash or a check is used as payment.
Consumers can bag their goods as they shop since it is not
necessary to remove the items for checkout, further eliminating
wasted checkout time.
FIG. 10 illustrates a toll collection system in which a toll gate
86 is equipped with a scanner 87 connected to a transaction manager
88. The scanner 87 includes an array of plasma loop readers 10, 30
as in the checkout lane scanners 56, 56a. The array is used to
rapidly sequentially scan for receiving passive loops oriented in a
range of angles on cars 81, 82, 83 passing underneath the toll gate
86.
Each car 81-83 that will use the system is assigned a unique
receiving sensor for identifying the car. The transaction manager
88 contains logic programming for determining whether a particular
car 81-83 has been scanned already or if it is unique from prior
scanned cars. The toll gate 86 may contain anti-fraud devices as
well, such as weight-triggered checks against whether a receiving
passive loop was detected or human toll collectors who can monitor
the system.
As will be appreciated, the horizontally and vertically oriented
scanners described above can be used in wide range of applications
where an object coded with a unique receiving passive loop passes
below or adjacent a scanning array of plasma loop sensors. Further,
the particular vertical or horizontal orientation shown in the
examples is not intended to be limiting, as the scanners could be
oriented to any fixed position which is more practical, subject to
ensuring the plasma loop readers in the scanner are oriented to
scan the appropriate area.
And, when a unique identification is not required, but merely
detection, the receiving passive loop in the object to be detected
does not need to include a unique code. The scanning array is used
to simply detect the presence of the receiving passive loop and
generate an alert, such as in a store security system or another
gated area for holding animals or objects carrying receiving
passive loops having a scanner at the gate.
As an example, in another embodiment of a scanning system, FIG. 11
shows a gate 91 having two walls containing scanners 92, 92a
connected to an alarm system 93. A person 95 has a card 97 or other
substrate carrying a receiving passive loop. If the person 95
passes through the gate 91 with the card 97, the plasma loop
sensors in the scanners 92, 92a will detect the presence of the
card 97 by interaction with the passive loop and the alarm system
93 will generate a response, such as shutting the gate 91, sounding
a siren or making a light flash. Such a scanning system can be used
for ensuring certain persons do not exit a gated area, provided
compliance with carrying the card 510 can be guaranteed.
Alternatively, the card 97 may contain a coded identifier for the
person 95. The card 97 may have a unique identifier, or simply
coded to indicate membership in a group or class. The card 97 can
be coded to permit access through some gates 91 without sounding an
alarm, while passing others will activate the alarm. In such cases
the scanners 92, 92a and alarm system 93 include a code table for
interpreting which card 97 is passing the gate 91 and determining
the permissions associated with the encoding on the card 97 before
sounding an alarm or preventing passage.
In FIG. 12, a further alternative sensor 60 configuration is
displayed. The sensor 60 is formed as a cube with sensor loops 10
provided on at least three panels 62, 64, 66 adjacent one vertex 65
of the cube. The sensor loops 10 are connected to a switch, such as
in the circuit of FIG. 4, for activating the sensor loops 10 in
cyclical succession. The sensor loops 10 may be controlled by
mechanical switches, plasma switches or solid state switches.
Preferably, the switch is a low resistance switch. The resistance
when the loop 10 is conducting, or closed, is preferably less than
1 Ohm, while in the open state, the resistance should be high. The
open state capacitance can be low.
It should be understood that any one or a combination of the plasma
loop sensor 10, metal loop sensor 30 with plasma section 31 or
multiple loop plasma sensor 71 can be used in the arrays and
scanning systems described herein.
The loop antennas described herein can be used effectively in both
near-field and far-field applications, as defined previously, using
magnetic induction or electromagnetic wave interaction between
sensor loops 10 and passive or active sensed loops. And, the loop
antennas are useful as RFID sensors, able to send and receive
electromagnetic wave signals at frequencies including radio
frequency up to Terahertz range frequencies. That is, each of the
sensors described herein as using only magnetic induction can also
rely instead upon electromagnetic wave interaction when the sensing
unit or other signal generator is properly driven, so that the
sensor system is expanded for use in far-field applications.
For example, in the toll collection system of FIG. 11, RF frequency
electromagnetic waves may be generated and interact with a unique
receiving sensor in each car to generate an RF return signal which
is interpreted in the same manner as the signal generated solely by
magnetic induction. A far-field sensor may be preferable in this
application in particular to permit higher vehicle speeds and to
provide more distance between a vehicle and toll barriers, since a
far-field sensor will be effective at a greater range.
Further, although the sensed loops 35 are referred to herein as
passive loops, it is envisioned that the sensed loops can be active
also, so as to produce their own electromagnetic field. For
example, a lithium battery source could be connected with the
sensed loop and frequency changing circuit like that shown in FIG.
2 to power the sensed loop and circuit. The principles of
near-field induction and far-field electromagnetic wave interaction
are not changed and the plasma loop sensors 10, 30, 71 can still
detect the presence or absence of such active sensed loops, as well
as receive information from the sensed loops.
An alternate reconfigurable antenna, which can be used as the
scanning element of any of the examples of FIGS. 9-11, among other
things, will now be described with reference to FIGS. 13A-17.
