U.S. patent number 3,665,480 [Application Number 04/793,266] was granted by the patent office on 1972-05-23 for annular slot antenna with stripline feed.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Matthew Fassett.
United States Patent |
3,665,480 |
Fassett |
May 23, 1972 |
ANNULAR SLOT ANTENNA WITH STRIPLINE FEED
Abstract
A stripline antenna having a radiating aperture in the form of
an annular slot provided in one of its conducting plates and a pair
of orthogonal narrow strip conductor feeds disposed between its two
conducting plates and terminating under the central disk formed by
the annular slot. Adjustment of the relative phase and amplitude of
the electromagnetic energy applied to the strip conductor feeds
permits radiation from the annular slot of circular, elliptical or
orthogonal linear polarizations into space or into a waveguide.
Inventors: |
Fassett; Matthew (Billerica,
MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
25159509 |
Appl.
No.: |
04/793,266 |
Filed: |
January 23, 1969 |
Current U.S.
Class: |
343/754;
343/700MS; 343/769; 342/376; 343/768 |
Current CPC
Class: |
H01Q
13/16 (20130101); H01Q 9/0435 (20130101); H01Q
21/24 (20130101) |
Current International
Class: |
H01Q
21/24 (20060101); H01Q 9/04 (20060101); H01q
003/26 () |
Field of
Search: |
;343/767,768,769,770,771,854,754 ;333/84M |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
I claim:
1. An antenna for radiating electromagnetic radiation comprising a
plurality of electrically conducting plates, at least one of said
plates being provided with an opening having a closed outline, an
electrically conducting member of smaller dimensions than said
opening supported within said opening such that the boundary of
said member which has substantially the same form as said closed
outline is aligned with the boundary of said opening to form a
radiating aperture, a plurality of feed elements supported by means
of at least one dielectric support structure in spaced relation to
said conducting plates to couple electromagnetic energy into said
radiating aperture, said feed elements being positioned about the
circumference of said radiating aperture to provide a continuously
variable polarization to radiation radiated from said aperture.
2. An antenna comprising two substantially parallel electrically
conducting plates in spaced apart relationship, a generally annular
radiating aperture of substantially uniform width provided in at
least one of said conducting plates, a plurality of feed elements
supported by means of a dielectric support structure in spaced
relation between said conducting plates to couple electromagnetic
energy into said radiating aperture, said feed structures being
positioned about the circumference of said radiating aperture to
provide a continuously variable polarization to radiation radiated
from said aperture.
3. An antenna comprising first and second feed elements supported
in spaced relationship between two substantially parallel
conducting plates by means of a dielectric support structure, at
least one of said conducting plates having a radiating aperture in
the form of a generally annular slot having substantially uniform
width and a circumference which permits said slot to support at
least one resonant mode of electromagnetic radiation radiated from
said aperture, said first feed element being positioned relative to
said slot to couple electromagnetic energy into said slot, said
second feed element being positioned at a null of the resonant
waveform excited in said slot by said first feed element so that
said second feed element couples electromagnetic energy into said
slot independently of the energy being coupled by said first feed
element.
4. The antenna of claim 3 including a plurality of electrically
conducting pins disposed with substantially equal spacing
circumferentially around said slot to reduce the propagation of
radiation between said conducting plates radially outwards from
said slot.
5. The antennas of claim 3 including a variable phase shifter
connected electrically between said first and second feed elements
whereby radiation having a first polarization received by said
first feed element is phase shifted by said variable phase shifter
and transmitted by said second feed element at a second
polarization.
6. An antenna comprising an electrically conducting plate having a
generally annular radiating aperture of substantially uniform width
enclosing a central disk which is supported by a metallic shorted
quarter-wave transmission line segment, a plurality of microstrip
feed elements in parallel spaced relationship to said conducting
plate which are supported by a plurality of metallic shorted
quarter-wave transmission line segments to couple electromagnetic
energy into said radiating aperture, said feed elements being
positioned about the circumference of said radiating aperture to
provide a continuously variable polarization to radiation radiated
from said aperture.
7. An antenna comprising first and second feed elements supported
by means of a dielectric support structure in parallel spaced
relationship to an electrically conducting plate, said conducting
plate having a radiating aperture in the form of a generally
annular slot having a substantially uniform width and adapted to
support at least one resonant mode of electromagnetic radiation
radiated from said aperture, said first feed element being
positioned to couple electromagnetic energy into said slot, said
second feed element being positioned at a null of the resonant
waveform excited in said slot by said first feed structure so that
said second feed element couples electromagnetic energy into said
slot independently of the energy being coupled by said first feed
element.
