U.S. patent application number 11/649046 was filed with the patent office on 2007-06-28 for distributed rfid antenna array utilizing circular polarized helical antennas.
Invention is credited to Gordon E. Hardman, Gary L. Overhultz, John W. Pyne, Edward J. Strazdes.
Application Number | 20070146230 11/649046 |
Document ID | / |
Family ID | 38668292 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070146230 |
Kind Code |
A1 |
Overhultz; Gary L. ; et
al. |
June 28, 2007 |
Distributed RFID antenna array utilizing circular polarized helical
antennas
Abstract
In accordance with the teachings described herein, RFID systems
are provided that include a distributed RFID antenna array
utilizing one or more circular polarized helical antennas. A
plurality of RFID tags may be used, with each RFID tag including a
linear polarized antenna for communicating RFID tag signals. One or
more receiver antennas may be used for receiving the RFID tag
signals from the RFID tags. An RFID tag signal reader may be used
to process RFID tag signals received by the receiver antennas. In
one example, the receiver antennas may include a circular polarized
helical antenna element. One or more transmitter antennas may be
used for transmitting an RF signal to the plurality of RFID tags,
the transmitter antennas including a circular polarized helical
antenna element. A transmitter may be used to generate the RF
signal for transmission by the transmitter antennas. In one
example, the RFID tag signal reader and the transmitter may be
included in a single reader/transmitter unit.
Inventors: |
Overhultz; Gary L.; (River
Forest, IL) ; Hardman; Gordon E.; (Boulder, CO)
; Pyne; John W.; (Erie, CO) ; Strazdes; Edward
J.; (Lafayette, CO) |
Correspondence
Address: |
Joseph M. Sauer;Jones Day
North Point
901 Lakeside Avenue
Cleveland
OH
44114
US
|
Family ID: |
38668292 |
Appl. No.: |
11/649046 |
Filed: |
January 3, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11417768 |
May 4, 2006 |
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11649046 |
Jan 3, 2007 |
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PCT/US05/37138 |
Oct 18, 2005 |
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11417768 |
May 4, 2006 |
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60625273 |
Nov 5, 2004 |
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Current U.S.
Class: |
343/895 ;
235/451 |
Current CPC
Class: |
G06K 7/10336
20130101 |
Class at
Publication: |
343/895 ;
235/451 |
International
Class: |
H01Q 1/36 20060101
H01Q001/36 |
Claims
1. A coreless helical antenna structure, comprising: a dielectric
base structure; an antenna element extending away from the
dielectric base structure in a spiral pattern and having a circular
polarized radiation pattern; and a plurality of support structures
that attach the antenna element to the dielectric base
structure.
2. The coreless helical antenna structure of claim 1, wherein the
plurality of support structures are configured to maintain the
spiral pattern of the antenna element.
3. The coreless helical antenna structure of claim 1, wherein the
antenna element includes a single radiating arm that extends away
from the dielectric base structure in the spiral pattern.
4. The coreless helical antenna structure of claim 3, wherein the
single radiating arm of the antenna element forms a single turn
helix antenna.
5. The coreless helical antenna structure of claim 3, wherein the
single radiating arm is formed from a single antenna wire.
6. The coreless helical antenna structure of claim 1, further
comprising: a metallic antenna backplane that adds directivity to
the circular polarized radiation pattern of the antenna
element.
7. The coreless helical antenna structure of claim 6, wherein the
metallic antenna backplane is integral to the dielectric base
structure.
8. The coreless helical antenna structure of claim 1, wherein the
plurality of support structures include openings and the antenna
element is supported within the openings.
9. The coreless helical antenna structure of claim 3, wherein the
plurality of support structures maintain a desired pitch of the
antenna element with respect to the dielectric base structure.
10. The coreless helical antenna structure of claim 9, wherein the
antenna element has a total length and the pitch is about equal to
1/5 the total length of the antenna element.
11. The coreless helical antenna structure of claim 1, wherein the
plurality of support structures are made of a dielectric
material.
12. The coreless helical antenna structure of claim 11, wherein the
plurality of support structures are plastic.
13. The coreless helical antenna structure of claim 1, further
comprising an amplifier circuit coupled to the antenna element and
operable to amplify a signal received by the antenna element.
14. The coreless helical antenna structure of claim 1, wherein the
coreless helical antenna structure is configured as a receiver
antenna for an RFID system.
15. The coreless helical antenna structure of claim 1, wherein the
coreless helical antenna structure is supported within an
enclosure.
