Feed Apertures With Crosspolarization Compensation for Linear Polarization

This invention relates to reflector antenna systems and, more particularly, to multibeam reflector antenna systems having cross-polarization compensation for linear polarization.

In multibeam antenna systems, multiple waveguide elements, or as used hereinafter feed horns are used in the focal region of a single reflector to produce a cluster of simultaneous multiple beams. If some of these beams use orthogonal polarizations in the same frequency band, a high degree of polarization purity with low crosspolarization is required to provide adequate signal suppression in adjacent beam footprint regions. In the prior art, the reduction of crosspolarization in reflector antennas has been accomplished by using a large F/D (focal length to diameter) ratio, by gridding the reflector or by using separate polarization screens or grids in front of the feed horn or in front of the entire reflector/feed horn assembly. For example, D. T. Nakatani and G. G. Kuhn in "Comstar I Antenna System," Digest International Symposium IEEE Antennas and Propagation Society, June 20-22, 1977, Stanford, California, (Ap-S Session 12, 0900, Tuesday, June 21) pp. 337-340, describe the use of polarizing screens placed in the aperture plane in front of each reflector to obtain polarization purity. These screens are formed of a parallel grating of conducting strips which are fabricated as a sandwich of aluminized Kapton.

The prior art systems for reducing crosspolarization, which require a relatively large F/D ratio, do not permit the use of a compact overall configuration with lower weight. The use of gridded reflectors or screens increases the weight and cost of the antenna system. Due to the need to provide room for the grids or screens, these prior art systems are not well suited for multibeam applications. Although the screens used by Nakatani and Kuhn are designed to provide low insertion loss, some loss is nevertheless present.

It is therefore the object of the present invention to provide improved means for compensating for crosspolarization which overcomes the drawbacks of the prior art systems.

To this end, the invention contemplates the incorporation of crosspolarization compensation means in the structure of the feed horn itelf. In one form of the invention, the compensation is provided by modifying the feed horn to have curved walls and septa which are empirically chosen to provide the proper amount of compensation. In a second form of the invention, the compensation is provided by employing shaped septa in a feed horn having straight, unmodified walls, the septa curvature being chosen to provide the necessary compensation of the crosspolarization components. The compensation may also be provided by a feed horn having curved walls and no septa. In the case of feed horns which are tilted with respect to the axis of the reflector, the compensation is provided by an asymmetric feed horn having curved septa and feed walls. The amount of asymmetry and the curvature of the walls and septa are chosen to provide the required compensation. For the case of feed horns having planar walls and a rectangular cross section normal to the horn axis which increases along the length of the horn, the crosspolarization compensation is provided by curved forward edges of the horn walls, the edges being convex for one type of compensation and concave for compensation in an opposite direction.

Additional objects, advantages and features of the invention will become more readily apparent from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is a diagram illustrating the presence of crosspolarization components in adjacent beam footprints of adjacent orthogonally polarized beams.

FIG. 2a is a diagram showing the required parallel polarization lines for zero crosspolarization with a symmetrical reflector.

FIG. 2b is a diagram showing the curved polarization lines of a typical, uncompensated feed horn with a symmetrical reflector.

FIG. 2c is a diagram showing a feed horn modified in accordance with a first embodiment of the invention to compensate for crosspolarization with a symmetrical reflector.

FIG. 3 is a perspective view of a partially assembled set of feed horns showing two adjacent feed horns modified in accordance with a second embodiment of the invention.

FIG. 4 is a diagram which illustrates the feed horns of the embodiments of FIGS. 2c and 3 arranged symmetrically with respect to the axis of a reflector.

FIGS. 5a and 5b are diagrams showing offset reflector feed horns for horizontal and vertical polarizations.

FIGS. 6a and 6b are diagrams illustrating why feed horns compensated for a symmetrical feed with horizontal and vertical polarizations, respectively, fail to compensate fully for the crosspolarization components of tilted feeds.

FIGS. 7a and 7b are diagrams showing the asymmetric modifications of the feed horns for horizontal and vertical polarizations, respectively, to provide proper compensation of the crosspolarization components for offset reflectors.

