Circularly Polarized Radio Frequency Antenna

This invention relates generally to radio frequency antenna and more particularly to radio frequency antenna adapted to operate with radio frequency energy having any one of a variety of polarizations.

As is known in the art, it is frequently desirable to use an antenna element which may operate with any one of a variety of polarization (i.e. linear or circular). One type of antenna element capable of such operation is sometimes referred to as a "double ridged" horn. One antenna element of such type generally would include a vertical feed and an independent horizontal feed, the phase centers of such feeds being coincident. For circular polarization the two feeds are fed with radio frequency energy having a quadrature phase difference. In order to provide efficient matching to free space over a relatively wide frequency band, say in the order of 3.5 to 1, it is generally required that the width of the horn be larger than half the wavelength at the nominal operating frequency of the antenna and sometimes be as large as one wavelength. In an array antenna, a plurality of antenna elements are provided in order to attain a relatively wide scan angle, say in the order of 120 degrees. In such array, it is generally required that the phase centers of adjacent ones of the plurality of antenna elements be displaced by less than one half wavelength. It follows then that while a double ridged horn antenna may be adapted to operate with radio frequency energy having circular polarization, such an antenna element may not be readily used, because of its size, in an array antenna having relatively wide scan angles.

In another type of array antenna adapted to provide a variety of polarization each one of the antenna elements includes an orthogonally disposed pair of printed circuit notch shaped antenna elements. One such type of antenna is described in U.S. Pat. No. 3,836,976 entitled "Closely Spaced Orthogonal Dipole Array," inventors George J. Monser, George S. Hardie, John R. Ehrhardt and Terry M. Smith, issued Sept. 17, 1974, and assigned to the same assignee as the present invention. While such antenna is adapted to operate with circularly polarized radio frequency energy over relatively wide scan angles and over a relatively wide band of frequencies, such antenna is limited in its power handling capability and hence is not suitable for use in those applications where such antenna is fed by a transmitter adapted to transmit relatively large amounts of power.

In accordance with the present invention an antenna element is provided having an open ended waveguide section coupled to a first feed means for establishing radio frequency energy having linear polarization, the electric field vector of such energy being disposed normal to a pair of opposing wall portions of the waveguide. The antenna element includes a microwave circuit means for establishing radio frequency energy having a linear polarization orthogonal to the polarization of the first mentioned radio frequency energy. The microwave circuit includes a dielectric and a conductive sheet disposed over the dielectric; such sheet providing one of the wall portions of the waveguide section. The conductive sheet has an array of notches formed therein, such notches being disposed adjacent the open end of the waveguide section. A second feed means is coupled to the array of notches for establishing radio frequency energy having a linear polarization, the electric field thereof being in the plane of the dielectric and hence normal to the polarization established by the first feed means.

With such arrangement, the first and second feed means are fed in quadrature and the phase centers of the orthogonally disposed polarized radio frequency energy at the open end of the waveguide are substantially coincident to thereby enable the antenna to establish circularly polarized radio frequency energy in free space. Further, the radio frequency energy fed to the second feed means is distributed to an array of notch shaped elements thereby increasing the power handling capability of the antenna element. Still further, the antenna element is relatively compact and suitable to be arranged in an array for providing relatively wide scan angle coverage.

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the description read together with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a radio frequency antenna system including an array of antenna elements according to the invention;

FIG. 2 is a perspective view of the array of antenna elements used in the antenna system of FIG. 1;

FIG. 3 is a top plan view of a member used to form a portion of one of the antenna elements of FIG. 2;

FIG. 4 is a bottom plan view of the member of FIG. 3;

FIG. 5 is a side elevation view of the member of FIG. 3;

FIG. 6 is an end elevation view of the member of FIG. 3;

FIG. 7 is an exploded, isometric view of a strip transmission line feed network used to form a portion of one of the antenna elements of FIG. 2;

FIG. 8 is a plan view of the member of FIG. 3 and the strip transmission line feed network of FIG. 7 disposed thereon;

FIG. 9 is an exploded isometric drawing, partly broken away, of one of the antenna elements in the array of FIG. 2;

FIG. 10 is a perspective view of the bottom portion of the member of FIG. 3;

FIG. 11 is an exploded cross-sectional side elevation view of a pair of members of FIG. 3 and a pair of strip transmission line networks of FIG. 7, such pair of feed networks, and pair of members forming the antenna element of FIG. 9, the cross-section of one of such members being taken along lines 11--11 of FIG. 3, one of such pair of members being the member shown in FIG. 3;

FIG. 12 is a cross-sectional side elevation view of the antenna element of FIG. 11; and

FIG. 13 is a cross-sectional view showing a portion of a feed probe used to feed the portion of the antenna element of FIG. 12 formed by the pair of members and also showing a portion of such pair of members, such FIG. 13 being of region 13--13 of FIG. 12.