16. A coreless helical antenna structure, comprising: a dielectric
base structure; an antenna element that includes a single radiating
arm extending away from the dielectric base structure in a spiral
pattern and having a circular polarized radiation pattern; and a
support structure that attaches the antenna element to the
dielectric base structure and that is configured to maintain a
desired pitch of the antenna element with respect to the dielectric
base structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/417,768, filed on May 4, 2006, which is a
continuation-in-part of International Patent Application No.
PCT/US05/37138, filed on Oct. 18, 2005, which claims priority from
U.S. Provisional Application No. 60/625,273, filed on Nov. 5, 2004.
These prior applications are incorporated herein by reference in
their entirety.
FIELD
[0002] The technology described in this patent document relates
generally to radio frequency identification (RFID) systems. More
particularly, the patent document describes a distributed RFID
antenna array that utilizes one or more circular polarized helical
antennas.
BACKGROUND
[0003] The RFID system described herein is related to the
inventions described in commonly assigned U.S. Patent Application
Pub. No. 2004/0056091, which is incorporated herein by reference in
its entirety. In that patent application, it was pointed out that a
need exists for an advertising compliance monitoring system that
provides versatility and flexibility by providing an RFID tag,
associated with a specific sign or product display, that
communicates tag data to an external reader.
[0004] U.S. Patent Application Pub. No. 2004/0056091 describes an
RFID system that may include RFID tags of various types (e.g.,
passive, semi-passive or active), backscatter reader transmitters
(BRT), and hubs. Typically, each BRT is a fully self-contained,
battery operated unit, and utilizes three antennas. Two medium-gain
patch antennas are used to read the tags, and a whip antenna is
used to report the received data over a wireless link to the hub.
This system functions well and is capable of detecting and
reporting tags in a variety of retail environments and at different
frequencies. It is desirable, however, to provide an even more
economical RFID system by centralizing some or all of the
electronics that have been distributed across areas or sub-areas in
a given facility, thereby reducing redundancy and cost. It is also
desirable to increase the read range of tags by the system to
reduce the number of antennas required and to increase the
reliability of tags being read under marginal conditions.
SUMMARY
[0005] In accordance with the teachings described herein, RFID
systems are provided that include a distributed RFID antenna array
utilizing one or more circular polarized helical antennas. A
plurality of RFID tags may be used, with each RFID tag including a
linear polarized antenna for communicating RFID tag signals. One or
more receiver antennas may be used for receiving the RFID tag
signals from the RFID tags. An RFID tag signal reader may be used
to process RFID tag signals received by the receiver antennas. In
one example, the receiver antennas may include a circular polarized
helical antenna element. One or more transmitter antennas may be
used for transmitting an RF signal to the plurality of RFID tags,
the transmitter antennas including a circular polarized helical
antenna element. A transmitter may be used to generate the RF
signal for transmission by the transmitter antennas. In one
example, the RFID tag signal reader and the transmitter may be
included in a single reader/transmitter unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 depicts an example RFID system that includes a BRT
hub that covers a designated area such as an entire commercial
sales facility.
[0007] FIG. 2 depicts an example RFID system that includes a
plurality of BRT hubs that are used in a plurality of designated
areas to cover a larger facility.
[0008] FIG. 3 depicts an example RF transmitter with a high power
amplifier and a band-pass filter.
[0009] FIG. 4 depicts an object having an RFID tag associated
therewith.
[0010] FIG. 5 is a graph illustrating example quadrifiler helix
antenna gain patterns to show that the antenna has a low gain on
the axis and a high gain on the sides.
[0011] FIG. 6 depicts an example switched backscatter tag (SBT)
illustrating the manner in which the switch is opened and closed to
accept or reject a BRT carrier signal.
[0012] FIGS. 7A and 7B depict an example transmitter antenna having
a circular polarized quadrifiler helix antenna element.
[0013] FIG. 8 depicts the example quadrifiler helix antenna
attached to an amplifier circuit.
[0014] FIGS. 9A and 9B depict an example receiver antenna having a
single turn helix antenna element.
[0015] FIG. 10 depicts the example single turn helix antenna
attached to an amplifier circuit.
[0016] FIGS. 11-13 depict another receiver antenna embodiment that
includes a single turn helix antenna element.
