Backlobe Reduction In Reflector-type Antennas

Abstract

The axial backlobe of a circular parabolic antenna dish is greatly reduced by providing variation of the phase of radiation diffracted at successive portions of the edge. The phase is varied by providing an edge configuration in which successive portions of the edge are at differing distances from the feed. A dual-polarized antenna employs a polygonal rim surrounding the round reflector. A large increase in front-to-back ratio is obtained. The theory of operation is described to enable use in other structures.

Description

This invention relates to directional microwave transmission, and more particularly to high-gain reflector-type antennas.

An important figure of merit in reflector-type antennas is the ratio, normally expressed in decibels, of the main or axial beam to the maximum radiation produced in any backward direction, commonly called the "front-to-back ratio." It has long been known that back radiation is produced by diffraction at the edge of the reflector, and has often been observed that the front-to-back ratio varies substantially with reflector shape.

Both for reasons of relative simplicity of manufacture and for reasons connected with the most desirable forward radiation characteristics, by far the most common form of reflector in widespread use is the circular "dish" paraboloid. The front-to-back ratio of such an antenna is relatively low because of a large on-axis backlobe where substantially the full aperture is sought to be used. Various forms of backlobe suppression have heretofore been used or attempted with circular dishes to achieve high front-to-back ratios. The simplest and most obvious of these, of course, is mere enlargement of the dish to the point where the illumination of the edge is very low compared to the central illumination, i.e., employment of a feed which is incapable of fully using the aperture, thus producing a high front-to-back ratio at the price of severe sacrifice of the forward pattern of which the aperture is capable when fully illuminated. (It will of course be understood by persons skilled in the art that terms such as "feed," "illumination," "radiator," etc., which appear to have a connotation of use in transmitting, rather than receiving, signals, are not actually limitative, the direction of travel of the energy being of no consequence.)

As another approach, various forms of shield-type additions have been made to the edges of fully illuminated circular reflectors to reduce the high axial backlobe with varying degrees of success. So far as is apparent from study, the primary effects of such additions as have been found beneficial are basically analogous to employment of only a part of the aperture, the benefit flowing from relocating the diffracting edge in a region of relatively low energy-reception. In addition to being fairly expensive, particularly for large-size dishes, all such known constructions add to the bulk and/or wind resistance of an antenna to an extent which is impractical for many or most purposes.

The present invention provides a novel form of construction for the edge of a circular parabolic dish antenna which achieves a large improvement in front-to-back ratio with no appreciable impairment of the forward beam, with wholly negligible increase of antenna dimensions and weight, and with only a relatively trivial addition to cost.

In the present invention, as in the addition of shield-type structures, a conducting extension is added to at least a portion of the perimeter of the circular dish. However in the present invention the conducting extension is of dimensions so small in relation to the overall dish dimensions as to be a negligible addition as compared with additions or extensions which have the effect of placing the diffracting edge in a region of very low illumination. Indeed, the addition to the circular configuration in the present invention is desirably sufficiently small so that the intensity of illumination of the entire radial extension is uniform, i.e., so that the field produced by the feed at the outermost portion of the extension is not importantly less than the field produced at the edge of the circular main body. Backlobe suppression structures of the prior art so dimensioned give negligible benefit. However in the present construction the outer edge of the extension is formed with a plurality of convolutions extending alternately closer to and farther from the feed so that the induced currents whose reradiation is the mechanism of diffraction are in differing phases. If one elemental portion of the convoluted edge is a half-wavelength further from the feed than a second, the diffracted radiations at the elemental radiators thus formed will be in opposite phase. If the direction of extension is wholly radial, the two elemental radiators thus defined are effectively at equal distances from any remote point on the axis, and the reinforcing back-radiations produced at the edge of the circular structure when used alone are converted to nulls on the axis for such pairs of points. As may be seen from study, it is possible to produce entire nulling of the axial backlobe by using a "symmetrical square wave" edge shape. Such a shape is of very limited utility, but it will serve to illustrate the utilization of the relative phase of portions of the diffracted radiation to control the back-radiation pattern which is the essence of the broader aspects of the invention. As will now be seen, the full cancellation of the axial beam would be more or less inconsistent with achievement of the maximum front-to-back ratios which the invention is capable of giving.