FIG. 13A shows an array 310 of linear variable conductive elements
320 on a dielectric surface 330. The array 310 of FIG. 13A
represents the foundation of the steerable antennas described
herein. The array is configurable, by energizing all, none or
specific ones of the elements 320, to filter selected frequencies
of electromagnetic radiation, including in the optical range. It
should be noted that elements 320 are dipoles. Feeds (not shown)
are provided to each element 320 in the array 310 using connectors
which are electrically small with respect to the dipole and
relevant frequencies.
Depending on the frequency range desired to be affected by the
array 310, the variable conductive elements 320 are formed by
different structures. In the RF frequency range, the variable
conductive elements 320 are a gaseous plasma-containing element,
such as a plasma tube. In the millimeter infrared or optical
region, the variable conductive elements 320 can be dense gaseous
plasma-containing elements or semiconductor elements. And, in the
optical region, the elements are photonic bandgap crystals. The
variable conductive elements 320 are referred to herein primarily
as gaseous plasma-containing elements or plasma tubes, but, unless
specifically stated otherwise, are intended to alternately include
semiconductor elements or photonic bandgap crystals, depending on
the desired affected frequency of the incident electromagnetic
waves. And, as used herein, plasma tube or plasma element is
intended to mean an enclosed chamber of any shape containing an
ionizable gas for forming a plasma having electrodes for applying
an ionizing voltage and current.
FIG. 13B illustrates an alternate embodiment of the array 310 of
FIG. 13A. In FIG. 13B, a second array 312 has slot elements 322 on
a dielectric substrate 330. Slot elements 322 may also be plasma
elements, photonic bandgap crystals or semiconductor elements,
depending on the filtered frequencies.
The arrays 310, 312 of the invention use plasma elements 320, 322
as a substitute for metal, as depicted in FIGS. 13A-B. When metal
is used instead for the elements 320, 322 each layer has to be
modeled using numerical methods and the layers are stacked in such
a way to create the desired filtering. Genetic algorithms are used
to determine the stacking needed for the desired filtering. This is
a complicated and numerically expensive process.
In contrast, arrays 310, 312 can be tuned to a desired filtering
frequency by varying the density in the plasma elements. This
eliminates much of the routine analysis involved in the standard
analysis of conventional structures. The user simply tunes the
plasma to get the filtering desired. Plasma elements 320, 322 offer
the possibility of improved shielding along with reconfigurability
and stealth. The array 310 of FIG. 13A, for example, can be made
transparent by simply turning the plasma off.
As the density of the plasma in a plasma element 320 is increased,
the plasma skin depth becomes smaller and smaller until the
elements 320, 322 behave as metallic elements and the elements 320,
322 create filtering similar to a layer with metallic elements. The
spacing between adjacent elements 320, 322 should be within one
wavelength of the frequency desired to be affected to ensure the
elements 320, 322 will function as an array.
The basic mathematical model for these arrays 310, 312 models the
plasma elements 320, 322 as half wavelength and full wavelength
dipole elements in a periodic array 310, 312 on a dielectric
substrate 330. Theoretically, Flouquet's Theorem is used to connect
the elements. Transmission and reflection characteristics of the
arrays 310, 312 of FIGS. 13A-B are a function of plasma density.
Generally, as plasma density increases in the elements 320, 322,
the arrays 310, 312 will block transmission and reflect incident
electromagnetic waves of increasing frequency.
In the array 310, 312 of FIGS. 13A-B, a scattering element 320, 322
is assumed to consist of gaseous plasma contained in a tube. It
should be noted that the plasma elements 320, 322 may be divided
along their lengths into segments 322a for the purpose of defining
current modes.
The arrays 310, 312 can be designed to be a switchable reflector.
By placing the elements 320, 322 closer together, a structure is
produced which acts as a good reflector for sufficiently high
frequencies. A reflective array 12, has the same general structure
as in FIG. 13B, but with the elements 322 more densely packed. For
this example, the length, diameter, vertical and lateral spacing
are 10 cm, 1 cm, 11 cm, and 2 cm, respectively.
The calculated reflectivity for the perfectly conducting case as
well as for several values of the plasma frequency using the values
above was determined. For frequencies between 1.8 GHz and 2.2 GHz
the array 12 operates as a switchable reflector, dependent upon the
plasma frequency in the scattering elements 322. By changing the
plasma frequency of the elements 322 from low (about 1.0 GHz) to
high (10.0 GHz or more) values, the reflector goes from perfectly
transmitting to highly reflecting.
The arrays 310, 312 can function in this manner based on the
understanding that the current modes induced in the plasma elements
320, 322 have the same form but different amplitude from those for
a perfect conductor. The reflectivity of the array 310, 312 is
directly proportional to the squared amplitude of the current
distribution induced in the elements 320, 322 by the incident
radiation. Based on this observation, it is clear the reflectivity
of a plasma array structure can be obtained from that for a
perfectly conducting structure by scaling the reflectivity with an
appropriately chosen scaling function.
The scaling function is defined based on the results of the exactly
solvable model of scattering from an infinitely long partially
conducting cylinder. The scaling of the current amplitude vs.
plasma frequency in the plasma FSS array is approximated as an
isolated infinitely long partially conducting cylinder.