8. In combination:
a source of electromagnetic energy emitting radiation having a
first polarization;
a plurality of antennas each of which comprises at least one
electrically conducting plate provided with an opening having a
closed outline, an electrically conducting member supported within
said opening such that the boundary of said member which has
substantially the same form as said closed outline is aligned with
the boundary of said opening to form an aperture for receiving
radiation having said first polarization and transmitting radiation
having a second polarization, said aperture being positioned to
receive a portion of the electromagnetic energy emitted by said
source of electromagnetic energy, first and second feed elements
having at least portions thereof substantially uniformly spaced
from said conducting plate to support respectively first and second
electromagnetic traveling waves which travel respectively from and
towards said aperture, said second feed element being positioned to
excite a resonant mode of electromagnetic radiation within said
aperture, said first feed element being positioned at a null of
said resonant mode to couple from said aperture radiation of said
first polarization;
a plurality of variable phase shifters, one of said variable phase
shifters being connected between an output of said first feed
element and an input of said second feed element to impart a phase
shift between said first and second travelling waves;
each of said antennas being positioned to transmit radiation having
said second polarization in a common direction to provide a
resultant radiation pattern formed in accordance with the phase
shifts imparted by said phase shifters.
9. In combination:
a source of electromagnetic energy emitting radiation having a
first polarization;
a plurality of antennas each of which comprises at least one
electrically conducting plate provided with a generally annular
radiating aperture having a circumferential length which permits
first and second orthogonal electric fields to be supported within
said aperture, a first feed element positioned relative to said
aperture for coupling electromagnetic energy from said first
orthogonal field, a second feed element positioned relative to said
aperture for coupling electromagnetic energy into said aperture to
excite said second orthogonal field, said first orthogonal field
being excited by radiation having said first polarization received
from said source of electromagnetic energy;
a plurality of phase shifters one of which is connected between
said first and second feed elements of each antenna whereby
radiation provided by said first feed element is phase shifted and
coupled to said second feed element;
each of said antennas being positioned to transmit radiation of a
second polarization emanating from each of said second orthogonal
fields in a common direction to provide a resultant radiation
pattern formed in accordance with the phase shifts imparted by said
phase shifters.
10. In combination:
antenna means comprising a plurality of electrically conducting
plates provided with radiating apertures and positioned to receive
radiation of a first polarization arriving from a common source, a
first feed element respectively positioned relative to each of said
apertures and extending substantially normally therefrom to couple
energy from received radiation of said first polarization, a second
feed element coupled to each of said apertures to excite radiation
of a second polarization in said apertures for transmission
therefrom, each of said second feed elements respectively spaced
along the periphery of each of said apertures from each of said
first feed elements whereby each of said first and second feed
elements are substantially electromagnetically decoupled;
a plurality of phase shifters respectively connected to each of
said first feed elements to impart phase shifts to radiation
provided by each of said first feed elements, the outputs of said
phase shifters being connected respectively to each of said second
feed elements whereby radiation of said second polarization is
phase shifted relative to the phase of radiation of said first
polarization for forming a combined radiation pattern of the
transmitted radiation of said second polarization.
11. In combination:
at least one antenna which comprises first and second feed elements
supported in spaced relationship between two substantially parallel
conducting plates by means of a dielectric support structure, at
least one of said conducting plates having a radiating aperture in
the form of a generally annular slot of substantially uniform width
and circumferential length to permit said slot to support at least
one resonant mode of electromagnetic radiation radiated from said
aperture, said first feed element being positioned relative to said
slot to couple electromagnetic energy into said slot, said second
feed element being positioned at a null of the resonant waveform
excited in said slot by said first feed element so that said second
feed element couples electromagnetic energy into said slot
independently of the energy being coupled by said first feed
element; and
waveguide means equipped with mounting means at one end thereof
whereby each of said antennas is mounted in a position to couple
electromagnetic energy into said waveguide means.
12. In combination:
a source of electromagnetic radiation;
a plurality of receiving antennas positioned to receive radiation
from said source of electromagnetic radiation, each of said
receiving antennas comprising at least one electrically conducting
plate provided with an opening having a closed outline, an
electrically conducting member of smaller dimensions than said
opening supported within said opening such that the boundary of
said member which has substantially the same form as said closed
outline is aligned with the boundary of said opening to form a
receiving aperture for receiving electromagnetic radiation, at
least one feed element positioned relative to said receiving
aperture whereby a component of the received field of the radiation
provided by said source of electromagnetic radiation induces a
received signal in said feed element, said receiving feed element
being spaced relative to said conducting plate whereby said
received signal propagates along said feed element from said
receiving aperture;
a plurality of phase shifters each of which is coupled respectively
to feed elements of said receiving antennas for imparting phase
shifts respectively to each of said received signals;
a plurality of transmitting antennas each of which comprises at
least one electrically conducting plate provided with an opening
having a closed outline, an electrically conducting member of
smaller dimensions than said opening supported within said opening
such that the boundary of said member which has substantially the
same form as said closed outline is aligned with the boundary of
said opening to form a transmitting aperture for transmitting
electromagnetic radiation, at least one transmitting feed element
positioned relative to said transmitting aperture to excite therein
a component of the tramsmitted field of electromagnetic radiation,
said transmitting feed element being spaced relative to said
conducting plate whereby electromagnetic energy for said
transmitted component field propagates along said transmitting feed
element to said transmitting aperture;
the outputs of each of said phase shifters being connected
respectively to each feed element of each of said transmitting
antennas for coupling thereto electromagnetic energy of each of
said components of said received radiation, said transmitting
antennas being positioned to radiate in a common direction whereby
the radiations from each of said transmitting antennas sum together
to provide at least one resultant radiation pattern which is formed
in accordance with the phase shifts imparted by said phase shifters
to said received signals.