The shifting of the relative phase, whether in the example just mentioned or in the more generally useful constructions to be described, does not affect the total back-radiated energy to any substantial degree. Energy removed from the axial beam thus appears in a pattern of off-axis lobes. It will be seen, for example, that the radial "square-wave" shape just discussed produces an off-axis pattern, details of which depend on the length selected for the alternated "steps" of opposite phase. For any given backward radiation, with no variation of the main beam, the best front-to-back ratio would be obtained with a back-radiation pattern of uniform dispersion throughout the solid angle generally considered as the backlobe region. Thus a null of the axial backlobe is in general undesirable, although the invention in its broad aspects may be utilized in this manner where only a narrow angle of back radiation is of concern.

Much greater improvement in front-to-back ratio is obtained by employing a continuum of relative phases in each edge convolution, i.e., in which each progression between minimum and maximum feed-distance is a slope or curve, rather than a single abrupt step-function.

An additional departure from the "half-wave square-wave" concept thus adopted as a conceptual reference relates to the difference between distances from the source (corresponding to the actual convolution dimensions in the case of the "focal-plane" feed and wholly radial backlobe-suppressor configurations). If only a half-wave overall variation is provided, producing 180.degree. between the most advanced and most retarded phase components, there is no opposite-phase elemental radiator to correspond to an intermediate point. Accordingly, it is generally desirable that the difference in distance from the feed correspond to approximately a full wavelength (exactness not highly critical). Multiples of a full wavelength may be used, but with little or no added benefit. Indeed, since illumination of the edge of a parabolic dish is normally in a region of substantial feed-pattern gradient (and certainly of the derivative of the gradient) the performance of the suppression may be substantially impaired by excessive size. In general, accordingly, the variation between the distance of the feed from the closest point on the convoluted edge to its distance from the farthest point on the convoluted edge should desirably be between about one-half wavelength and about three wavelengths at the frequency of the feed, with approximately one wavelength being the preferred difference.

The discussion thus far neglects the effects of polarization direction of the signal. It is found that this is in fact a major factor in practical design of structures for best utilization of the invention. Experimental results appear to indicate that the backward diffraction can be treated as occurring in proportion to the electric field polarization component perpendicular (in the plane of the conductor) to the conductor edge. Stated otherwise, for any given intensity of feed radiation incident on an elemental edge portion, the backward diffraction from that elemental portion varies as the cosine of the angle between the edge and the direction of electric field polarization.

If so desired, the backlobe suppression addition of the invention may be made an integral part of the reflector, formed (except for the shaping of the edge) in the same operation in which the reflector is given its parabolic contour. In general, however, it is much more simple and economical to add the suppression construction to an existing dish, and the invention in its narrower aspects employs a simple and easily fabricated adjunct for this purpose.

Although the invention was conceived and developed with the specific object of increasing the front-to-back ratio of a circular parabolic antenna with uniform edge illumination, its broader teachings will be seen from study of the more detailed description below to be adaptable to other applications wherein the suppression of undesired radiation backlobes produced by diffraction at a conductor edge is required.

With the general purpose and mode of operation of the structure of the invention thus described, the embodiment illustrated in the drawing, together with the further novel features and advantages of the invention, and its more detailed theory, will be readily understood.

IN THE DRAWING:

FIG. 1 is a front view in elevation of an antenna incorporating the invention;

FIG. 2 is a diametric sectional view of the antenna, with the feed structure shown in elevation;

FIG. 3 is an enlarged fragmentary front view of the peripheral portion of the antenna;

FIG. 4 is a fragmentary sectional view taken along the line 4-4 of FIG. 3;

FIG. 5 is a view in elevation of the feed employed in the particular embodiment illustrated, taken along the line 5-5 of FIG. 2;

FIG. 6 is a fragmentary sectional view taken along the line 6-6 of FIG. 3;

FIG. 7 is a fragmentary sectional view taken along the line 7-7 of FIG. 5;

FIG. 8 is a set of comparative partial radiation patterns; and

FIG. 9 is a schematic view illustrating certain aspects of the theory underlying the invention.

The embodiment of FIGS. 1 through 7 is a cross-polarized antenna 10 having a circular parabolic reflector 12. The feed assembly, generally designated at 14, comprises a dual-polarized horn feed 16 fed by waveguides 18 and 20 oriented in conventional "buttonhook" fashion to extend through vertex plate 22 and provided with coupling flanges 24 and 26 at the rear of the reflector. Preferably the feed 16 is closely adjacent to the plane of the front edge of the reflector 12, the configuration thus constituting a "focal-plane" feed. The particular feed construction employed is of the type disclosed and claimed in the U.S. Pat. application of Richard F. H. Yang and Laurence H. Hansen, Ser. No. 641,348, filed May 25, 1967. As shown in FIGS. 5 and 7, it comprises a square horn 28, surrounded by a square sleeve 30 mounted on a round reflector plate 32 to form a choke annulus 33 set slightly back from the mouth of the horn. A shielding skirt 34 around the perimeter of the plate 32 and intermediate circular vane 36 complete the feed. The design and construction of the feed are not directly relevant to the present invention, which is advantageously used with any feed construction, and the purpose, dimensioning, etc., of the elements of the feed are accordingly not further described beyond stating that it produces a very wide solid angle of uniform dish illumination.