DETAILED DESCRIPTION
[0017] FIG. 1 depicts an example RFID system that includes a
backscatter reader/transmitter (BRT) hub (called a "Spider") that
covers a designated area of a facility. The RFID system may, for
example, be used to detect and report the presence and location of
radio frequency (RF) tags across selected zones in a retail
environment. The RFID system may also be used to centralize RF
transmission and receiving functions to reduce the expense of
recurring components. A single BRT hub ("Spider") may be used that
includes antennas attached to multiple transmit and receive ports
to cover a designated area of a facility. In small facilities, a
single BRT hub may be used to cover the entire facility as the
designated area. The Spider may, for example, be connected to AC
power to eliminate the cost and maintenance of batteries, as well
as allowing more read cycles, if desired. This also may permit
higher wattage to be used in the transmit function, potentially
increasing the size and reliability of detection zones.
[0018] In FIG. 1, a small facility 10 is shown in which the
designated area 12 to be covered by a BRT hub 14 includes the
entire facility. The BRT hub 14 is coupled to a plurality of
transmitters (TX 1, 2) 16-18 and a plurality of receivers (RX 1-10)
20-38, for example using coaxial cable. The plurality of receivers
20-38 are positioned to provide coverage of the entire designated
area 12 (the entire facility 10). Preferably, only one TX and one
RX are active at a time. It will be noted that RX 22 is able to
receive data from RFID tags 60, 62, and 64 at different distances
in the sub-area covered by RX 22, as illustrated by concentric
circles 54, 56, and 58. Also it will be noted that the transmitter
TX 16 has concentric rings 48, 50, and 52 that illustrate the
transmitter-to-tag zones covered by the range of transmitter TX 16,
thus showing that the transmitting antenna TX 16 is positioned to
illuminate at least a portion of the RFID tags (in the RX zones
covered by RX 20, 22, 26, 30, 34, 36, and 38) in the designated
area. In like manner, TX 18 shows corresponding concentric rings
illustrating illumination coverage ranges and representing
transmitter-to-tag zones covering at least a portion of the RFID
tags. Between the two transmitters TX 16 and 18, all of the RFID
tags in the designated area (the facility 12) are capable of
illumination.
[0019] Each of the transmitters TX 16 and 18 is coupled to the BRT
hub 14, for example with coaxial cable. In like manner, each of the
receiver antennas in each sub-area is coupled to the BRT hub 14,
for example using coaxial cable. Of course, wireless connections,
or other well-known known types of connections could be used
instead of coaxial cable.
[0020] When the transmitting antenna 16 illuminates RFID tags
within its range, one of the RF signal receiving antennas, such as
RX 22, receives the modulated tag signals and conveys them to the
BRT hub 14 over coaxial cable (such as 42) for transmission to a
remote server. A modulated RFID tag signal may be received by more
than one RX antenna when read sequentially (for example RX 26 and
RX 28). In such cases, the BRT hub (Spider 14) may forward both RX
events to the server, and may ascertain a location within a store
using closest zone readings, received signal strength indicator
(RSSI) readings, antenna intersection, or other algorithms. One
preferred method is disclosed in commonly assigned copending
application Ser. No. 11/418,319, entitled "Systems and Methods for
Approximating the Location of an RFID Tag," filed on even date
herewith, the subject matter of which is incorporated herein in
full.
[0021] The transmitting antennas 44 and 46 associated with
respective transmitters TX 16 and 18 should be omni-directional in
order to illuminate tags over a large area. A shaped beam with low
gain on axis and a high gain to the sides is ideal. For example, a
quadrifiler helix antenna, as illustrated in FIGS. 7 and 8, may be
used for the transmitting antennas 44 and 46. Quadrifiler helix
antennas have been the choice in orbiting spacecraft communications
for years. A quadrifiler helix antenna has circular polarization
and a shaped beam for high gain when the spacecraft is farthest
away on the earth's horizon, and low gain when the spacecraft is
closest or overhead. Also, when used in an RFID system as described
herein, the low profile of an quadrifiler antenna is equally
advantageous. To a consumer or other observer in the facility, a
quadrifiler helix antenna will typically look like a small white
paper towel tube that hangs down a few inches vertically from the
ceiling.
[0022] Typically, the transmit beam gain from TX 16 to RX 38 would
be lower than the transmit beam gain from TX 16 to RX 22.
Quadrifiler helix antennas are range compensating. The gain of the
antenna is higher for objects farther away, which compensates for
free-space power loss due to distance. This is illustrated in FIG.
5 which shows power vs. antenna angle. Higher power levels (gain)
at 70 degrees are offset by the bore sight of the antenna.