The reflectivity for a perfectly conducting array, obtained by the
Periodic Moment Method, is then scaled to obtain the reflectivity
of the plasma array vs. plasma frequency. The results of these
calculations support the concept that switchable filtering behavior
can be obtained with the use of the plasma array 310, 312 of FIGS.
13A-B.
With respect to FIGS. 13A-B, it should be observed that while the
arrays 310, 312 have been described as elements 320, 322 supported
on dielectric 330, the arrays 310, 312 may be formed in reverse as
well. That is, permanent slots may be formed through a variable
conductive area, such as a plasma body, surrounding the slot. The
effective size of the slot can be changed with respect to
electromagnetic waves by modifying the properties of the variable
conductive area surrounding the slot. For example, by switching a
plasma body between conducting and non-conducting states, and/or
changing the frequency and plasma density in the plasma body, the
effective size of the slots can be changed. Changing the effective
size of the slots permits the array to filter different
frequencies.
An example of the utility of this feature is found in connection
with radomes, which are conventionally formed as metal shells with
bandpass slots tuned for the enclosed radar antenna operating
frequency. A radome is improved by forming the radome structure
from the substrate 330 and providing an array 310, 312 with slots
surrounded by variable conductive regions on the substrate 330.
Unlike a conventional radome, the array 310, 312 of the invention
can include fixed slots in this embodiment, but is also
reconfigurable to pass different frequencies electronically rather
than mechanically. By changing the conductivity of the variable
conductive regions surrounding the slots, the effective slot size
is changed, and the radome is "retuned" to a different frequency.
Thus, a multiple frequency radar antenna could be housed in a
radome formed by an array 310, 312 of the invention.
In a further variation of this embodiment, the dielectric substrate
330 could be replaced by a conductive metal substrate. Depending
whether the array 310, 312 is formed by plasma elements or slots
surrounded by variable conductive regions, the result is either a
single frequency or tunable frequency bandpass filter. But, in such
case, it should be understood that the limitations of using
conductive metal as the substrate will apply to the function of the
arrays 310, 312 used alone or together.
FIGS. 13C and 13D illustrate further embodiments of the arrays 310
in which the plasma-containing elements 320 have different
configurations to produce different effects.
FIG. 13C shows an array 314 which can function as a polarizer.
Variable conductive scattering elements 324 in the polarizing array
314 are star-shaped. Polarization on different axes is effected by
changing the conductivity of the several spokes 324a-f of each
element 324 in the array 314. By coordinating the conductivities of
each spoke 324a-f of the several elements 324 in the array 314, a
wave passing through the array can be polarized. More importantly,
the polarization of an incident signal can be controllably changed
simply by changing the conductivities of the spokes 324a-f.
In FIG. 13D, the array 316 on substrate 330 is composed of variable
conductive elements 326 which are sized progressively smaller in
each row of the array 316. That is, the top row of elements 326 are
largest, while the bottom row of elements 326 are the smallest.
An array 316 as shown in FIG. 13D will produce progressive phase
shifting, for example, when the array 316 is positioned 1/8
wavelength above a ground plane (not shown). A standing wave is
developed between the dielectric substrate 330 and array 316 and
the ground plane. Depending on the effective length of the elements
forming the array 316, a phase shift is produced which causes the
reflection angle to change. By electrically reconfiguring the
length of the variable conductive elements 326 in the array 316, a
flat, variable phase shift, steerable antenna is produced having
characteristics otherwise similar to a parabolic steerable antenna
with fixed phase shifts.
When multiple arrays as shown in FIGS. 13A-D are used in
combination, selective filtering and other effects can be produced.
Any of the arrays 310-316 can be driven by feeds as well to act as
a transceiving antenna, rather than simply powered for producing
particular effects. For example, a driven array 310 of dipoles as
in FIG. 13A, can be combined with a polarizing array 314 as in FIG.
13C, a bandpass array 310, 312 of FIG. 13A or 13B and a phase
shifting array 316 of FIG. 13D to transmit polarized
electromagnetic waves at selected frequencies in specific,
changeable, radial directions. The arrays 310-316 used should all
be spaced within one wavelength of the transmitted frequency of
each other. Alternatively, as discussed herein, the arrays 310-316
can be combined for use with other driven antennas to control their
radiation patterns.
While the variable conductive elements 320, 322, 324, 326
illustrated in FIGS. 13A-D are preferably dipoles or the shapes
indicated, the arrays 310-316 may be formed by elements 320-326 of
different geometric shape. Alternate elements may have any antenna
or frequency selective surface shape, including dipoles, circular
dipoles, helicals, circular or square or other spirals, biconicals,
apertures, hexagons, tripods, Jerusalem crosses, plus-sign crosses,
annular rings, gang buster type antennas, tripole elements, anchor
elements, star or spoked elements, alpha elements, and gamma
elements. The elements may be represented as slots through a
substrate surrounded by variable conductive surfaces, or solely by
variable conductive elements supported on a substrate. The slots
may be filled by a dielectric, or simply be open and filled by
air.
FIG. 14A shows a steerable antenna 410 of the invention composed of
an omnidirectional antenna 400 surrounded by an annular shield 420.