The reflector 12 is, except for the peripheral addition later to be described, of conventional construction, being an aluminum sheet formed to paraboloidal concavity by a spinning operation and having a rearwardly extending flange portion 38 and a reinforcement ring 40 (formed from segments not shown) rigidizing the flange 38 by welds securing the ring 40 to the main body of the rear of the dish and to the flange 38.

To the periphery of the circular dish structure, there is secured a rim generally indicated at 42. The rim 42 is formed from angle-stock bent to conform to the circular perimeter of the dish. One of the webs of the angle serves as the support web 44, being welded to the flange 38. The other web, which extends wholly radially outwardly in the plane of the circular edge of the dish, is formed to produce distribution of the phase of the diffracted radiation and constitutes a diffraction ring 46. The ring 46 is generally in the form of a regular polygon having sides 48 tangent to the circular dish. As seen in FIG. 3, the ring 46 has slight departures from the configuration of a regular polygon at the intersections of the sides 48, whereat there are short flat portions 50 for reasons which will be mentioned later.

With this construction, the distance of points on the edge of the diffraction ring 46 from the feed 16 (i.e., from its effective phase center) varies continuously from a minimum at the points of tangency to the circular configuration to a maximum on the flats 50. It will be seen that although the sides 48 are straight, the portions extending in each direction from the points of tangency are effectively opposite "slopes" for purposes of the manner in which distance from the radiation source varies alternatingly with angle around the circular structure.

As indicated in FIG. 4, the maximum radial extension of the diffraction ring 46 is approximately one wavelength at the frequency of operation of the feed 16. The construction illustrated employs a diffraction ring in the form (except for flats 50) of a 12-sided regular polygon (dodecahedron). A specific antenna constructed as illustrated has a 10-foot diameter reflector and is operated at 6.175 GHz. As later seen, it is these parameters which determine the number of sides of the polygon in accordance with the narrower aspects of the invention. The purpose and operation of certain features of the illustrated structure will best be understood by first considering the underlying theory and experimental findings in a manner somewhat amplifying the general statements earlier made.

In FIG. 8, there are reproduced fragmentary radiation patterns of an antenna with which experimentation was performed in the development of the invention. E-plane patterns with and without diffraction phase dispersion are shown at 52 and 54 and the corresponding H-plane patterns at 56 and 58, each over a range of plus and minus 10.degree. from the axial backward direction. Before considering the effects of the addition of the invention, shown at 54 and 58, there can be pointed out the features of the "no rim" patterns of the prior art construction, and the manner in which they lead to a "model" of the diffraction phenomena in such a structure which permits applying the invention not only to the present purposes but also to the improvement of many other microwave transmission devices where analogous problems are encountered.

It will be seen that the H-plane pattern at 56 has a single large peak in the axial direction. (As will be obvious, the ordinate values on the vertical scale represent ratio in decibels to the axial front lobe, not shown). Except for minor lobes close to the axis, the back radiation is below the recording sensitivity. In the E-plane, however, as shown at 52, there are found a large number of peaks of substantial amplitude at all angles, even the axial central peak being far less sharp than the single "pip" in the H-plane pattern. The dissimilarity of the patterns in the two planes, upon analysis, has led to a qualitative portrayal of the mechanism of backlobe formation, and thus to the manner in which the large central lobe is suppressed in the present invention.