[0023] Further, quadrifiler helix antennas are typically
inexpensive. The antennas 44 and 46 shown in FIG. 1, for example,
may be constructed of materials, such as PVC piping, #12 copper
wire, and a small circuit card to maintain proper phasing between
the elements. This type of antenna has been experimentally tested
in a retail environment with very successful results.
[0024] Under FCC rules, part 15, a conducted RF output power of 1
Watt is allowed. The BRT's that are used in the system disclosed in
commonly assigned U.S. Patent Application Publication No.
2004/0056091 are battery powered and have a maximum output power of
200 mW to conserve battery life while "illuminating" tags (e.g.,
reflect and receive backscatter modulated signals produced by the
tags). Increasing conducted transmitter power will illuminate tags
in a larger area and better illuminate tags marginally located in
existing zones. The use of the quadrifiler helix antenna enables a
gain of approximately 6 dbic translating into an effective
isotropic radiated power (EIRP) of +36 dBm or 4 W. This is an
increase of approximately 9 dB over the BRT patch antenna disclosed
in the above identified published and commonly assigned co-pending
patent application. This translates into an increase of 8 times the
power.
[0025] The performance of an RF reader may be affected by
transmitter power being coupled into the BRT receiver through the
receiver antenna. The backscattered signal from the RFID tag is
extremely small, and its detection can easily be overwhelmed by the
backscatter transmitter carrier wave signal. Therefore, the
separation of the TX antenna and the RX antenna, as shown in FIG.
1, improves performance because the deployment system allows for
excellent separation.
[0026] Also, the use of the switched backscatter RFID tag (SBT) 102
shown in FIG. 6 also improves the signal communications between the
SBT and the BRT. In one example, the SBT 102 has an antenna in
which each side 104 and 106 of the antenna is approximately
1/4.lamda. (i.e., 1/4 wavelength). In the case of a 915 MHz tag,
each side is about 3.2 inches long. For a 2.45 GHz tag, these
lengths would be approximately 1.2 inches long. Thus, for different
frequencies the antenna lengths also would be different. A
backscatter generator 110 produces a sub-carrier frequency that
contains data, such as a tag ID. This backscatter signal opens and
closes the RF switch 108 that connects the resonant 1/4.lamda.
antenna elements 104 and 106. When the switch 108 is in the closed
position, the antenna acts as a 1/2.lamda. element, which is not a
good receiver, and that reflects a higher percentage of the reader
carrier frequency back to the reader.
[0027] When the switch 108 is in the open position, as shown, each
antenna side is 1/4 of the wavelength of the carrier frequency,
which makes it a good receiver, and therefore absorbs more of the
reader carrier frequency so it is not reflected back to the reader.
This combination results in a substantial increase in the ratio of
a "mark" (a 1 in binary state monitoring) to "space" (a 0 in binary
state monitoring) signal received by the BRT. The increased ratio
results in a dramatic improvement in the reader's ability to track
the modulated signal containing the tag data across much larger
distances. It also allows tags to be read more easily under
marginal conditions, such as when they are close to liquid or metal
(conditions well known in the art to be quite challenging for tags
in the UHF band). In one example, the tag has improved performance
because the antenna is T-shaped, with the antenna elements across
the top of the tag, pointing out and away from other circuitry on
the printed circuit board. This increases the effectiveness of the
available frequency aperture and reduces antenna de-tuning.
[0028] The clean switching between "on" and "off" of a resonant
aperture increases the mark-to-space ratio of the backscatter data
as received by the BRT. It is this increased ratio that improves
the BRT's ability to detect tags in a specific area of the store
area being monitored using a carrier frequency, thereby allowing
tags with a cleanly-switched resonant aperture to be detected at a
much greater distance than tags without a cleanly-switched resonant
aperture.
[0029] The system shown in FIG. 1 is well-suited for a small
commercial sales establishment, such as a drug store, but a single
Spider would likely be insufficient for larger-format retailers,
such as grocery or mass merchandiser outlets. In such cases,
several Spiders, each with separate Webs, could be used to cover
the establishment. Connectivity to phone lines and redundant
external communication electronics across multiple Spiders in a
store could be circumvented by centralizing those functions into
one master Spider 84. Such a system is shown in FIG. 2.
[0030] Note in FIG. 2 that the selected location, or retail sales
facility 10, is too large for one Spider. Therefore, in this
example, four designated areas 72, 74, 76, and 78 are used to cover
the entire facility 10. Each of the systems in each of the
designated areas 72-78 is identical to the system shown in FIG. 1
and operates in an identical manner as described above. However,
each of the Spiders 80, 82, 84, and 86 could be electronically
coupled to a master hub 88 as shown.