Referring now to FIG. 1 a multibeam radio frequency antenna system 10 adapted to operate over a relatively wide band of frequencies, here 4.8 GHZ to 18.0 GHZ is shown to include a radio frequency lens 12 having a plurality of feed ports 14a-14n disposed along a portion of the periphery of such lens 12 and a plurality of here eight array ports 16.sub.1 -16.sub.8 disposed along an opposing portion of the periphery of the lens 12, the plurality of array ports 16.sub.1 -16.sub.8 being coupled to an array 20 of a plurality of, here eight, identically constructed antenna elements 22.sub.1 -22.sub.8 through a power distribution network 24 the details of which will be described hereinafter. Sufficient to say here, however, that the shape of the lens 12, the construction of the power distribution network 24 and the arrangement of the antenna elements 22.sub.1 -22.sub.8 are selected such that n collimated beams of radio frequency energy are formed in free space by the antenna system 10, each one of such n beams having a different direction and each one of such n beams having circularly polarized radio frequency energy.

Referring to FIG. 2 array 20 is shown to include a plurality of identically constructed conductive members 26.sub.1 -26.sub.10, an exemplary one thereof, here member 26.sub.1 being shown in detail in FIGS. 3-6 and 10 and a plurality of, here nine, identically constructed microwave circuits, here strip transmission line circuits 27.sub.1 -27.sub.9, an exemplary one thereof, here circuit 27.sub.1 being shown in detail in FIG. 7. A pair of members 26.sub.1 -26.sub.10 and a pair of circuits 27.sub.1 -27.sub.9 form one of the identically constructed antenna elements 22.sub.1 -22.sub.8. Thus, as shown FIGS. 9, 11 and 12 an exemplary one of the antenna elements 22.sub.1 -22.sub.8, here antenna element 22.sub.1 is shown to include conductive members 26.sub.1 and 26.sub.2 and strip transmission line circuits 27.sub.1 and 27.sub.2. (It is noted that constituent parts of member 26.sub.1 and circuit 27.sub.1 are designated by a subscript 1 and constituent parts of member 26.sub.2 and circuit 27.sub.2 are designated by a subscript 2). More particularly, as shown in FIGS. 9, 11 and 12, the upper surface of the antenna 22.sub.1, is formed, in the frontal portion thereof, by the bottom surface 28.sub.1 of strip transmission line circuit 27.sub.1 and in the rearward portion thereof by the bottom surface 30.sub.1 of conductive member 26.sub.1 ; whereas the lower surface of antenna elements 22.sub.1 is formed, in the frontal portion thereof, by the upper surface 32.sub.2 of strip transmission line circuit 27.sub.2 and, in the rearward portion thereof, by the upper surface 34.sub.2 of conductive member 26.sub.2.