13. In combination:
a source of electromagnetic radiation;
a plurality of receiving antennas which are positioned to receive
said radiation, a plurality of phase shifters, a plurality of
transmitting antennas, each of said receiving and transmitting
antennas having an annular slot radiating aperture disposed within
a conducting plate, said aperture being coupled to a pair of
orthogonal feed elements which are uniformly spaced with respect to
said plate for exciting within said aperture orthogonal electric
fields having sinusoidal amplitude distributions around the
circumference of said radiating aperture;
the two feed elements in each of said receiving antennas being
connected, respectively, to a pair of phase shifters to couple
respective components of the received electromagnetic field from
the radiating aperture into the respective phase shifters to
provide a corresponding pair of phase shifted electromagnetic
signals;
the two feed elements in each of said transmitting antennas being
positioned to couple corresponding pairs of said phase shifted
electromagnetic signals from pairs of said phase shifters into the
radiating aperture in each of said transmitting antennas to excite
within the aperture of each of said transmitting antennas
corresponding components of an electromagnetic field for subsequent
transmission;
each of said transmitting antennas being positioned to radiate in a
common direction whereby the radiations from each of the orthogonal
electric fields within the radiating apertures of said transmitting
antennas sum together to provide at least one resultant radiation
pattern which is formed in accordance with the phase shifts
imparted by said phase shifters to said received signals.
14. An antenna for radiating electromagnetic radiation comprising
at least one electrically conducting plate provided with an opening
having a closed outline, an electrically conducting member of
smaller dimensions than said opening supported within said opening
such that the boundary of said member which has substantially the
same form as said closed outline is aligned with the boundary of
said opening to form an aperture which can support a resonant mode
of electromagnetic radiation having a wavelength approximately
equal to the length of said aperture, at least one feed element
having at least a portion thereof substantially uniformly spaced
from said conducting plate to support an electromagnetic wave which
propagates toward said aperture, said feed element terminating
adjacent one point of said aperture to couple electromagnetic
energy from said feed element to said aperture such that the
electromagnetic radiation resulting from the interaction of said
feed element and said aperture is polarized in a single
direction.
15. In combination:
a plurality of receiving antennas positioned to receive
electromagnetic radiation from a common source of electromagnetic
radiation and a plurality of transmitting antennas positioned to
radiate at least a portion of their individual radiations in a
common direction, each of said receiving antennas and each of said
transmitting antennas comprising at least one electrically
conducting plate provided with an opening having a closed outline
larger than a wavelength of said radiation, an electrically
conducting member of smaller dimensions than said opening supported
within said opening such that the boundary of said member which has
substantially the same form as said closed outline is aligned with
the boundary of said opening to form a radiating aperture which can
support a resonant mode of said radiation;
a plurality of feed elements positioned relative to said aperture
for coupling electromagnetic radiation to said aperture, said feed
elements being spaced relative to said conducting plate to permit
electromagnetic radiation to propagate along said feed elements to
couple with said aperture; and
a plurality of groups of phase shifters, each of said group of
phase shifters interconnecting one of said receiving antennas to
one of said transmitting antennas such that each of said phase
shifters in said group of phase shifters interconnects a feed
element of said receiving antenna with a corresponding feed element
of said transmitting antenna for coupling electromagnetic energy
from said receiving antenna to said transmitting antenna, each of
said phase shifters imparting individual phase shifts to
electromagnetic energy coupled by said phase shifter for varying
the polarization of said radiation transmitted by said transmitting
antennas relative to the polarization of radiation received by said
receiving antennas.
Description
BACKGROUND OF THE INVENTION
This invention relates to stripline and microstrip antennas and
more particularly to an annular slot type of stripline antenna
employing orthogonal feed structures.