[0031] Multiple Web antennae are connected to a single backscatter
transmitter/receiver in the Spider, for example through coaxial
cables. These coaxial cables pass through a switch matrix. This
matrix and the long coaxial cables combine to create additional
attenuation, thereby lowering the received signal level. To
overcome this loss, a low noise amplifier (LNA) is positioned at
each RX antenna. These amplifiers draw small amount of current
(.apprxeq.15 mA) through the coaxial cable using bias tees.
Locations in retail environments that are difficult or expensive to
monitor via coaxial cable, such as external fuel pump signage,
could still be served by the previously-designed BRT's with
distributed reader/transmitter electronics by forwarding their data
wirelessly to the master Spider.
[0032] FIG. 3 is a block diagram of an example quadrifiler helix
antenna 90. The antenna 90 is coupled to the Spider through a
coaxial cable 92 and has an associated high power amplifier 94 to
recover coaxial cable signal attenuation. The antenna 90 also has
an associated ISM (Industrial, Scientific, and Medical) band pass
filter 96 to reduce noise or harmonics.
[0033] FIG. 4 depicts an example object 98 having an PFID tag 100
associated therewith. The object may be a permanent display, Point
of Purchase (POP) temporary display, signage, advertising material,
stock-alert sensors, merchandising material, category section
marker, individual product, or other material desired to be
monitored by retailers, manufacturers, or point-of-sale producers
(collectively a "display"). The object may also be a consumer (or
movable object) to which an RFID tag is associated so that the
shopping (movement) pattern of the consumer can be monitored. In
this manner, consumer exposure to a given display may be tracked.
An RFID tag given to a consumer may, for example, be a small active
transmitter tag (ATT) that uses the same frequency and protocol as
the reflection from the semi-passive backscatter tags.
[0034] FIGS. 7-10 depict example circular polarized antenna
configurations that may be used as transmitter and receiver
antennas in an RFID system, as described herein. It has been
determined that for both economic and performance reasons the
optimal solution for the antennas in an RFID system is to use
circular polarized antennas for the transmitters and receivers and
to use linear polarized antennas for the RFID tags. The switched
backscatter RFID tag (SBT), described herein, is one example of an
RFID tag having a linear polarized antenna.
[0035] Using a linear polarized tag in an RFID system is typically
more economical than using a tag with circular polarization. A
linear polarized tag can typically be made smaller than a tag using
circular polarization because a linear polarized antenna needs to
operate in only one axis. However, from a system standpoint the
radiation patterns of the antennas in the transmitter, receiver and
tag should all be aligned or coplanar to achieve the most robust
link and the best performance. This is most easily achieved in a
retail environment using circular polarized antennas because
maintaining coplanar antenna alignment between linear antennas in a
retail environment is often impractical. A good compromise is the
use of circular polarized antennas for the receivers and
transmitters and linear polarized antennas for the RFID tags. In
this manner, a high level of overall system performance may be
maintained, while reducing the cost of the RFID tags.
[0036] FIGS. 7A and 7B depict an example transmitter antenna 200
that includes a quadrifiler helix antenna element 202. FIG. 7A is a
side view of the antenna structure 200 and FIG. 7B is an exploded
view in which the antenna element 202 and dielectric core 204 are
depicted separately. The dielectric core 204 is a cylindrical
structure formed from a non-conducting material. The antenna
element 202 includes four radiating arms that are joined at a
common junction 206 and that extend from the common junction in a
helical pattern. In one example, the antenna element 202 may be
formed from two antenna wires that are joined at the common
junction 206, for instance by soldering, and that are shaped to
form the four radiating arms of the quadrifiler helix structure. In
another example, the two wires forming the antenna element may be
in physical contact, but not mechanically joined, at the common
junction 206.
[0037] In the illustrated example, the antenna structure 202 is
attached to the dielectric core 204 using a plurality of holes 208
in the dielectric core 204. As illustrated in FIG. 7A, the antenna
structure 202 may be attached through the holes 208 in the
dielectric core 204, such that the common junction 206 is within
the cylinder of the core 204 and the spiral portions of the
radiating arms extend through an upper set of holes 208 and along
the outside of the dielectric core 204. The four radiating arms may
also extend through a lower set of holes 208 such that the four end
portions 210 of the radiating arms extend from inside of the
dielectric core 204. In addition, the antenna element 202 may be
further secured to the dielectric core 204, as well as protected
from environmental conditions, by covering the radiating arms on
the outside of the core 204 with a protective material, such as a
heat shrink, as shown in FIG. 7A.