Antenna 400 is a dipole, and can be a radiating plasma tube, a
conventional metal dipole antenna, or a biconical plasma antenna
for broadband radiation. Shield 420 is composed of variably
conductive elements which can be switched between conducting and
non-conducting states, and made to conduct at different
frequencies. In one embodiment, the shield 420 may be formed by a
cylindrical array formed by curling one or more of any of arrays
310-316 illustrated in FIGS. 13A-D. In a preferred embodiment,
illustrated in FIGS. 14B and 14C and discussed in greater detail
below, the shield 420 is composed of vertically oriented
plasma-containing elements 422, such as plasma tube elements. The
plasma tubes 422 form a simple array of one row and multiple
columns surrounding the antenna 400. The plasma tubes 422 may be
mounted in a substrate or other electromagnetically transparent
material to assist maintaining their placement.
The configuration of antenna 410 becomes a smart antenna when
digital signal processing controls the transmission, reflection,
and steering of the internal omnidirectional antenna 400 radiation
using the shield 420 to create an antenna lobe in the direction of
the signal. Multilobes may be produced in the case of the
transmission and reception of direct and multipath signals. The
shield 420 is opened or made electrically transparent to the
radiation emitted by the omnidirectional antenna 400 using controls
to switch sections or portions of the shield 420 between conducting
and non-conducting states, or by electrically reducing the density
or lowering the frequency of the shield elements 422.
The distance between omnidirectional antenna 400 and plasma shield
420 is important, since for given frequencies, the antenna 410 will
be more or less efficient at passing the transmitted frequencies
through apertures in the shield 420. Specifically, the release of
electromagnetic antenna signals from antenna 400 depends upon the
annular plasma shield 420 being positioned at either one wavelength
or greater from the antenna 400, or at distances equal to the
wavenumber times the radial distance, or kd, to interact with the
transmitted signals effectively. Thus, an electromagnetically
effective distance between the shield 420 and antenna 400 is one
wavelength or greater of the transmitted frequencies the shield is
intended to act upon, or at distances corresponding to kd are
satisfied, as discussed further herein.
It is envisioned that multiple annular plasma shields 420 can be
positioned around the antenna 400 to provide control over
transmission of multiple frequencies. For example, only the shield
420 corresponding to a desired transmission frequency could be
opened along a particular radial, while all other frequencies are
blocked through that aperture by other shields 420.
FIGS. 14B and 14C illustrate two embodiments of the antenna 410 of
FIG. 14A. The antenna 410 in each case is comprised of a linear
omni-directional antenna 400 surrounded by a cylindrical shell of
conducting plasma elements 422 forming plasma shield 420.
Preferably, the plasma shield 420 consists of a series of tubes 422
containing a gas, which upon electrification, forms a plasma.
Fluorescent light bulbs, for example, can be used for tubes 422.
The plasma is highly conducting and acts as a reflector for
radiation for frequencies below the plasma frequency. Thus when all
of the tubes 422 surrounding the antenna are electrified and the
plasma frequency is sufficiently high, all of the radiation from
omnidirectional antenna 400 is trapped inside the shield 420.
By leaving one or more of the tubes 422 in a non-electrified state
or lowering the frequency below the transmission frequency of
antenna 400, apertures 424 are formed in the plasma shield 420
which allow transmission radiation to escape. This is the essence
of the plasma window-based reconfigurable antenna, or plasma window
antenna (PWA). The apertures 424 can be closed or opened rapidly,
on micro-second time scales in the case of plasma, simply by
applying and removing voltages.
FIG. 14B shows the configuration when the PWA 410 has seven active
conductors 422 in the shield 420. The following simple geometric
construction for creating the plasma shield 420 is used. For
forming a complete shield 420, N cylinders 422 are placed with
their centers lying along a common circle chosen to have the source
antenna 400 as its center. Some distance from the origin d is
selected as the radius. The distance can be calculated to produce
optimal results for a given PWA 410 frequency, but should be within
one wavelength to be effective. Then, the circle of radius d is
divided into equal segments subtending the angles:
.PSI..sub.1=2.pi.dN where the integer 1 takes on the values -1, 0,
1, . . . N-1. The apertures 424 are modeled by simply excluding the
corresponding cylinders (plasma tube 422) from consideration. Thus,
for example, the mathematical model of FIG. 14B was generated by
first constructing the complete shield 420 corresponding to N=8.
Then, the illustrated structure having one aperture 424 was
obtained excluding the cylinder corresponding to 1-2 where we have
numbered the cylinders assuming the angle to be measured from the
positive x-axis (i.e, extending 90.degree. to the right).
In the following analysis, it is convenient to specify the cylinder
radius through the use of a dimensionless parameter .tau. which
takes on values between zero and unity. More explicitly, the radius
of a given cylinder (all cylinder radii assumed to be equal) is
given in terms of the parameter .tau., the distance d of the
cylinder to the origin, and the number of cylinders needed for the
complete shield N by the expression: a-d.tau. sin(.pi.N)
It should be noted that there is no need to restrict the steerable
antenna 410 to configurations of touching conductor cylinders. When
the plasma tubes 422 are powered to sufficiently high plasma
density that the frequency exceeds the transmission frequencies,
the size of any gaps between the tubes 422 and distance from the
omnidirectional antenna 400 determine the extent of signal
reflection caused by the plasma tubes 422. When spaced properly and
powered sufficiently, plasma tubes 422 produce a perfectly
reflective shield 420 that prevents electromagnetic signals from
omnidirectional antenna 400 from escaping and transmitting, even
when gaps between tubes 422 are present.
