Annular slot antenna with stripline feed

[57]

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.

Claims, 14 Drawing Figures

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PATENTED MAY 2 3 1972

3,665,480

SIILEI 1 OF 4

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PATENTED MAY 2 3 1972

3,665,480

SHFEF 2 OF 4

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ATTORNEY

PATENTED MAY 2 3 1972

3,665,480

SHEET 3 OF 4

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BY

INVENTOR

MATTHEW FASSETT

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ATTORNEY

PATENTED MAY 2 3 1972

6,665.480

SHEET U OF 4

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FIG. 14

INVENTOR

MATTHEW FASSETT

ATTORNEY

3,665,480

ANNULAR SLOT ANTENNA WITH STRIPLINE FEED 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 radiat- 25 ing 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 com- 30 bines 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 40 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 ⁵⁵ 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 hom radiator in which phase shifting elements are intercon₆₅ nected 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, 75 polyphenylene oxide, though other materials can be used such

3,665,480 as well-known Teflon-fiberglass, composed of two layers, respectively 26A and 26B. Two feed elements in foe 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 foe 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 foe feed elements 28 and 30. Operation in foe frequency range of approximately 10 to 11 gigahertz is obtained with foe embodiment of FIG. 1 having the following dimensions: an outer diameter of the annular slot 22 of 0.40 inches; an taner diameter of 0.28 tachos; ₃„ and a spacing between foe front plate 20 and back plate 24 of tAW rsrerl ΊΑ a..1.2-1-___-1.___ · ν-π*»· — - _

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 35 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. 40 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 foe electric field and the ⁴⁵length 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 foe 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 foe resultant electric field vector shown in the center of FIG. 4. In foe same way, foe 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 foe 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 ₆o beneath the slot 22 at a point where foe 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 eleone 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 ............ _ _{Slol ana} of the antenna, therefore has a polarization given by the vec- 70 tromagnetic field in which the electric field tor 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 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 ters 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 26 A 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 ^Pr°7'^ ^a c°^{ntinuous slot 48} having edges which are outline or closed curve in the form of a square. The Mme 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 elec-------------1 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.

• · * j , 7 - ., J» ts located at a null of the component waveform excited hv s introduced into coaxial adaptors 32 and 34, the slot 75 the other feed element so that foe two feed XentT^e ment 28 passes beneath the slot 22. Since the field excited by 65 uniformly spaced apart, each edge of the stot« beVgZfo^d elements 28 and 30 are utilized here as in the preceding emhnAimoMfr TLa ____ ____it-. . ... » « . ®

3,665,480 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 35 such that one feed element is ρ^βΓω’^ direction’^f 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 30 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 ' . ' ______________ ./ 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

--------------------„.· receiving electromagnetic energy while the second feed element, being electromagnetically decoupled from the first feed element, is capable of simultaneously transmitting elec, _ tromagnetic energy. Electromagnetic signals received by the

------------j 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 ₅₀ 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 55 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 ₆₀ outer conducting plates of the phase shifter. The resulting radiation pattern which is formed in accordance with wellknown 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 electromagReferring 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 ------------------------ 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 con20 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 _______________________ „„ _{o ulc sccunu} enclosed by the annular slot 22, as shown in FIG. 7. The 40 feed element. In each antenna, one feed element is used for 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 approxi. . . ^r AC ---------- OlHliaia 1CUC1VCU uy me

Λ ^^{Pen C} provided by the feed elements 28 and 45 _{first feed element in each of antennas 18 are} conducted by 30 OI FIG. 1. mblpc fitfb tn variable nboeo QI :____* _i____

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 ₆₅ netic signals by each of the variable phase shifters 82.

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 posi- 70 and clamps 96for“ holding *e ^nid^slot ^ten^ie” tion whereby the center conducting electrode 74 of adapter .......

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 ____________________j___, auucuuu s embodiment can readily be modified by the inclusion of a 75 veniently energized through coaxial cables 97, partially shown

3,6' 7 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_U 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 horntype 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° 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° so that a portion ofthe 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 ofthe 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.