[0038] FIG. 8 depicts the example quadrifiler helix antenna 200
attached to an amplifier circuit 220. As illustrated, the end
portions 210 of the antenna element 202 may extend through a
dielectric material 222, such as a printed circuit board, to couple
the antenna 202 to the amplifier circuit 220. The dielectric
material 222 may also incorporate an antenna backplane (e.g., a
metallic surface) to shield the antenna 202 from the amplifier
circuit 220 and to provide directivity to the circular polarized
radiation pattern of the helical antenna element 202.
[0039] The amplifier circuit 220 may, for example, be attached to
the ceiling of a retail environment such that the antenna 200
extends downwardly from the ceiling. In addition, the amplifier
circuit 220 may be coupled to other components in the RFID system
via an external connector 224, such as a coaxial cable connector.
In one example, the amplifier circuit 220 may include two or more
gain settings that may be used to tune the amplifier circuit 220
for use in different sized retail environments. For example, a
higher gain setting for the amplifier 220 may be used for a larger
retail environment.
[0040] FIGS. 9A and 9B depict an example receiver antenna 230 that
includes a single turn helix antenna element 232. FIG. 9A is a
prospective view of the antenna structure 230 showing both the
antenna element 232 and the dielectric core 234, and FIG. 9B shows
only the antenna element 232. The dielectric core 234 is a
cylindrical structure formed from a non-conducting material. In the
illustrated example, the antenna element 232 is attached to the
dielectric structure 234 using a hole 236 in a bottom portion of
the dielectric core 234 and a slot 238 in an upper portion of the
core 234. As illustrated in FIG. 9A, an upper end portion 240 of
the antenna element 232 may extend trough the slot 238 and a lower
end portion 242 of the antenna element 232 may extend through the
hole 236, such that the spiral portion of the antenna element
extends along the outside of the dielectric core 234.
[0041] FIG. 10 depicts the example single turn helix antenna 230
attached to an amplifier circuit 250. As illustrated, the lower end
portion 242 of the antenna element 232 may extend through a
dielectric material 252, such as a printed circuit board, to couple
the antenna 232 to the amplifier circuit 250. The dielectric
material 252 may also incorporate an antenna backplane (e.g., a
metallic surface) to shield the antenna 232 from the amplifier
circuit 250 and to provide directivity to the circular polarized
radiation pattern of the helical antenna structure 232. FIG. 10
also illustrates a conductive patch 245 that may be included to
tune the antenna and possibly to help adhere the antenna element
232 to the dielectric material 252. The element 232 may be adhered
to the outside of the patch 245.
[0042] The amplifier circuit 250 may, for example, be located in
the ceiling of a retail environment, for example above the ceiling
tiles. In addition, the amplifier circuit 250 may be coupled to
other components in the RFID system via an external connector 254,
such as a coaxial cable connector.
[0043] FIG. 11 depicts another preferred embodiment of receiver
antenna 300 that includes a single turn helix antenna element 302.
In this example, the antenna element 302 is not supported by a
dielectric core. Rather, the antenna element 302 is attached to a
dielectric material 304, such as a printed circuit board, using a
plurality of support structures 306 made of a dielectric material,
such as plastic. In addition, an end portion of the antenna element
302 is coupled to an amplifier circuit 310 through a hole 308 in
the dielectric material 304. The dielectric material 304 may also
incorporate an antenna backplane (e.g., a metallic surface) to
shield the antenna element 302 from the amplifier circuit 310 and
to provide directivity to the circular polarized radiation pattern
of the helical antenna structure 302. Also illustrated is a
connector 312, such as a coaxial cable connector, for coupling the
amplifier circuit 310 to other components in the RFID system. In
one example, the antenna element 302 may be about 1.lamda. in
length with a pitch of about 0.2.lamda. The openings 307 in the
supports 306 serve to fix the pitch at the beginning portion of the
element 302 at its critical beginning location.
[0044] FIG. 12 is an exploded view of an example enclosure 330, 335
for housing the receiver antenna 300. The antenna housing 330, 335
may, for example, be secured in the ceiling of a retail
environment, for example above the ceiling tiles. FIG. 13 shows how
the antenna structure 302 fits within the housing portions 330,
335.
[0045] This written description uses examples to disclose the
invention, including the best mode, and also to enable a person
skilled in the art to make and use the invention. The patentable
scope of the invention may include other examples that occur to
those skilled in the art.
* * * * *