Referring now to FIG. 7 an exemplary one of the identically constructed strip transmission line circuit 27.sub.1 -27.sub.9, here strip transmission line circuit 27.sub.1 is shown in detail to include a pair of dielectric support structures 40.sub.1, 42.sub.1 of any suitable material, here Teflon Fiberglas material having a dielectric constant of 2.56. Initially, each one of the dielectric support structures 40.sub.1, 42.sub.1 has a sheet of conductive material, here copper clad on the upper and lower surfaces thereof. The sheet of conductive material on the lower surface of dielectric support structure 42.sub.1 is removed entirely with a suitable chemical etchant whereas a plurality of, here four, flared notches 44.sub.1 are etched into the conductive material 32.sub.1 clad onto the upper surface of such dielectric support structure 40.sub.1 using conventional photolithographic-chemical etching techniques. Each one of the notches 44.sub.1 has a narrow portion 46.sub.1 and a wider portion 48.sub.1. The notches 44 are separated from each other a distance less than a half wavelength at the smallest operating wavelength of the antenna. More particularly, here the center-to-center spacing of the notches 44 is 0.350 inches. The width of the wide portion 48 is here 0.260 inches and the widths of the narrow portion 46 is here 0.050 inches. The length of the wide portion 48 is here 0.130 inches and the length of the narrow portion 46 is here 0.842 inches. Considering now the second one of the pair of dielectric support structure 42.sub.1 a similar pattern of four flared notches 50.sub.1 is etched into the conductive sheet 28.sub.1 clad to the bottom surface of such dielectric support structure 42.sub.1. Each one of the slots 50.sub.1 is identical to the slots 44.sub.1 formed on the conductive sheet 32.sub.1 clad to the upper surface of dielectric support 40.sub.1. The conductive sheet clad to the upper surface of the dielectric support structure 42.sub.1 is etched to form a feed network 52.sub.1. The feed network 52.sub.1 is a strip transmission line circuit having strip conductor 54.sub.1 disposed between a pair of ground plane conductors formed by the conductive sheets 28.sub.1, 32.sub.1, and separated from such sheets 28.sub.1, 32.sub.1 by the dielectric support structures 40.sub.1, 42.sub.1. The feed network 52.sub.1 includes a first two-to-one power divider section 56.sub.1 the output of which into turn feeds a pair of two-to-one power divider sections 58.sub.1, 60.sub.1. Each one of the three power divider sections 56.sub.1, 58.sub.1, 60.sub.1 includes a step-matching transformer section 62.sub.1. Thus, power fed to the strip transmission line feed network 52.sub.1 is divided equally, and in phase, to each one of four feed lines 64.sub.1. Each feed line 64.sub.1 is disposed underneath the narrow portions 46.sub.1 of a pair of notches 48.sub.1, 50.sub.1 as shown in FIG. 8, the notches 44.sub.1 on conductive sheet 32.sub.1 being in registration with the notches 50.sub.1 formed in conductive sheet 28.sub.1. When strip transmission line feed network 54.sub.1 is fed radio frequency energy from a coaxial connector 66.sub.1 (FIG. 8) having a center conductor 68.sub.1 electrically connected to a strip conductor 54.sub.1 of the strip transmission line feed network 52.sub.1, 54.sub.1 and outer conductors 70.sub.1 electrically connected to conductive sheets 32.sub.1, 28.sub.1, the strip transmission line circuit 27.sub.1 couples energy to the feed lines 64.sub.1 and then to notches 44.sub.1 and 50.sub.1 whereupon such feed energy is then radiated into free space with an electric field vector disposed in the plane of the strip transmission line circuit 27.sub.1 as shown by the vector E.sub.1 in FIGS. 1 and 8. Thus, the energy radiated by the notches 44.sub.1, 50.sub.1 is linearly polarized; more particularly, here horizontally polarized.