FIGS. 8a and 8b are diagrams showing a side elevation and an end view, respectively, of a feed horn having planar walls and a rectangular cross section of increasing area and having one type of edge modification.

FIGS. 9a and 9b are diagrams showing a side elevation and an end view, respectiely, of a feed horn having planar walls and a rectangular cross section of increasing area and having another type of edge modification.

As illustrated in FIG. 1, two adjacent beams, Beam 1 and Beam 2, of a multibeam reflector antenna system of the prior art include signals in the same frequency band. These signals, are radiated simultaneously at a single reflector by a pair of feed horns of a set of multiple feed horns located in the focal region of the reflector and positioned adjacent each other with the feed horn axes pointed at the reflector. The radiated beams are orthogonally polarized as indicated by the vertically polarized signal component 21 in Beam 1 and the horizontally polarized component 22 in Beam 2. Each of these polarizations is sometimes referred to as the dominant linear polarization of the beam. The reflected beams are radiated by reflector/feed horn assemblies of the prior art which inherently radiate a crosspolarization component. Thus, when vertically polarized Beam 1 is radiated, it includes a spurious horizontally polarized component of lower amplitude, some of which will appear as a horizontal crosspolarization component 21.times. in Beam 2. Likewise, when horizontally polarized Beam 2 is radiated, it includes a spurious vertically polarized component of lower amplitude, some of which appears as a vertical crosspolarization component 22.times. in Beam 1. A portion of the signal intended for Beam 1 thus appears in Beam 2 and vice versa, resulting in inadequate signal separation. It is therefore desirable to improve the polarization purity and reduce the crosspolarization of the adjacent orthogonally polarized beams to provide adequate signal suppression in adjacent beam footprint regions.

FIG. 2A illustrates the desired polarization condition which results in no crosspolarization. Each of the vertically polarized (in this case) lines 24, 25 and 26 are parallel and have no lateral components. However, as shown in FIG. 2b, a typical, uncompensated feed horn 30 of the prior art radiates the vertically polarized signal 32 as a set of polarization lines, some of which are curved and thus include crosspolarization components. Depending on the reflector geometry, the F/D ratio, the feed horn size and the feed horn shape, the outer polarized lines may be convex, as shown by lines 24a and 26a, or concave, as shown by dash lines 24b and 26b. For a given reflector geometry there may exist an uncompensated feed horn which provides parallel polarization lines. However, such a feed horn will not, in general, coincide with the feed horn required to provide acceptable antenna efficiency and radiation characteristics.

According to the present invention, the feed horns themselves are structurally modified to provide the required polarization line (E-field) compensation. The standard feed horn is, for example, modified to include curved feed horn walls, curved septa within the horn, or both. In the embodiment shown in FIG. 2c, modified feed horn 34 includes curved feed horn walls 35, 36, 37 and 38, of which opposed walls 35 and 36 are convex and opposed walls 37 and 38 are concave. A set of spaced septa 40, 41 and 42 extend across feed horn 34. Septa 40 and 42 are curved, while the central septum 41 is generally straight. The spacing and number of septa are selected to cut off all but the fundamental linear polarization mode. The septa are spaced somewhat less than 1/2 the free space wavelength; the number of septa is determined by the size of the feed horn opening. The curvature of the feed horn walls and the septa may be selected empirically and/or analytically to produce appropriate curvature of the field lines 44, 45 and 46 in the horn to compensate the radiated curved polarized lines 24a or 24b and 26a or 26b of FIG. 2b and produce the desired straight polarization lines 24, 25 and 26 as seen in FIG. 2c. The transformation (or mapping) of the feed polarization into the reflector aperture can be visualized by comparing the polarization lines of FIGS. 2b and 2c. Although the embodiment shown in FIG. 2c is vertically polarized, for reasons of geometric symmetry, the feed horn modification shown in FIG. 2c is applicable also for horizontally polarized waves by rotating the feed horn as seen in FIG. 2c by 90.degree. about the reflector axis.