In many applications, small lightweight antenna elements are
desirable, such as for use in phased array antenna applications,
and particularly aircraft and missile antennas, as well as for use
in coupling electromagnetic radiation into waveguides. These
antenna elements are preferably in the form of stripline or
microstrip antenna elements which are readily fabricated to have a
suitably small lightweight structure. It is also preferable that
these antenna elements have an electrical structure which permits
polarization of their electromagnetic radiation at any polarization
angle so that the antenna can be used, for example, to excite
within a waveguide modes of radiation which are coupled to the
antenna aperture.
Stripline antennas formerly have frequently employed configurations
of slot radiators and conducting strip radiators which have a
capability for producing radiation having, typically, a fixed
polarization rather than a capability for providing desirable
polarization diversity. Other conventional forms of stripline and
microstrip antennas have a crossed slot radiating aperture which
provides a measure of polarization diversity capability. In this
crossed slot form, the radiation emitted from one slot is polarized
perpendicular to the radiation emitted from the other slot, the
amplitude of each radiation being controllable so that radiation
from the two slots combines to form a resultant radiation having
the desired polarization. However, the crossed slot configuration
introduces mutual coupling between the feed structures for each of
the two crossed slots with a resultant decrease in the extent of
polarization control.
Still other forms of stripline and microstrip antennas for use in
polarization diversity applications avoid undesirable mutual
coupling by having the two slots perpendicularly disposed in spaced
apart relation, rather than as a crossed slot, so that the fields
of radiation of the two slots are essentially decoupled from each
other. However, the radiation from these two slots have separate
phase centers which cause relative phase shifts between the fields
radiated from the two slots in directions off the antenna axis
while the phase centers are coincident in the case of the crossed
slot antenna.
Accordingly, it is an object of the present invention to provide an
improved stripline or microstrip-type antenna which provides
radiation components having a common phase center and which permits
varying the polarization of its radiation. A further object is to
provide a lightweight, readily fabricated antenna element of either
the stripline or microstrip type which can serve as a
coaxial-to-waveguide coupling device for exciting waveguide modes
of differing polarizations.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided an antenna for
radiating electromagnetic energy, the antenna comprising two
substantially parallel electrically conducting plates supported in
spaced-apart relationship by means of a dielectric support
structure, a substantially annular slot radiating aperture provided
in at least one of the conducting plates, and at least one
stripline feed element having the form of an elongated electrical
conducting strip embedded within the dielectric structure
substantially equidistantly between and parallel with the
conducting plates, and terminating at the annular slot. In the
preferred embodiment, the annular slot circumference is
approximately one wavelength of the free space radiation and there
are provided two stripline feed elements which extend approximately
one-tenth wavelength of the dielectric radiation under the central
disk formed by the annular slot and have a circumferential spacing
of substantially ninety degrees between the terminations which
spacing corresponds to approximately one-quarter wavelength of the
free space radiation. Accordingly, an electric field excited within
the annular slot radiating aperture by one feed element has a null
at the location of the termination of the other feed element to
provide substantial electromagnetic decoupling between the two feed
elements. Due to the ninety degree circumferential spacing of the
feed elements, the radiation excited by one feed element has a
polarization which is perpendicular to the polarization of the
radiation excited by the other feed element. The direction of the
resultant polarization of the sum of the two radiations is
determined by the independently adjustable phases and amplitudes of
electromagnetic signals applied to the two stripline feeds.
Alternative embodiments are disclosed showing a radiating aperture
in each conducting plate; an antenna structure in which one
conducting plate is deleted; and antennas having various forms of
dielectric structures supporting the feed elements in relation to
the conducting plates.