Referring now in more detail to members 26.sub.1 -26.sub.10, each one of such members 26.sub.1 -26.sub.10 is constructed from a block of electrically conductive material, here aluminum, here having outer dimensions of 4.037 inches (length) and 2.250 inches (width). Such block has machined therein S-shaped side wall portions 74.sub.1, 76.sub.1 (FIG. 3) and a rear wall portion 78.sub.1 having a recess notch or 80.sub.1 formed therein. The depth of the side wall and rear wall portions is here substantially 0.325 inches. Also machined into the upper surface 34.sub.1 of the member 36.sub.1 is a tapered ridge 82.sub.1, as shown, here having a width of 0.20 inches. The tapered ridge 82.sub.1 has an aperture 84.sub.1 formed in the upper, flat top portion 86.sub.1 thereof, the flat top portion 86.sub.1 terminating in a tapered portion 88.sub.1, (FIGS. 3, 9) as shown. The length from the end of the tapered ridge 82.sub.1 to the end of the member 36 is here 2.2 inches. The length of the tapered portion 88.sub.1 is here 0.9 inches. The depth of the notch 80.sub.1 formed in the rear wall portion 78.sub.1 is here 0.075 inches, such notch 80.sub.1 having a length along the rear wall portion 78.sub.1 of, here, 0.588 inches. It is noted that the separation between the side wall portions 74.sub.1, 76.sub.1 disposed laterally to the tapered portion 88.sub.1 is relatively constant, here 1.5 inches; however, such separation decreases, here along curved paths, 90.sub.1, 92.sub.1, as such rear wall portions 74.sub.1, 76.sub.1 extend towards the rear wall portion 78.sub.1. As will be discussed in more detail hereinafter, the converging of the side wall portions 74.sub.1, 76.sub.1 as they extend towards the rear wall portion 78.sub.1 in the region behind the aperture 84.sub.1 (such aperture being the area where the antenna element 22.sub.1 formed by such member 26.sub.1 together with member 26.sub.2 is fed by the coaxial connector in a manner to be described) improves the impedance matching between the coaxial connector and the antenna element 22.sub.1. Member 26.sub.1 also has holes 100.sub.1 drilled through it, such being used for bolting the members together with bolts and screws 107.sub.1 as shown in FIG. 2. A hole 108.sub.1 is formed partly into the upper surface of the member 26.sub.1 and is used to receive an alignment pin 109.sub.2 (FIGS. 4-6) formed on the bottom surface of member 26.sub.2, as shown also in FIG. 10. Disposed along the curved regions 90.sub.1, 92.sub.1 (FIG. 3) of the side wall portions 74.sub.1, 76.sub.1 are open ended channels 110.sub.1, 112.sub.1. Channels 110.sub.1, 112.sub.1 are here formed of curved conductive strips 114.sub.1, 116.sub.1 here aluminum, having ends 118.sub.1, 120.sub.1 spaced from, and affixed to, side walls 74.sub.1, 76.sub.1 respectively. The spacing is provided by aluminum spacers 122.sub.1, 124.sub.1 such ends 118.sub.1, 120.sub.1 and spacers 122.sub.1, 124.sub.1 being affixed to the side wall portions 74.sub.1, 76.sub.1 through any convenient means as by bolts or a suitable electrically conductive epoxy, not shown. The spacers 122.sub.1, 124.sub.1 here have a thickness of 0.1 inches and the length of the channels 110.sub.1, 112.sub.1 is here 0.6 inches. The channels 110.sub.1, 112.sub.1, are effective in removing unwanted surface currents produced along the side wall portions 74.sub.1, 76.sub.1 such currents being associated with radio frequency energy having a frequency of here 13.2 GHZ. That is, it was noted that there was a significant loss of gain the antenna at about 13.2 GHZ. It was also noticed that the channels 110.sub.1, 112.sub.1 removed the loss of gain in the region of 13.2 GHZ. It is noted that the length of the channels 110.sub.1, 112.sub.1 is here about three quarters of the wavelength of the frequency associated with the unwanted surface currents.

The side wall portion 74.sub.1, 76.sub.1 (FIG. 3) disposed between the tapered ridge 82.sub.1 and the frontal end of the member 26.sub.1 are flared outwardly along a nonlinear path 131.sub.1 to increase the surface length of the side wall portions 74.sub.1, 76.sub.1 from the tapered ridge 82.sub.1 to free space within the fixed longitudinal length of the antenna element 26.sub.1 thereby providing a relatively compact antenna element with a side wall length sufficiently long to provide an adequate transition region between the tapered ridge and free space.

Referring now to the bottom surface 30.sub.1 of member 26.sub.1 (shown more clearly in FIGS. 4, 5, 6 and 10) such surface 30.sub.1 also has a tapered ridge 126.sub.1 formed thereon; here, however the flat portion 123.sub.1 of the ridge 126.sub.1 has a turret shaped conductive post 122.sub.1 (here shown) press fit therein by a pin shaped end 127.sub.1 as shown in FIG. 13. Post 122.sub.1 has a hole 128.sub.1 drilled therein as shown for receiving the center conductor 142.sub.1 of a coaxial connector 140.sub.1 (FIG. 11) in a manner to be described in detail in connection with FIG. 13. It is noted from FIG. 5 that the tapered ridges 82.sub.1, 126.sub.1 formed on the upper and lower surfaces of member 26.sub.1 are in alignment or registration with each other. Further, it is evident that the post 122.sub.1 of member 26.sub.1 fits into the aperture 84.sub.2 of member 26.sub.2 as shown in FIGS. 9, 11 and 12.