As the plasma density, and therefore, the frequency, are decreased,
in a particular plasma tube 422, that tube becomes transparent for
electromagnetic signals generated by the omnidirectional antenna
400. Thus, if a single plasma tube is powered down so as to be
transparent to a particular frequency or all frequencies, an
electromagnetic signal transmitting from omnidirectional antenna
400 will be permitted to escape or broadcast along the radials
passing through the aperture formed by the transparent plasma tube
422 and any adjacent gaps. The antenna signal can be steered by
simply opening and closing apertures by powering and unpowering the
plasma tubes 422. The amount of radiation released will depend in
part upon the distance of the plasma tube ring from the antenna 400
times the wavenumber of the antenna radiation.
A multi-frequency steerable antenna can be created by adding
further rings of plasma tubes 422 spaced apart and at radial
distances from antenna 400 to optimally affect particular
frequencies. An antenna of this configuration permits selectively
transmitting specific frequencies along specific radials.
As a further expansion of the frequency bandwidth of the antenna,
the transceiving antenna 400 can be a nested antenna. That is, a
smaller, higher frequency antenna can be nested inside a larger,
lower frequency antenna. The nested construction is possible
especially when using plasma antennas, as the plasma chambers
forming each antenna are separated from each other and can be
individually made active to transmit or receive. Higher frequency
signals from the encased antenna will pass through the plasma of
the lower frequency antenna. The individual antennas making up the
nested antenna can be turned on and off, providing additional
control over the transceived frequencies of the reconfigurable
antenna 410.
And, the nested antenna configuration can also be used to permit
simultaneous transmission and reception by the reconfigurable
antenna 410. For example, one frequency can be transmitted by one
nested antenna, while a second frequency band is monitored for
reception by a second one of the nested antennas. Multiple antennas
beyond two can be nested together to transmit and/or receive on
other frequencies.
A more complex application of the arrays of FIGS. 13A-D is shown by
FIGS. 15A and 15B, in which several of the arrays are arranged in
stacked layers 810-818. In each case, the layers 810-818 are
selected to produce a particular effect in conjunction with each
other on the signal broadcast through the surrounded antenna 402.
The antenna 402 shown is a biconical, center-fed antenna, which
type of antenna is particularly useful for broadband applications.
The biconical antenna 402 is preferably a plasma-filled cone
antenna, so that the advantages gained thereby are obtained,
including the broad frequency range resulting from different plasma
densities along the length of each end of the antenna 402. A
transceiver 800 is attached to the antenna 402 through a feed for
generating and interpreting signals transmitted through and
received from antenna 402.
The array layers 810-818 are arranged concentrically around the
antenna 402, and are spaced within one wavelength of the
transmitted signals of each other. The optimal spacing between
layers, and elements in each layer, can be calculated, as with the
shield 120 of FIG. 14A, above. The spacing between antenna 402 and
the layers 810-818 is the same as with the shields 420 of FIGS.
14A-C, above. The layers 810-818 are selected to produce a
particular effect, such as a selective bandpass filter, polarized
transmission, phase shifting, and steering the transmitted signals
by using one of the array types of FIGS. 13A-D for each layer
810-818. The substrate 330 of each array type used is preferably
formed into a cylinder, so that the array is equidistant from the
antenna 402 at each radial.
For example, each layer 810-818 may be a frequency filter, such as
the array of FIG. 13A or 13B. Different frequencies can be
selectively filtered by choosing different element 320, 322
configurations in the arrays 310, 312 forming the layers 810-818.
That is, for higher frequency filters, more rows and columns of
elements 320, 322 should be used in array like that of FIG. 13A or
13B, while lower frequencies require fewer elements 320, 322 to
block. Biconical antenna 402 can generate several different
frequencies due to the changing cross-section of the antenna
shape.
The frequency filter formed by layers 810-818 can be used to pass
or block particular frequencies within the range affected by the
filter on selected radials, while others are permitted to pass. In
a preferred arrangement, layer 810 is an array for reflecting, or
blocking, the highest frequencies transmitted or received, while
layer 818 is an array for reflecting the lowest frequencies. Layers
812-816 are selected to reflect progressively lower frequencies
between those affected by layers 810 and 818. It should be
appreciated that higher frequencies will continue to pass through
lower frequency tuned arrays, even when those arrays are active.
But, to pass the lowest frequency signals, all of the shield layers
810-818 must be effectively opened along the desired radial(s) by
making the array elements non-conducting in the window where the
low frequency signal is transmitted. When the arrays are
sufficiently large, it is possible to control transmission and
reception in both the radial and azimuth axes by creating a window
in the shield layers 810-818 and sequentially opening and closing
the window.
Alternatively, one of the layers 810-818 may be a polarizer or
phase shifter array, such as illustrated by FIGS. 13C and 13D. The
shield layers 810-818 work in the same manner as above with respect
to received signals. Thus, inclusion of a phase shifter array
permits reflection and scattering of certain received signals, such
as to avoid active detection of the antenna 402. For example, the
layers 810-818 may be designed to deflect incident electromagnetic
signals at non-backscattering angles, so as to produce no, or only
a very small, radar cross-section. A phase shifter array provides
one arrangement for steering incident signals. A further use of the
layers 810-818 and antenna 402 is to act as a repeater station, for
propagating a received signal along all or selected radials.