BRIEF DESCRIPTION OF THE DRAWINGS
The aforementioned objects and other features of the invention are
explained in the following description taken in connection with the
accompanying drawings, wherein:
FIG. 1 is a plan view, partially cut away, of one embodiment of the
stripline antenna of the invention;
FIG. 2 is a sectional view of the embodiment of FIG. 1 taken along
line 2 -- 2;
FIG. 3 is a sectional view of another embodiment of the invention
having an aperture disposed in each conducting plate;
FIG. 4 is the distribution of the electric field vector within an
annular slot as provided by excitation from one feed element;
FIG. 5 is a further embodiment of the invention having a generally
annular radiating aperture;
FIG. 6 is a partial plan view of an embodiment of the invention of
FIG. 1 modified to utilize an alternative form of feed element
having a perpendicularly projecting end tab;
FIG. 7 is a partial plan view of another embodiment of the
invention showing a feed element which is shorted to a central disk
formed by the annular slot in one conducting plate;
FIG. 8 is a sectional view of the embodiment shown in FIG. 7 taken
along the line 8 -- 8;
FIG. 9 is a sectional view of another embodiment of the invention
utilizing an air dielectric and support structures formed by
shorted quarter wavelength transmission lines;
FIG. 10 is an auxiliary view of the embodiment of FIG. 9 taken
along the line 10 -- 10;
FIG. 11 shows a method for exciting the feed elements whereby the
phase and amplitude of the electromagnetic signal applied to one
feed element can be varied with respect to the signal applied to
the other feed element;
FIG. 12 shows in diagrammatic form the antenna of FIG. 1 in which
individual phase shifters are connected to a pair of antennas of
the invention so that radiation of a first polarization received on
one feed element of each antenna is phase shifted prior to
radiation at a second polarization;
FIG. 13 is an isometric view of the antenna of FIG. 1 affixed to
the end of a waveguide for exciting modes of radiation in the
waveguide; and
FIG. 14 shows in diagrammatic form a stripline antenna system for
dissecting and reforming the radiation pattern of a horn radiator
in which phase shifting elements are interconnected with three
input and three output antennas of the form shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2, there is shown respectively, a plan and
sectional view of an embodiment of the invention in the form of a
stripline antenna 18 comprising a front conducting plate 20 having
a radiating aperture formed by an annular slot 22 and a back
conducting plate 24 which is held in parallel relationship to the
front plate 20 by means of a dielectric, polyphenylene oxide,
though other materials can be used such as well-known
Teflon-fiberglass, composed of two layers, respectively 26A and
26B. Two feed elements in the form of longitudinal conducting
strips, respectively 28 and 30, extend radially outward in
perpendicular directions from approximately the perimeter of
central disk 31 formed by the annular slot 22 and are embedded
along the interface of the two dielectric layers 26A and 26B in
parallel relationship to the plates 20 and 24 and spaced from these
plates to support traveling electromagnetic waves by which
electromagnetic energy is propagated to the annular slot 22
respectively from identical stripline to coaxial cable adapters 32
and 34.
The annular slot 22 has an inner circumference, namely the
circumference of disk 31, which is slightly less than the free
space wavelength of the highest frequency radiation which is
radiated from the antenna. The outer circumference of the annular
slot 22 has a circumference which is slightly longer than the free
space wavelength of the lowest frequency radiation which is
radiated from the antenna. In this embodiment the inner and outer
radius of the slot 22 differ by approximately 20 per cent to
provide approximately a 10 per cent bandwidth of the radiated
frequencies. A larger bandwidth can be provided by a still further
increase in the width of the slot 22. The slot 22 supports a
resonant electromagnetic field composed of two component fields
each of which is excited by energy coupled into the slot 22
respectively from the feed elements 28 and 30. Operation in the
frequency range of approximately 10 to 11 gigahertz is obtained
with the embodiment of FIG. 1 having the following dimensions: an
outer diameter of the annular slot 22 of 0.40 inches; an inner
diameter of 0.28 inches; and a spacing between the front plate 20
and back plate 24 of 0.125 inches and a feed element width of 0.085
inches. The plates 20 and 24 and the feed elements 28 and 30 are
formed from copper foil having a weight of 1 to 2 ounces per square
foot. The two feed elements 28 and 30 extend beneath the disk 31 a
small distance of 0.60 inches, corresponding to approximately
one-tenth wavelength of the radiation propagated in the dielectric,
for improved coupling of electromagnetic energy into the slot. An
annular slot can also be placed in the back plate 24 as shown in
the alternative embodiment of FIG. 3 to provide an antenna having
two radiating apertures.
A component of the electromagnetic field in the slot 22, excited by
the feed element 28, is shown diagrammatically in FIG. 4 in which
the arrows or vectors, directed radially across the slot 22 show
the direction of the electric field and the length of the arrows
represents the magnitude of the electric field. It is readily
apparent from FIG. 4 that the magnitude of a single component field
varies substantially sinusoidally with position around the
circumference of the slot 22. Due to the sinusoidal variation in
the amplitude, each electric field vector has the same direction
and magnitude as the vector located in the diametrically opposite
portion of the slot 22. The summation of these vectors produces the
resultant electric field vector shown in the center of FIG. 4. In
the same way, the component field, not shown, excited by feed
element 30 which is perpendicular to feed element 28 has a
resultant electric field vector which is perpendicular to the
aforementioned resultant electric vector excited by feed element
28. It is further shown in FIG. 4 that one end of feed element 30
is positioned beneath the slot 22 at a point where the field
excited by feed element 28 has essentially zero magnitude. By
symmetry, it is apparent that the electric field excited by feed
element 30 will have essentially zero magnitude at the point where
feed element 28 passes beneath the slot 22. Since the field excited
by one feed element has essentially zero magnitude at the other
feed element, the two electric fields are substantially decoupled
from each other and accordingly can be generated independently of
each other. The radiation radiated from slot 22 of the antenna,
therefore, has a polarization given by the vector sum of the two
resultant electric fields due to feed elements 28 and 30.