When members 26.sub.1, 26.sub.2 and strip transmission line circuits 27.sub.1, 27.sub.2 are affixed together (here by screws 107 (FIG. 2) and conductive epoxy, not shown, disposed on the portions of the copper conductive sheets 28, 32 of circuits 27.sub.1 -27.sub.9 (FIGS. 9 and 10) which contact portions of the conductive members 26.sub.1 -26.sub.10), the lower surface 30.sub.1 of member 26.sub.1, and the upper surface 34.sub.2 of member 26.sub.2 form opposing upper and lower wide surfaces of the rear portions of a hollow rectangular, open ended waveguide structure; the bottom ground plane conductor 28.sub.1 of circuit 27.sub.1 and the upper ground plane conductor 32.sub.2 of circuit 27.sub.2 form opposing upper and lower wide wall portions of the forward portion of the rectangular waveguide structure and side wall portions 74.sub.2, 76.sub.2 and rear wall portion 78.sub.2 form narrow side and rear walls of such open ended, rectangular waveguide. More particularly, the affixed members 26.sub.1, 26.sub.2 formed a tapered ridge rectangular waveguide antenna element 22.sub.1. Surfaces 145.sub.1 of member 26.sub.1 contact surfaces 149.sub.2 of member 26.sub.2 as shown in FIG. 11. The side and back edges of the circuits 27.sub.1 -27.sub.9 are covered with a conductive epoxy (not shown) to electrically connect the side and back edges of the conductive sheets 28, 32. Further, circuits 27.sub.1, 27.sub.2 (FIG. 3) fit into a groove 146.sub.1, 148.sub.1, 152.sub.1 formed in the conductive members 26.sub.1 -26.sub.8 so that the flat portions 86.sub.2, 123.sub.1 of the ridges 126.sub.2, 82.sub.1 are separated a distance "d" (FIG. 13), and the wide walls of the waveguide, i.e. conductive sheets 28.sub.1, 32.sub.2, are separated a distance "b" (FIG. 12). The distances "b" and "d" are designed so that the waveguide propagates even in the TE.sub.10 mode. Here "d" is 0.045 inches and "b" is 0.325 inches. The tapered ridge waveguide antenna elements 22.sub.1 -22.sub.8 are fed by the coaxial transmission line through coaxial connectors 140.sub.1 -140.sub.8 (FIGS. 1, 9, 11 and 12) having a center conductor 142.sub.1 (FIG. 13) passing through hole 144.sub.1 (FIGS. 5, 11, 13) and the end of such center conductor 142.sub.1 press fit to post 122.sub.1 to provide electrical and mechanical contact to post 122.sub.1. The outer conductor 145.sub.1 is electrically and mechanically connected to the member 26.sub.2 through screws 141.sub.1 as shown in FIGS. 9 and 11. The inner conductor 142.sub.1 is separated from the walls of the hole 144.sub.1 by a dielectric sleeve 143.sub.1 as shown in FIG. 13. A ferrite ring 147.sub.1 is disposed around the inner conductor 142.sub.1 between the dielectric 143.sub.1 and the post 126.sub.1, as shown in FIG. 13 to provide impedance matching between the coaxial connector 142.sub.1 and the post 122.sub.1. Radio frequency energy fed to the antenna element 26.sub.1 via connector 140.sub.1 thus launches radio frequency energy into cavity 148.sub.1 (FIG. 12), such energy travelling towards the open end 160.sub.1 of the cavity in the TE.sub.10 mode having an electric field vector extending between the wide surfaces of the waveguide as shown by arrow E.sub.2 in FIGS. 1 and 12.

It is noted then that each one of the antenna elements 22.sub.1 -22.sub.8 is adapted to transmit radio frequency energy having vertical polarization, (as when feeding radio frequency energy to connector 140.sub.1 -140.sub.8 or horizontally polarized radio frequency energy (as when feeding radio frequency energy to connector 66.sub.1 -66.sub.9). Further, if radio frequency energy is fed equally to both connectors 140.sub.1 -140.sub.8 and 66.sub.1 -66.sub.9 and the phase of the energy fed to such connectors differs by ninety degrees, such antenna element will transmit circularly polarized radio frequency energy in free space.