It should be understood as within the scope of this invention that
the antenna 400 of FIGS. 14B and 14C or antenna 402 of FIGS. 15A-B
can be substituted for each other, or other antennas may be used.
One alternative antenna configuration which is contemplated
combines two or more antennas in the same manner as the arrays
310-316 which are stacked in layers 810-818. That is, a
conventional omnidirectional dipole may be surrounded by a
co-axially oriented helical antenna, or a plasma biconical antenna
may consist of two plasma biconical antennas formed to have one
antenna inside the other, in a nested configuration. A greater
range of different frequencies may be transceived using the nested
antennas or dual biconical antenna by producing a higher plasma
density in the inner antenna and a lower density in the outer
antenna. The higher frequencies produced in the inner plasma
biconical antenna will pass easily through the lower plasma density
of the outer biconical antenna.
In the case of combining a helical antenna co-axial with another
antenna, such as a dipole, a multi-axis antenna is formed when the
frequencies are properly selected. The helical antenna will
transceive primarily along radiation lobes oriented extending on
the longitudinal axis of the helix, while an omnidirectional dipole
located along that axis will transceive mainly in a donut shaped
region radially surrounding the dipole antenna. The frequencies
must be selected similarly to the arrays to ensure proper
transmission of higher frequencies through lower ones.
In a further embodiment, the layers 810-818 may consist of
transmitting arrays arranged to produce an arbitrary bandwidth
antenna. In such case, the layers 810-818 can be used in
conjunction with a shield 420 or other filtering array 310-316. The
transmitted frequency of layer 810 should be the highest and that
of layer 818 the lowest. The layers 810-818 may be turned on and
off to produce single and multi-band effects. When used as
transmitters, the layers 810-818 need not be within one wavelength
of the adjacent layers 810-818, and can be more effective when
spaced greater than one wavelength apart from the adjacent layers
810-818. Such spacing does not significantly increase the footprint
size of the transmitting antenna in most cases, for example, when
used in the millimeter or microwave bands and higher frequencies,
such as used by personal or portable electronics.
Further, any of the arrays 310, 312, 314, 316 on substrate 330 may
be arranged co-planar or bent to have a particular curvature, such
as for parabolic reflectors, or into cylinders, as described above.
The arrays 310-316 may alternatively be arranged on the surfaces of
one or more planar substrates 330 to form volumetric shapes
surrounding an antenna 400 other than cylinders, including closed
or open end triangles, cubes, pentagons, etc. While it is preferred
that the substrates and arrays form the walls of geometric shapes,
the arrays may be conformed to any surface for use, provided the
appropriate calculations are done to ensure proper location of the
elements for the desired purpose.
Resonant waves set up between layers of elements 320-326 as shown
in FIGS. 13A-D will cause the reconfiguration in progressive phase
shifting to provide reconfigurable beam steering from an antenna,
such as a horn antenna or similar feed.
In a further modification, the reflective shield can include
annular tubes stacked perpendicular around the plasma tubes 422, to
provide additional control over the size of aperture created. When
specific annular tubes are unpowered in combination with certain
plasma tubes 422, a transmission window through the reflective
shield is formed along a particular radial and at a particular
elevation. Thus, steering in the vertical direction can be combined
with radial steering.
Further, the powered plasma tubes in any cylinder may act as a
parabolic reflector for the affected frequencies, thereby
strengthening the transmitted signal through an aperture.
Similarly, the plasma densities can be adjusted to produce plasma
lenses for focusing the transmitted antenna signal beam.
Preferably, the apertures will be at least one wavelength in arc
length to permit effective transmission. It should be noted that
Fabry-Perot Etalon effects may occur for the release of
electromagnetic radiation through the antenna while powering the
plasma tubes 422, but at lower plasma densities than for signal
reflection.
FIGS. 16 and 17 illustrate transmission radiation lobes which can
be produced using the antenna 410 of the invention. FIG. 16 shows
how the reflective shield 420 can include a layer of annular plasma
tubes 426 oriented perpendicular to vertical shield elements. Thus,
in FIG. 16, a transmission radiation lobe 430 is produced along a
particular radial and at an elevation selected by unpowering the
upper ones of the annular plasma tubes 426.
Similarly, in FIG. 17, two different transmission radiation lobes
430 are produced by creating apertures on each side of antenna 410
and at different elevations. The transmission radiation lobes 430
illustrated have side lobes 430a.
The steerable antennas illustrated in each of FIGS. 14A-17 can be
substituted for the loop sensors 10, 30, 71 in each of the examples
above. The antennas described in FIGS. 14A-17 are particularly
useful in far field applications, where the tags which are being
sensed are likely located outside of an effective near field range.
While the loop sensors 10, 30, 71 can be used in far field
applications as electromagnetic wave transceivers, they are
preferred for use in near-field applications, and the steerable
antennas of FIGS. 14A-17 are preferably used in far field
application.