A partial assembly of a set of feed horns of a multibeam reflector antenna system is shown in FIG. 3 in which a pair of feed horns 54 and 55 are illustrated positioned so that their forward openings are adjacent each other. A mounting plate 48 includes a plurality of holes for the feed horns; only holes 49, 50 and 51 are visible in the figure. The feed horns are mounted over these openings and secured in place by means of mounting screws 52. Two adjacent feed horns 54 and 55 are shown mounted on board 48. These horns have straight walls but are modified in accordance with the invention by the inclusion of septa, some of which are curved to compensate for the cross-polarization components. Thus, feed horn 54 contains a curved septum 56, a generally straight septum 57 and a curved septum 58. These are so spaced and curved that the radiated polarized wave is compensated for crosspolarization components. The orthogonally polarized adjacent feed horn 55 also includes three septa 60, 61 and 62; of these septa 60 and 62 are curved, while septum 61 is generally straight. Again, the septa are so spaced and curved that the wave radiated by feed horn 55 is compensated for crosspolarization components.

The embodiments shown in FIGS. 2c and 3 are intended for use in a feed arrangement which is symmetrically positioned with respect to the parabolic reflector. Referring to FIG. 4, the set of feed horns are indicated at 64, the feed horns extend axially of reflector 68. The axis of the set of feed horns, the focal point 66 of the reflector and the vertex 70 of the reflector are generally in alignment.

In the case of an offset reflector, somewhat different modifications are required. FIGS. 5a and 5b show, respectively, offset feed horns for horizontally and vertically polarized waves. In FIG. 5a, the feed horn 74 is located at the focal point 76 of the reflector but is tilted towards the center of the offset aperture of reflector 78 to provide symmetrical reflector illumination. The axis of feed horn 74 no longer is aligned with focal point 76 and vertex 80 of reflector 78. In like mannr, the feed horn 84 in FIG. 5b is located at the focal point 86 of reflector 78 but is tilted towards the center of the offset aperture of reflector 78. Again, the axis of horn 84 is not aligned with focal point 86 and the vertex 80 of reflector 78. In compensating such offset reflector arrangements for crosspolarization, it is not sufficient to tilt the feed, as compensated for a symmetrical reflector; the polarization line orientation must be maintained with respect to the principal planes of the reflector.

The problem is illustrated in FIG. 6a for the horizontally polarized wave. A feed horn 92 is compensated in the manner suitable for a symmetrical reflector with curved feed horn walls 93, 94, 95 and 96 and curved septa 97 and 99 on opposite sides of generally straight septum 98 to produce field lines 100, 101, and 102 suitable for compensating the crosspolarization components for the symmetrical reflector. Due, however, to the tilt of feed horn 92 and its asymmetric relation to reflector 78, the resulting radiated wave includes curved polarization lines 103 and 105 on opposite sides of a generally straight polarization line 104. Accordingly, the problem of cross-polarization will remain. The same situation applies with respect to the vertically polarized arrangement of FIG. 6b in which feed horn 110 is compensated for a symmetrical reflector by suitably curving feed horn walls 111, 112, 113 and 114 and by including septa 116, 117, and 118, of which septa 116 and 118 are curved. The field lines 119, 120 and 121 in feed horn 110 are suitable for compensating the crosspolarization components for the symmetrical reflector. Again, the resulting radiated wave includes curved polarization lines 122, 123 and 124 with the problem of crosspolarization remaining.

In order to maintain the polarization line orientation with respect to the principal planes of the reflector when the feed horn axes are offset from the reflector vertex, the modifications shown in FIGS. 7a and 7b are employed. In FIG. 7a, feed horn 125 is modified to bring the polarization line orientation asymmetrical with respect to the feed horn center. The arrangement of curved walls 126, 127, 128 and 129 is now asymmetrical with wall 127 near the center of reflector 78 much longer than wall 129 near one edge of the reflector. The pattern of the septa is also changed. The two septa 131 and 132 closest to the center of the reflector are both curved, while septum 133 closest to the edge of the reflector is generally straight. The field lines 135, 136 and 137 within feed horn 125 are not properly shaped to compensate for the crosspolarization components, and the radiated lines 141, 142 and 143 are now straight and parallel.