Thus, referring again to FIG. 1 the details of the power distribution network 24 will now be described. As mentioned above in connection with FIG. 1 the array ports 16.sub.1 -16.sub.8 are coupled to the array 20 through a power distribution network 24. The power distribution network 24 includes a plurality of, here eight, hybrid couplers, 150.sub.1 -150.sub.8 having a pair of input terminals 152a.sub.1, 152b.sub.1 -152a.sub.8, 152b.sub.8 and a pair of output terminals 154a.sub.1, 154b.sub.1 -154a.sub.8, 154b.sub.8. One of the input terminals 152a.sub.1 -152a.sub.8 is coupled to a corresponding one of the array ports 16.sub.1 -16.sub.8, as shown and the other one of such input 152b.sub.1 -152b.sub.8 terminals is terminated in a matched load 156.sub.1 -156.sub.8 as shown. Hence the power fed to the couplers 150.sub.1 -150.sub.8 is divided equally between output ports 154a.sub.1, 154b.sub.1, through 154a.sub.8, 154b.sub.8 respectively, but the signals at such output ports differ in phase from one another by ninety degrees. That is, considering coupler 150.sub.1 for example, the signals at output ports 154a.sub.1, 154b.sub.1 differ in phase by ninety degrees. The signals produced at ports 154a.sub.1 -154a.sub.8 are fed to two-to-one power dividers 160.sub.1 -160.sub.8, respectively as shown. The power in the signals fed to dividers 160.sub.1 -160.sub.8 is divided equally and in phase. The pair of signals produced at the outputs of dividers 160.sub.1 -160.sub.8 are fed to two-to-one power combiners 162.sub.1 -162a.sub.1 as shown. It is noted that one of the inputs of combiners 162.sub.1, 162.sub.9 are terminated in matched loads 164, 166. The outputs of power combiners 162.sub.1 -162.sub.9 are fed to coaxial connectors 66.sub.1 -66.sub.9 which feed strip transmission line circuits 27.sub.1 -27.sub.9. Thus, if the radio frequency signal phases at output ports 154 a.sub.1 -154a.sub.8 are represented as: .phi., 2.phi., . . . 8.phi. respectively, the signal phases at connectors 66.sub.1 -66.sub.9 may be represented as: .phi./2, 3.phi./2, 5.phi./2 . . . 15.phi./2, 17.phi./2, respectively and the signal phases at connectors 140.sub.1 -140.sub.8 may be represented as: .phi.+(.pi./2) . . . 8.phi.+(.pi./2) respectively.

Considering first an exemplary pair of the pair of inner connectors 66.sub.2, 66.sub.3 through 66.sub.7, 66.sub.8, here for example the pair of connectors 66.sub.6, 66.sub.7 it is noted that such connectors feed the strip transmission line circuits 27.sub.6, 27.sub.8 which form a portion of the upper and lower wide walls of the antenna element 22.sub.6. Further, the signal phases fed to such terminals 27.sub.6, 27.sub.8 may be represented as: 11.phi./2 and 13.phi./2 respectively. Therefore, in the region between the circuits 27.sub.6, 27.sub.8 at the open end of the antenna element 26.sub.6, the signals combine so that the resulting signal phase may be represented as: 6.phi. in the region between the circuits 27.sub.6, 27.sub.8. Further, such signal has a horizontal polarization. Still further, the signal phase fed to connector 140.sub.6 of such antenna element 22.sub.6 may be represented as: 6.phi.+(.pi./2), has a vertical polarization, and has a phase center coincident with the phase center of the horizontally polarized energy of the signal produced by the pair of strip transmission line circuits 27.sub.6, 27.sub.8. Thus, the energy associated with antenna element 22.sub.6, in free space, is circularly polarized. It is noted that the above discussion applies to antenna elements 22.sub.2 -22.sub.7, however there is some distortion with the antenna elements 22.sub.1, 22.sub.8 which are at the end of the array 20. However, with a large array, i.e. an array having 16 elements or more the effect of the end elements is minimized.

Having described a preferred embodiment of the invention it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is believed therefore that this invention should not be restricted to the disclosed embodiment but rather should be limited only by the spirit and scope of the appended claims.