FIG. 18 illustrates how the steerable antennas can be used in a
scanner 850 to scan an area for ID tags 900. The ID tags 900 can be
both passive and active, or activatable ID tags 900 as will be
further described. The scanner 850 consists of reconfigurable
antenna 410 and transceiver 800. The reconfigurable antenna 410 of
scanner 850 emits a radiation lobe 430 through an opened section of
the antenna shield 420 (not shown in FIG. 18). The radiation lobe
430 interacts with the ID tags 900 to sense their presence, or read
data from the tags 900, and, in some cases, write date to the tags
900 as well. The radiation lobe 430 can be made to sweep a full
circle around the antenna 410 by controlling which radials of the
shield are opened and closed, so that scanning is intentionally
limited to a single direction at a time, even though the actual
transceiving antenna used in reconfigurable antenna 410 is an
omnidirectional antenna. Transceiver 800 may contain switching and
control programs for operating the shield and reconfigurable
antenna 410 to this end.
Alternatively, the transceiver 800 may be two distinct units
connected to different antennas within reconfigurable antenna 410.
For example, the reconfigurable antenna 410 may use plasma nested
antennas, stacked arrays, and plasma shields around an
omnidirectional antenna as plasma filters or plasma frequency
selective surfaces as individual layers or two or more layers to
create large bandwidths or multi-bandwidth radiation patterns, so
that one antenna transmits while the other receives, and no
switching is necessary to control the transceiver 800. The
arrangement permits simultaneous transmission and reception of
signals, and the antenna 410 can operate continuously, if desired.
The shield 420 still must be controlled to adjust the radial on
which the antenna 410 transmits and receives simultaneously.
The ID tags 910, 920, 930 in FIG. 18 represent different versions
of tags which can be sensed by the antenna 410. ID tags 930 are
simply any type of antenna capable of interaction with the scanner
850 operating frequency. For example, ID tags 930 can be conductive
metal loops, or other known RFID tags.
ID tags 910 and 920 are more complex versions which include an
antenna 900, a code 902 and a power source 905, 907. The code 902
is connected with and transmitted by antenna 900 so that ID tag
910, 920 can provide more information to scanner 850 than simply
indicating its presence, as with tags 930. The power sources 905,
907 operate differently, depending on the type of ID tag 910,
920.
ID tag 920 is shown in the active state, in which it transmits a
tag radiation lobe 908 that interacts with the scanner radiation
lobe 430. ID tag 920 is continuously powered by power source 905,
so that it continuously generates radiation lobe 908. Power source
905 may be a battery sufficient to power antenna 900 or other power
source with similar ability. Code 902 can include a controller for
switching the power source 905 on and off, for example, when
antenna 900 is a plasma loop 10, 30, 71, and a memory for storing
an identifier and possibly for receiving and writing data
transmitted by a scanning signal. ID tag 920 thus has two
states--on and off. In the off state, it is electromagnetically
invisible to the scanner 850 and cannot be activated without
application of significant external power. In the on state, ID tag
920 actively provides information to scanner 850.
ID tag 910 represents yet another embodiment in which the antenna
900 is a plasma loop 10, 30, 71 that is weakly ionized or weakly
powered by power source 907. The power source 907 may be a
radioactive seed, a weak battery or other voltage source, or other
known power sources. When scanner radiation lobe 430 impinges on
antenna 900, the power transmitted by scanner 850 is sufficient to
activate plasma loop 10, 30, 71 so that code 902 can be read by the
scanner 850. One scanning antenna suitable for energizing the
activatable ID tags uses pure neon gas plasma with a mercury
additive. The antenna produces a plasma with high current at about
6 Torr pressure, without requiring a significant power increase to
the scanning antenna.
Alternatively, the antenna 900 may be activated by an external
power source other than the radiation lobe 430. In the weakly
ionized state, ID tag 910 is electromagnetically invisible and does
not interfere with other devices.
It should be noted that both ID tags 910 and 920 can be provided
with or without code 902. Thus, the ID tags of the invention may be
active or inactive transmitting tags (have a code 902), active or
inactive passive tags (no code 902--sensed by interference only),
and active or inactive activatable tags (have a weakly powered
plasma antenna, with or without code 902).
FIG. 19 demonstrates a further application of the antennas
described herein used in a scanner for determining the contents of
a ship 1000 entering a port or at dockside 990. The scanner again
consists of an antenna 410 like that of FIGS. 14A-17 and a
transceiver. The antenna 410 is mounted to a tower or building 980,
which may include a control room for monitoring the scanning. The
scanner transmits along a radiation lobe 430 in a direction
selected by configuration of the antenna 410. In one embodiment,
the radiation lobe 430 may be kept fixed, for example, as the ship
sails past the antenna 410. Alternatively, the radiation lobe 430
can be swept through between angles parallel to the dock 990 and
crossing all of the containers 950 on the ship 1000. In such case,
the ship 1000 can remain stationary or move past the antenna
410.
In order for the scanner to be effective, a modification must be
made to conventional shipping containers 950 to permit
electromagnetic radiation to penetrate the container. The walls of
the containers must have slots 960, similar to those used in arrays
310-316. The slots 960 are formed, for example, by dielectrics in
the metal sides, which permits the scanning signals to interact
with ID tags 900 on goods in the containers 950. The slot 960
configuration in the container 950 walls will determine what
bandwidth of scanning frequencies can be used effectively to read
and/or write to ID tags 900 on the container contents.