In FIG. 9 there is shown a diagrammatic representation of the manner of formation of the relatively intense axial backlobe in a circular dish antenna. There are shown two shaded peripheral arcs 60 and 62. These are considered as "effective radiators," excited in equal phase by the component of the electrical field vector perpendicular to the edge. If the intensity of the excited reradiation at the two ends of the electric-field diameter be taken as E.sub.O, the radiation from any other point is E.sub.o cos .theta., where .theta. is the angle formed with the polarization direction by the radius to the point. The diffraction radiation is thus maximum at the ends of the E-vector diameter, shown as the sides of the dish in FIG. 9, and zero at the top and bottom. By more or less arbitrary selection of points where the radiation may be considered to become negligible, there results a model consisting of two arcuate "arrays" of vertically stacked in-phase short radiators, horizontally spaced by an amount very large compared to a wavelength. The H-plane pattern for the two "arrays" is the same as the pattern for either of them, i.e., a single strong beam perpendicular to the plane of the array. In the E-plane, however, there appears a diffused and complex "interference pattern" produced by the interaction of two curved arrays thus widely spaced. Because of the curvature of the "arrays," none of the peaks in the intricate side patterns in the E-plane approaches the amplitude of the axial backlobe, the axis remaining the only place where the radiation from all elemental radiators arrives in the same phase.

By altering the in-phase relation of the elementary radiators, the axial backlobe may be reduced (or even eliminated) by distributing the energy-radiation which it represents over a large number of smaller lobes. The simplest way of "breaking up" the phase uniformity is to add to the circular diffracting edge conducting extensions for the purpose of delaying the incidence of the phase-front from the feed on the edge portion at one rotational angle with respect to the time of its incidence on the edge portion at another. It will be observed that mere edge-shaping the paraboloidal surface itself will avoid the phase identity, i.e., that the extending conducting structure may follow the paraboloidal internal contour of the reflector in the form, for example, of notches forming a toothed aperture. Although such an employment of the invention is within the contemplation of its broadest aspects, it has numerous disadvantages as compared with the plane radial diffraction ring earlier described, in addition to the obvious disadvantage of complexity of fabrication. Such a construction substantially alters the forward pattern, in addition to the desired beneficial effect on the back radiation. Additionally, the convolutions of the edge required for any given performance will necessarily be much larger than in the case of the illustrated construction. With a direction of extension following the parabolic cross section, the necessary position difference between two points required to give any given difference of phase of excitation is enormously greater than the mere portion of a wavelength corresponding to that phase difference. Further, except in the case of "deep dishes" and "shrouded" construction, wherein the feed is rearward of the plane of the diffracting edge, the effects of the difference in phase of excitation will be partially cancelled by the axial displacement of the planes in which the points of different phase lie; to any extent that a difference in phase of excitation results merely from a backward axial component of difference in distance from the feed, the advantage is largely cancelled.

It is accordingly desirable that the conducting extension depart outwardly from the parabolic shape of the main body of the reflector, irrespective of whether it is constructed as an integral extension of the reflector surface or a separate addition. To minimize the dimension required for any given maximum difference of phase of excitation, the diffracting-edge addition is desirably reasonably close to parallelism with the direction of exposure to the feed. Thus in the case of the focal-plane antenna, the wholly radial plane of the edge of the circular aperture forms an ideal location. In addition to the minimization of required dimensions for production of any given phase difference in excitation, the entire "array" of elemental radiators now excited in different phases remains in the same plane perpendicular to the axis.

Where the antenna is of a design wherein the feed is substantially forward of the plane of the edge of the circular dish, the best orientation for the diffraction ring must be determined by experiment in each case. If the wholly radial plane is employed, dimensions required for any given phase difference of excitation are increased, and in addition the angle of incidence on the added forward-facing conductor areas thus enlarged is such that the sidelobe production in the forward direction may in some cases become objectionable. In this case, tilting back of the ring may be employed.

It will of course be understood that the advantages of the invention may in part be obtained by somewhat different entire location of the phase-diffusing edge. For example, it will be obvious from the teachings of the invention to modify the outer edges of large "back-radiation shields" to produce variation of the phase of the diffracted radiation, and thus render such shields truly effective. Although it is most desirable that the invention be utilized in a manner minimizing the size of the structure added to the antenna, it may of course be employed in such a less advantageous manner while still utilizing its more basic aspects.

As will by now be obvious, a diffracting ring extending all around the circumference of the dish is not required for an antenna of single polarization, only the segments which oppose each other in the direction of E-polarization being of substantial effect. An entire ring is used in the illustrated embodiment largely because of the dual polarization of the feed. However, the added cost of an entire ring, as compared with mere opposed segments, is essentially negligible, and it is contemplated that entire rings may be used routinely in dishes of general utility.