Unlike the situation applying for the symmetrical reflector arrangements, it is no longer possible simply to rotate the feed horn 90.degree. to provide for the vertically polarized beam. Due to the asymmetry, as shown in FIG. 7b, the modifications of the feed horn 144 for the vertically polarized wave are different. Curved feed horn walls 146, 147, 148 and 149 are again asymmetrical with wall 147 near the center of reflector 78 longer than wall 149 near the edge of the reflector. Septa 151 and 153 are curved asymmetrically and center septum 152 is generally straight. The curvature of the feed horn walls and the spacing and curvature of the septa are such that vertically polarized field lines 155, 156 and 157 in feed horn 144 compensate for the crosspolarization components; and feed horn 144 radiates a vertically polarized wave having straight & parallel polarization lines 161, 162, 163 and 164.

Another technique, suitable for the case of a symmetrical reflector, for crosspolarization compensation is shown in the embodiments of FIGS. 8a and 8b and FIGS. 9a and 9b. This technique involves modifications of feed horns which are of the flared horn type, i.e which have planar walls and whose cross section normal to the horn axis increases along the length of the horn. In FIGS. 8a and 8b, a rectangular waveguide 168 feeds a flared feed horn with rectangular cross section 170, having flat side walls 171, 172, 173 and 174. The required amount of crosspolarization compensation is introduced by modifying the forward edges of side walls 171, 172, 173 and 174. In particular, in the example of FIGS. 8a and 8b, the modification involves inwardly curving the forward edges as shown at 175, 176, 177 and 178. As seen in FIG. 8b, this results in polarization lines 181, 182 and 183 within the horn which will contribute the proper amount of direction of compensation for the crosspolarization components. If compensation of the opposite type is required, the modification shown in the embodiment of FIGS. 9a and 9b is employed. In FIGS. 9a and 9b, a rectangular waveguide 188 feeds a flared horn 190 with rectangular cross-section, having flat side walls 191, 192, 193 and 194. The forward edges of these walls are curved outwardly, as shown at 195, 196, 197 and 198 to provide the proper amount and direction of compensation. As shown in FIG. 9b, the polarization lines 201, 202 and 203 are bowed in a direction to provide compensation for crosspolarization components directed oppositely to those compensated for in the embodiment of FIGS. 8a and 8b.

In designing the feed horns of the invention, it has been found to be more effective to proceed empirically. Although an analytic approach is also possible, involving solutions of the Maxwell equations for given boundary conditions, it is difficult to take into account all side effects. When proceedig empirically, the magnitude of the crosspolarization component is checked for different configurations until the best result is obtained. In the case of the embodiment shown in FIG. 2c, this would involve adjustment of both wall shape and septa shape. Because the embodiment of FIG. 3 requires adjustment of the septa shape only, this embodiment is preferred as requiring a less complicated empirical procedure.

Tests of embodiments of the invention have verified a marked reduction in the size of the crosspolarization component. In particular, a first test using a feed horn of the prior art having no septa, such as the feed horn 30 shown in FIG. 2b, resulted in a signal having a crosspolarization component which had a measured power magnitude 22.2 dB below the measured power magnitude of the dominant polarization with an offset reflector. The test was then repeated using a feed horn with flat walls and shaped septa, as shown in FIG. 3. The crosspolarization component had a measured power magnitude of 29.5 dB below the measured power magnitude of the dominant polarization, demonstrating a 7.3 dB improvement by using shaped septa to compensate for the crosspolarization component.

The principles of the invention are applicable in all linearly polarized microwave radiating systems in which crosspolarization is a relevant factor. The invention will find utility, for example, in microwave point-to-point relay systems, radar systems with rain cancellation features for airport surveillance systems, and especially in satellite communication system antennas, both for ground and satellite equipment, where frequency reuse requires good polarization isolation. The invention may also be used in high performance test equipment antennas which are used for testing such systems.

Although the invention has been described with reference to particular embodiments, various changes and modifications which are obvious to a person skilled in the art to which the invention pertains are deemed to be within the spirit and scope of the invention as set forth in the appended claims.