In operation, therefore, when electromagnetic radiation whose
wavelength approximates the circumference of slot 22 is introduced
into coaxial adaptors 32 and 34, the slot resonates and emits
radiation into the region in front of the aperture and particularly
in a direction substantially perpendicular to the front and back
plates. Radiation from the annular slot is provided with orthogonal
linear polarizations or a continuously variable polarization such
as circular or elliptical polarization in accordance with the
relative phase and amplitude of electromagnetic signals applied to
the strip conductor fields.
The antenna is readily fabricated. For example, the embodiment
shown in FIG. 1 is fabricated from two sections of copper clad
dielectric board using well-known photographic etching techniques.
Thus, with reference to FIGS. 1 and 2, a first section comprising
the front plate 20, slot 22, dielectric layer 26A and the two feed
elements 28 and 30 is formed from a single piece of copper clad
dielectric, the annular slot 22 being etched out of the copper foil
on one side of the dielectric material, the remaining foil
including central disk 31 and serving as the front plate 20. The
two feed elements 28 and 30 are formed on the other side of the
dielectric material by etching away the portions of the copper foil
surrounding the feed elements. The second section of the antenna
comprising dielectric layer 26B, the back plate 24, and the
mounting ports 36 for each of the two well-known
stripline-to-coaxial cable adapters 32 and 34 is formed from a
second piece of copper clad dielectric, the copper foil on one side
of the dielectric being completely etched away, and the two
mounting ports 36 being formed on the other side by etching the
copper foil which serves as the back plate 24. The two sections are
then pressed together to form the antenna, and coaxial cable
adapters 32 and 34, which are shown in FIG. 2, are attached to the
mounting ports 36 by means of metallic bolts 38 and nuts 39 which
also serve to bind the two sections together. The mounting port 36
being an aperture in the copper foil leaves an insulated region of
back plate 24, as shown in FIG. 2, by which the center conductor 40
of a coaxial cable adapter, such as adapter 32, can pass through
the copper foil. A hole 42 drilled through dielectric layer 26B is
also provided through which center conductor 40 makes electrical
contact with a feed element such as the feed element 28. As the two
sections are pressed together, the feed elements 28 and 30 are
pressed into the dielectric layers 26A and 26B along both sides of
the interface of these two layers, producing a slight recession 44
in the dielectric in the immediate vicinity of each feed element,
the recession 44 having little or no effect on the operation of the
antenna.
It is desirable to insert electrically conducting pins 46 around
the annular slot 22, as shown in FIGS. 1 and 2, to retain the
electromagnetic energy in the vicinity of the slot 22 rather than
permitting the energy to propagate between the front plate 20 and
the back plate 24 away from the annular slot 22. These pins 46
extend from the front plate 20 to the back plate 24 and are spaced
apart with a spacing which is typically less than one-quarter
wavelength of the radiation propagated within the dielectric layers
26A and 26B. The metallic bolts 38 which form a part of adapter 32
and of adapter 34 also serve in the same manner to retain the
electromagnetic energy in the vicinity of the coaxial cable
adapters 32 and 34.
Referring now to FIG. 5, there is shown a portion of an isometric
view of an alternative embodiment of the invention having a
generally annular radiating aperture of uniform width provided by a
continuous slot 48 having edges which are uniformly spaced apart,
each edge of the slot 48 being a closed outline or closed curve in
the form of a square. The same feed elements 28 and 30 are utilized
here as in the preceding embodiments. The generally annular slot 48
functions in the same manner as the annular slot 22 and supports an
electromagnetic field in which the electric field vectors are
transverse to the slot and have a magnitude which varies
sinusoidally with circumferential distance along the slot. Thus,
each feed element of FIG. 5, as with the embodiment of FIG. 1, is
located at a null of the component waveform excited by the other
feed element so that the two feed elements are orthogonal and are
substantially electromagnetically decoupled. The individual field
vectors of the embodiment of FIG. 5 sum together to give a
resultant polarization and directivity pattern to the radiated
electromagnetic energy which is substantially the same as that
provided by the antenna of FIG. 1.
Referring now to FIG. 6, there is shown a portion of a plan view of
an alternative embodiment of the invention having a pair of feed
elements 50 each with a right angle tab or bend 51 which forms the
terminus of the feed element 50 at the radiating aperture. In this
embodiment, the elements 50 extend only partially beneath the
annular slot 22 and couple electromagnetic energy into the slot.
The dimensions of a feed element 50 and its spacing relative to the
slot 22 differ from the dimensions and spacing of the feed elements
28 and 30 of the preceding embodiments, and, therefore, the
impedance presented to coaxial adapters, not shown, by the feed
element 50 differs from that presented by the elements 28 and 30.