It will be seen upon study that the dispersion of the axial backlobe of the curves 52 and 56 into numerous sidelobes does not require intricate and precise design of the outer edge of the diffracting structure. Further, it will be seen that the most complex and continuous (and therefore the most uniform and thus lowest-maximum) radiation pattern will be produced by having radiation elements of a large number of phases, rather than merely the single phase of the circular structure or mere opposed phases. A simple and convenient shape for addition to a circular structure to achieve continuously varying radius is a circumscribed polygon. The rear patterns of curves 52 and 56 of FIG. 8 were obtained with a 12-foot focal-plane dish operated at 6.425 GHz. The desired variation of one wavelength being approximately 2 inches, a polygon was constructed of 15 lengths of 2-inch width aluminum tape arcuately shaped on the inner edge, this number of sides producing a very close "fit" (no substantial flats) around the 12-foot diameter. The patterns thus produced in the respective planes are shown at 54 and 58. (The lobes shown in these rearward 20.degree. plots were the highest that appeared in the backward region.) As may be seen in FIG. 8, there was an improvement of about 5 db. in the front-to-back ratio.

It will be noted that the E-plane pattern 54 was somewhat asymmetrical near the axis. This was ascribed to the employment of a polygon with an odd number of sides. Although symmetry of the radiation in the backward region is of little importance, it is normally considered preferably. Since there is no substantial advantage to a null on the axis (note that this is actually a null point in curves 54 and 58 although there are peaks very closely adjacent), it was believed, and verified by experiment, that the flats 50 in the embodiment illustrated would have no adverse effect, and subsequent constructions, designed for commercial use, employ polygons with equal numbers of sides. For the band centered at 6.175 GHz., there were constructed a 12-foot reflector antenna with a polygonal backlobe suppressor with 14 sides and a 10-foot antenna with a polygon of 12 sides (illustrated in the drawing). Fabrication of the diffraction ring is made in a very simple manner. Aluminum angle stock with a 2-inch web is prebent to circular or semicircular form and the sides of the polygon are formed by simple linear machining, either before or after attachment to the dish.

Reasonably accurate calculation of the backlobe patterns may be made by summing at each far-field point the phased contributions of discrete "elemental radiators" considered to be at half-wave intervals along the outer edge of the ring. The reference amplitude of radiation is taken as the amplitude at opposite points in the direction of the E-vector, and the amplitude at other points is determined in accordance with the cosine of the angle formed with this diameter (where the number of sides of the polygon is large, the error introduced by continuing to use the cosine of the radius angle is small). Phase is assigned in accordance with radial location. Negligible error is introduced by considering only portions of the ring forming angles of 45.degree. or less with the E-vector diameter.

Calculations of the reduction in back radiation of the 10-foot antenna described in the H-plane indicated that the directly axial backlobe should be reduced by about 10 db., but that there should now be maximum lobes in the region of about 7.degree. from the axis, and a 7.8 db. reduction in maximum backlobe was accordingly indicated. The reduction of maximum backlobe in the E-plane was calculated as being even greater, the central backlobe and lobes at approximately 13.degree. being those of greatest amplitude and approximately equal. The predicted improvement in front-to-back ratio taken overall was accordingly the 7.8 db. of the H-plane. In these calculations, the 2-inch maximum extension was considered as a wavelength for purposes of representing the frequency band. Measurements made on the antenna before and after the addition of the diffraction or suppression ring showed highly satisfactory correspondence between the theoretical and experimental improvement when considering the approximations used in the calculations.

At 5.925 GHz., the front-to-back ratio before addition of the ring was 60 db. and after addition of the ring was 67.5 db. At 6.175 GHz., the ratio was 57 db. before the addition and 66.5 db. after the addition. At 6.425 GHz., the ratio was 60 db. before the addition and 71 db. after the addition. In each case, the measurements were made separately on the two polarizations of the feed. Differences in the degree of improvement of the two polarizations were minor despite the asymmetry introduced by the waveguide feed. Although generally similar improvement is shown in the 12-foot dish, it is presently believed, although measurements have not yet been made, that the difference in orientation of the suppression ring with respect to the two polarizations, inherent in using a number of sides which is not divisible by four, will produce considerable difference of location, but only minor difference of amplitude, of the maximum backlobes of the two polarizations.