Further, it is envisioned that the interiors of the containers will
be lined with electromagnetic absorptive material or absorbing
dielectric cones. Such interior lining will prevent resonant
signals from building within the container and causing unwanted
interference with the scanning signal. As a further alternative,
the ship hull, or when applied on land, a truck or airplane body,
can be formed with dielectric slots for permitting specific
frequencies to penetrate the hull and scan the contents for ID
tags. The dielectric slots may be formed as described herein in
connection with the arrays 310-316 as well. That is, the slots can
be variable dielectric slots formed by variable conductive elements
which either permit or block electromagnetic waves from passing,
slots surrounded by variable conductive regions, or a constant
dielectric material selected and arranged to permit a particular
frequency band to pass.
While the example of FIG. 19 is described using the plasma window
antenna 410, it should be understood that any of the reconfigurable
antennas disclosed herein could be used. The same scanning can be
done using the plasma loop sensors 10, 30, 71 in near or far field
operation, as the distance between antennas and ID tags
requires.
The scanners disclosed herein in each of the examples of FIGS.
9-11, 18 and 19 can use any of the antennas disclosed as the
scanning element. That is any scanner disclosed can have plasma
window antenna 410, stacked arrays 810-818, or arrays of plasma
loop sensors 10, 30, 71 as the scanning element which broadcasts
the scanning signal connected to a transceiver or similar
component. Whichever antenna type is selected as the scanning
element, a radiation lobe is generated based on information from
the transceiver 800 for interaction with ID tags in the effective
range of the scanner. Thus, while multiple plasma loops 10, 30, 71
are sequentially activated to scan multiple directions in one
embodiment, the same scanning can be done using the plasma window
antenna 410 by sequentially opening a transmission window to direct
the transmission lobe along selected radials.
In all of the applications discussed above, plasma-containing
elements used as plasma antennas or passive plasma elements can be
operated in the continuous mode or pulsed mode. During the pulse
mode, the plasma antenna or passive plasma elements can operate
during the pulse, or after the pulse in the after-glow mode. To
reduce plasma noise, the plasma can be pulsed in consecutive
amplitudes of equal and opposite sign. Phase noise can be reduced
by determining whether the phase variations are random or discrete
and using digital signal processing. Phase noise, thermal noise,
and shot noise in the plasma can also be reduced by digital signal
processing.
It is recommended that AC bipolar pulses operated at a frequency
above the ion acoustic wave frequency in the plasma be applied to
the plasma for ionization and transmission purposes be used so as
to reduce noise in any of these plasma antenna systems, including
plasma antennas, plasma arrays including stacked plasma arrays both
active and passive, plasma nested antennas, plasma shields, and any
plasma readers or plasma antenna tags both active and passive.
During the pulse cycle, the time between pulses called the
afterglow state is the least noisy state.
All of the plasma elements described herein can be operated in the
afterglow state using AC bipolar pulses with frequencies above the
ion acoustic wave frequencies to minimize noise. This technique
also reduces power requirements for the plasma elements. To
maximize the amount of time the plasma antenna or plasma shields
are in the low noise afterglow region, the pulse width in time
should be minimized and the time between pulses should be
maximized. During the pulse, the electron beam from the electrodes
in the plasma tube containing the plasma can transfer energy into
waves in the plasma which in time create nonlinearities and noise.
Some of these waves are at or near the plasma frequency. Some of
these waves are in the range of between 2 KHz to 15 KHz, which are
in the range of ion acoustic waves. Much of the noise created by
the transfer of energy from the electron beam from the electrode to
waves in the plasma can be controlled by controlling the electron
beam. In practice the amount of energy from the electron beam
feeding these waves can be controlled by chopping the electron beam
and creating a pulse.
Other designs that can reduce noise in the plasma include providing
electrodes with enough energy spread or energy jitter to reduce the
transfer of energy from the electron beam from the electrodes to
the waves in the plasma. Still other ways of controlling the noise
in the plasma include using plasma antennas or plasma tubes without
electrodes for any of the plasma elements. Mechanisms for coupling
energy into the plasma if electrodes are not used include
capacitive sleeves around the plasma tubes, inductive couplers into
the plasma tubes, or remote ionization. Remote ionization can be
achieved by lasers, other antennas, acoustics, or other means.
Each of the scanners described above can be mounted within a
suitable casing for permitting the antennas to operate as
described. It is envisioned as well that the components making up
the antennas of each scanner can be embedded within a material
having a dielectric constant which approximates air. For example, a
synthetic foam including a large volume of air bubbles used to
support the antenna elements can have a dielectric constant which
approximates that of air. That is, the plasma loops or
reconfigurable antenna can be held in place by a rigid, air-filled
foam. The foam can further be formed to have external cones, like
those used in an electromagnetic anechoic chamber, which reduce
reflection. When such a supporting structure is used, the scanner
can be fully encased and protected from damage, but still operate
normally, as the casing material does not adversely affect the
ability of the antennas to function. Other materials having similar
properties can be used, while those with different dielectric
constants can also be used, but are less preferred due to their
adverse affect on signal strength.
While a specific embodiment of the invention has been shown and
described in detail to illustrate the application of the principles
of the invention, it will be understood that the invention may be
embodied otherwise without departing from such principles.
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