As is well known, the impedance depends on the geometry of the feed
elements and the slot, and is, therefore readily selected by an
appropriate choice of dimensions such as the slot width and the
spacing between the slot and a feed element. Selection of the feed
element is a convenient way of providing the correct impedance for
maximum power transfer from the feed element to the radiating
aperture. As is well known, the antenna is reciprocal with respect
to transmitted and received power so that a feed structure which
provides for maximum coupling of power to the annular slot on
transmission is also well suited for coupling received
electromagnetic energy from the annular slot to the feed
structure.
The annular slot antenna 18 can alternatively be constructed in a
further embodiment, as shown in the plan view and sectional view
presented respectively in FIG. 7 and FIG. 8, having a solid
dielectric 52, a low-loss material such as a well-known mixture of
Teflon-fiberglass which supports front plate 53, feed element 54
and back plate 55. In this embodiment the feed element 54 is
terminated by means of a shorting bar 56 in the form of an
electrically conducting rod in electrical contact with both the
feed element 54 and the central disk 57 enclosed by the annular
slot 22, as shown in FIG. 7. The distance along element 54 between
bar 56 and slot 22 is approximately one-quarter wavelength of the
radiation propagated within the dielectric medium so that the
impedance of the bar 56 reflected back to the slot 22 approximates
the open circuit provided by the feed elements 28 and 30 of FIG.
1.
Each of the embodiments which have been described are of the
stripline form; that is, they contain two uniformly spaced
electrically conducting plates which can be flat or curved and a
central conducting strip uniformly spaced between the two plates.
With reference to FIG. 1, the antenna is still operable if the back
plate 24 is removed, leaving an embodiment of the invention in the
form of microstrip, that is, a single flat or curved conducting
plate containing a radiating aperture and a feed element uniformly
spaced relative to the plate to support an electromagnetic
traveling wave.
An example of a microstrip embodiment of the invention is presented
in FIGS. 9 and 10 which show, respectively, a sectional view and an
auxiliary view of such embodiment. In addition, this embodiment
serves as an example of construction utilizing an air dielectric
and shorted quarter-wavelength transmission line support elements
58 to position feed element 60 and electrically conducting disk 62
in conducting plate 64. The plate 64 and feed element 60 are
constructed from rigid electrically conducting material, preferably
copper, and are uniformly curved and positioned with respect to
each other to support a traveling electromagnetic wave from coaxial
cable adapter 66 to annular slot 68. The adapter 66 is affixed to
plate 64 by means of mounting bolts 70 and nuts 72 in a position
whereby the center conducting electrode 74 of adapter 66 makes
electrical contact with feed element 60. Since this embodiment of
the antenna has one feed element, the antenna radiates linearly
polarized radiation. It is understood that this embodiment can
readily be modified by the inclusion of a second feed element and
coaxial cable adapter, not shown, whereby the antenna would radiate
radiation having a variable polarization.
Referring now to FIG. 11, there is shown a means, partially in
block diagram form, for supplying electromagnetic energy to the
antenna 18 of FIG. 1 comprising a source of electromagnetic energy,
such as x-band signal source 76, a variable attenuator 78 for
attenuating the signal provided by source 76 and transferring the
attenuated signal via coaxial cable 80 to coaxial cable adapter 32,
and a well-known variable phase shifter 82 for phase shifting the
signal provided by source 76 and transferring the phase shifted
signal to a second attenuator 84 which attenuates the phase shifted
signal and transfers the attenuated phase shifted signal via
coaxial cable 86 to coaxial cable adapter 34. By means of the two
attenuators 78 and 84, a relative difference is provided in the
amplitudes of the two signals applied, respectively, to the
adapters 32 and 34. Thereby, two x-band signals differing in
amplitude and phase, but having the same frequency, are applied to
the antenna, one signal provided respectively for each feed
element. By a well-known variation in the phase and amplitude, the
polarization of radiation from the antenna can be made to vary from
circular to elliptical to orthogonal linear polarizations.
Referring now to FIG. 12, there is shown a novel method for
combining the radiation patterns from an array of annular slot
antennas 18, two of which are shown in FIG. 12, utilizing the
capability for independently exciting the feed elements of each
pair of feed elements in the antennas 18. A source of microwave
energy, such as the source 76 which is adapted to radiate through
horn 88 connected to source 76 by waveguide 90, transmits radiation
having a fixed linear polarization simultaneously to the antennas
18. Each antenna is positioned such that one feed element is
parallel to the direction of polarization, that is, parallel to the
electric field of the radiation, and the second feed element is
perpendicular to the polarization whereby radiation is received by
the first feed element while substantially no radiation is received
by the second feed element. In each antenna, one feed element is
used for receiving electromagnetic energy while the second feed
element, being electromagnetically decoupled from the first feed
element, is capable of simultaneously transmitting electromagnetic
energy. Electromagnetic signals received by the first feed element
in each of the antennas 18 are conducted by cables 80 to variable
phase shifters 82 which impart phase shifts to each of these
signals. The phase shifted signals are then conducted via cables 86
to the second feed element in each of the antennas 18 for
subsequent radiation from each of the antennas 18. Alternatively, a
more compact form of phase shifter, not shown, can be employed
which is constructed of well-known stripline components, usually by
photoetching techniques to provide inductive and capacitive
elements from conducting materials, such as copper foil embedded in
a dielectric material between two conducting surfaces, so that, for
example, the stripline form of phase shifter can conveniently be
incorporated within the structure of antenna 18 in which the plates
20 and 24 shown in FIG. 1 also serve as the outer conducting plates
of the phase shifter. The resulting radiation pattern which is
formed in accordance with well-known principles from contributions
of radiation from each of the antennas 18 in the array can be fixed
or varied at will in accordance with the phase shifts imparted to
the electromagnetic signals by each of the variable phase shifters
82.