Although the discussion herein has been limited to the type of microwave transmission equipment in the course of development work on which the invention was made, many other uses of the teachings of the invention will be apparent wherever analogous problems must be solved. As an obvious example, the invention is not limited to wholly circular reflectors, but can be advantageously employed where only a part of the reflector aperture is a circular edge, as in certain forms of paraboloidal reflector geometries. As another example, in the case of a line-feed reflector antenna with one or more linear edges, a similar approach may be made to elimination of the central planar backlobe by addition of phase-dispersing diffracting extensions of the edges. The feed may of course itself include a reflector, as in the case of a Cassegrain. Also, it will be obvious that the described embodiments do not necessarily purport to give the utmost in backlobe reduction which can be attained by the invention, the polygonal construction being used for simplicity of fabrication in view of its adequacy where complete optimization is unnecessary. Diffraction-pattern shaping optimized for various needs may be obtained by edge shapes of diverse configurations reached by calculation and simple experiment. Further, the invention in its broader aspects may be embodied in forms which, although superficially wholly different, will be found to utilize the essential novelty; for example, persons skilled in the art will devise other means for providing the relative delay than the source-distance variation. Accordingly, the scope of the protection to be afforded the invention should not be limited by the particular embodiments herein described.

Claims

What we claim is:

1. In microwave transmission apparatus including at least one feed radiator and a conducting reflector of predetermined reflecting surface shape having at least a portion of its boundary exposed to feed radiations of substantially equal phase, the improved construction characterized by having a conducting extension on at least a portion of the boundary of the reflector so exposed, the extension departing from the reflecting surface shape and having means thereon for producing, at remote rearward locations, diffracted radiation fields of progressively advanced and progressively retarded phase from successive points along the edge of the extension, each progression of relative phase including a plurality of phases intermediate between the most advanced and the most retarded, the difference between the latter phases being greater than 180.degree., the rearwardly diffracted energy being widely diffused in directionality by the interaction of the numerous phases.

2. The improved microwave transmission apparatus of claim 1 wherein there are a plurality of such progressions, the phase produced at successive points being alternately advanced and retarded along the edge.

3. The improved microwave transmission apparatus of claim 1 having delay means varying the transmission time of radiations from the feed to successive portions of the edge to excite the diffracted radiation in differing phase.

4. The improved microwave transmission apparatus of claim 3 wherein the edge of the extension has successive portions at differing distances from the feed and constitutes the delay means varying the transmission time to the successive portions.

5. The improved microwave transmission apparatus of claim 4 wherein the edge of the extension has portions of alternatingly increasing and decreasing distance from the feed.

6. The improved microwave transmission apparatus of claim 4 wherein the difference between the greatest and smallest distance of any portions of the edge from the feed is between 1/2 wavelength and 3 wavelengths at the operating frequency of the feed.

7. The improved microwave transmission apparatus of claim 6 wherein the difference is approximately one wavelength.

8. The improved microwave transmission apparatus of claim 6 wherein the extension extends in a direction close to parallelism with the direction of the radiations from the feed incident thereon, so that the difference of linear distances from successive portions of the edge to the perimeter of the reflecting surface is substantially equal to the difference of distances of such portions of the edge from the feed.

9. The improved microwave transmission apparatus of claim 1 wherein the boundary of the reflector is circular and the feed radiations have their electric field linearly polarized across a diameter, the reflector having at least one such conducting extension on the portions in the region of each end of said diameter.

10. A suppressed-backlobe parabolic dish antenna comprising the improved microwave transmission apparatus of claim 9 wherein the reflector is a circular parabolic dish having the feed at the focus.

11. The parabolic dish antenna of claim 10 wherein the extensions on said portions comprise arcuate ring-segments of alternately increasing and decreasing radial dimension on the periphery of the dish aperture.

12. The parabolic dish antenna of claim 11 wherein the outer edge of at least a portion of each ring-segment is linear and substantially tangent to the circular aperture.

13. The parabolic dish antenna of claim 12 wherein each ring-segment has a plurality of such tangent linear edges forming a generally polygonal segment-edge shape.

14. The parabolic dish antenna of claim 11 wherein the maximum difference of distance of portions of the edges from the feed is from 1/2 wavelength to 3 wavelengths at the operating frequency.

15. The parabolic dish antenna of claim 14 wherein the feed is closely adjacent to the front plane of the dish and the extensions extend substantially wholly radially in that plane so that the difference in distance from the feed is substantially equal to the difference of radial extension.

16. The parabolic dish antenna of claim 15 having thereon an entire circular ring so formed.

17. The parabolic dish antenna of claim 16 wherein the outer edges of the ring are in the general form of a polygon circumscribed about the circular aperture.

18. The parabolic dish antenna of claim 17 wherein the ring has a maximum radial extension of approximately one wavelength, the maximum extension being at short flat regions at the vertices of the polygon.