Referring now to FIG. 13, there is shown a relatively simple method
of mounting an annular slot antenna to radiate into the end portion
of a circular waveguide. For example, a portion of waveguide 92 is
shown having a mounting flange 94 and clamps 96 for holding the
annular slot antenna 18 in position for radiating into the
waveguide 92. In this way, the antenna 18 serves to launch one or
more waveguide modes which have a polarization corresponding to the
polarization of the radiation emitted by the antenna. The antenna
is conveniently energized through coaxial cables 97, partially
shown in FIG. 13, with signals provided by well-known signal
sources, not shown. In this embodiment of the invention, the
antenna 18 serves as a dual coaxial cable to waveguide transition
for orthogonal TE.sub.11 modes in circular waveguide.
Referring now to FIG. 14, there is shown a diagrammatic
representation of a novel method for dissecting and reforming a
radiation pattern. This method utilizes the capability of the
annular slot antenna 18 for permitting independent excitation of
its two feed elements as well as the antenna capability for
transmitting and receiving radiation having linear, circular and
elliptical polarizations. As shown in FIG. 14, three annular slot
antennas 18 are positioned in an array 98 to receive radiation, and
a second group of three annular slot antennas 18 are positioned in
a second array 100 to transmit radiation. A source of
electromagnetic energy, preferably having a horn-type radiator,
such as the source 102 emits radiation towards the array 98 of the
three antennas 18 which are positioned to receive portions of the
rays of radiation indicated by arrows 104. The perpendicularly
disposed feed elements in each antenna 18 of array 98 resolve the
received radiation in perpendicular components such as for example,
horizontal and vertical components which are then passed through
phase shifters respectively 106A and 106B. These phase shifters
which are preferably of the well-known variable phase type
providing a phase shift in response to a control signal, not shown,
impart a phase shift to the horizontal and vertical components of
the received radiation which are then fed respectively to the
horizontally and vertically disposed feed elements of antennas 18
in array 100 for subsequent transmission. The resulting pattern of
the radiation from the antennas of the array 100 is composed from
contributions of radiation from each of the antennas 18 in the
array 100 and can be fixed or varied at will in accordance with the
relative positions of the antennas 18 within the two arrays, and in
accordance with the phase shifts imparted by the phase shifters
106A and 106B to the horizontal and vertical components of the
radiation.
The use of phase shifters 106A and 106B of the variable type is
demonstrated by an example in which the source 102 transmits a
clockwise circular polarization and each phase shifter 106A is
adjusted in a well-known manner to impart a phase shift which
differs by 180.degree. from that imparted by each shifter 106B to
give a resultant radiation from the antennas of array 100 having a
counterclockwise circular polarization. As a further example,
attenuators, not shown, can now be connected to the outputs of
phase shifters 106B to alter the relative amplitudes of the
horizontal and vertical components to provide in a simple manner
elliptical polarization to radiation from the antennas of array
100. Alternatively, one or more pairs of phase shifters 106A and
106B can impart equal phase shifts while other pairs of shifters
106A and 106B impart phase shifts differing by 180.degree. so that
a portion of the clockwise circularly polarized radiation of source
102 is modified to counterclockwise polarized radiation while the
remaining portion retains the clockwise polarization. In this
manner, circularly polarized radiation of one sense can be readily
converted into radiation having circular polarization of both
senses. As a still further example, the directivity patterns of the
horizontal and vertical components of the radiation emitted from a
given arrangement of the stripline antennas 18 in array 100 can be
individually varied in response to the differences in phase shift
imparted respectively by the shifters 106A and 106B. In this
manner, substantially separate directivity patterns can be formed
which are respectively characterized by the horizontal and vertical
components of a received radiation.
It is understood that the above-described embodiments of the
invention are illustrative only and that modifications thereof will
occur to those skilled in the art. Accordingly, it is desired that
this invention is not to be limited to the embodiments disclosed
herein but is to be limited only as defined by the appended
claims.
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