CA1226936A - Multibeam antenna with reduced sidelobes - Google Patents

Abstract

MULTIBEAM ANTENNA WITH REDUCED SIDELOBES
Abstract The present invention relates to a multibeam antenna feed arrangement comprising a plurality of feeds, each feed being capable of receiving a different input signal and including a separate waveguide means connected between the feed and an associated local transmitter or receiver. The feed arrangement further comprises cross-coupling paths disposed between predetermined pairs of the waveguide means for (a) coupling out a portion of the signal propagating in a waveguide means associated with a first one of the feeds, (b) adjusting the amplitude and phase of the coupled-out signal in a predetermined manner, and (c) introducing the resultant adjusted signal into the waveguide means associated with a second one of the feeds where the sidelobe of the second one of the feeds exceeds a predetermined sidelobe level in the direction served by the first one of the feeds thereby reducing signal interference in the second one of the feeds. For N feeds, where N is an even number, only N cross-coupling paths may be needed. Where N is an odd number, only N-1 cross-coupling paths may be needed.

Description

MULTI BEAM ANTENNA WITH REDUCED SIDE LOBES

Technical Field The present invention relates to a multi beam antenna with reduced side lobes and, more particularly, to a multi beam earth-station antenna comprising multiple feeds and a network arrangement disposed at the outputs of the feeds for coupling to the output path of each feed of interest a signal from each other feed aiming a beam in the same direction as a side lobe which is above a predetermined side lobe level at the feed of interest, thereby substantially reducing the effects of interfering signals from other beams at the feed of interest.
Background of the Invention In a circularly symmetric antenna for use at, for example, an earth-station, one of the chief causes of enlarged side lobes is beam blockage. Blockage, in turn, is caused by obstacles located in front of the main reflector.
Typical obstacles are feeds and struts in a front-fed design, or a sub reflector and struts in a Cassegrainian design.
In multi beam antennas, a single feed is normally used for each beam Feeds in, for example, an earth-station antenna are located such that the beam from each separate feed is aimed at, for example, a separate satellite, which is closely spaced on the geosynchronous arc with other satellites associated with beams from other feeds. The main signal passing through each feed is intended for transmission to, or reception from, only one satellite. However, side lobes of a given beam generally point towards adjacent satellites Consequently, a small part of each main signal is inadvertently radiated toward the wrong satellites. Conversely part of the signal radiated by each satellite, which is intended for just one associated feed of an earth-sta~ion antenna, is inadvertently received by adjacent feeds. Such inadvertent transmission and reception is defined as interference.
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Various techniques have been devised to reduce side lobes. For example, to provide separation for o'er-lapping area coverage and spot beams in the far field of an antenna, the transmitting antenna includes networks which couple a portion of the area coverage signal into each of the spot beam feeds to provide cancellation in a selected area in the far field of the antenna. In this regard see, for example, US. patent 4,145,658 issued to A. Acampora et at on March 20, 1979.
Another technique for reducing side lobes in a single beam antenna is to provide, for the principal feed of interest, means in the plane of the side lobes of the principal feed to suppress such side lobes only. Such means can comprise two auxiliary feeds and associated networks, in the plane of the side lobes to be suppressed, for launching separate beams of the signal being launched by the principal feed whereby the central ray of each beam impinges the same point on the main reflector as the central ray of the principal feels beam as shown in US.
patent 4,364,052 issued to E. A. Ohm on December 14, 1982. Such means can also comprise suppression means disposed adjacent and on symmetrically opposite sides of the main antenna in the plane of the side lobes to be sup-pressed at approximately the width of the aperture of the main reflector. The suppression means can alternatively comprise separate auxiliary antennas ox subsections of the main reflector as disclosed in US. patent 4,376,940 issued to H. Madame on March 15, 1983.
Still another technique is the use of adaptive interference suppression arrangements as disclosed, for example, in US. patent 4,320,535 issued to D. M. Brady et at on March 167 1982. There a main antenna picks up the desired and interfering signal while a small auxiliary antenna essentia71y picks up only the interfering signal.
A feedback arrangement adaptively adjusts the phase and amplitude of the interfering signal at the output of the auxiliary antenna to cancel the interfering signal in the overall signal received by the main antenna.
The problem remaining in the prior art is to provide a ~ultibeam antenna capable of receiving beams from a plurality of remote transmitters at separate feeds with reduced interference at each feed using simple and cost effective means for new antennas or for retrofitting existing antennas.

The foregoing problem in the prior art has been solved in accordance with the present invention which relates to a multi beam antenna with reduced side lobes in directions served by each of the feeds and, more particularly, to a multi beam earth-station antenna comprising multiple feeds and a network arrangement disposed at the outputs of the feeds fur coupling to the output path of each feed of interest a signal from each other feed that aims a beam in the same direction as a side lobe which is above a predetermined side lobe level at the feed of interest, thereby substantially reducing effects of interfering signals from nearby satellites at the feed of interest. The network arrangement functions to couple a weighted signal, which is 180 degrees out of phase from the output of each other feed, to the output of the feed of interest when each side lobe of the feed of interest exceeds a predetermined side lobe level in the direction served by one of the other feeds.
In accordance with an aspect of the invention there is provided a multi beam antenna feed arrangement comprising: a plurality of N feeds capable of receiving signals from a plurally of N remote, spaced-apart, transmitters and transmitting the received signals along N
separate associated paths characterized in that the feed I

arrangement further comprises: network means comprising a plurality of at least N cross-coupling paths Norway Lo is an even integer, or N-l cross-coupling paths where is an odd integer, each cross-coupling path including means for providing a predetermined phase and amplitude adjusted version of a signal propagating in an output path of an associated first one of the plurality of N feeds to an output path of an associated second one of the plurality of N feeds where a side lobe associated with the second one of the feeds exceeds a predetermined side lobe level in a beam direction associated with the first one of the reeds for substantially reducing interference from the signal destined for the associated first one of the feeds which is also received by the associated second one ox the feeds via the side lobe associated with the second one of the feeds.
It is an aspect of the present invention to provide a multi beam antenna comprising an interference reducing network at the output of the Leeds of the multi beam antenna. The network functions to couple to a feed of interest, a weighted signal 180 degrees out of phase from the output line of each other feed, where each side lobe of the feed of interest is above a predetermined side lobe level in the direction served by one of the other feeds. In general, in an N-beam antenna, maximum of N-l cross-coupling paths are needed to couple to each feed of interest signals from feeds that aim teams in directions in which swaddles ox a feed of interest exceed predetermined levels. However, in a particular embodiment, for an Beam antenna, where N is an even integer then a total of only cross coupling paths are needed, while where N is an odd integer then a total of only N-l cross-coupling paths are needed.
It is a further aspect of the present invention to provide an antenna feed arrangement or use with either Jo - pa -front-fed or subreflector-included antennas wherein the antenna employing the feed arrangement can be used to receive signals from closely spaced satellites in geosynchronous orbit with reduced interference at earn feed associated with a separate one of the satellites.
Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawings.
Brief Description of the Drawings Referring now to the drawings, in which like numerals represent like parts in the several views:
It. 1 is an illustration of an exemplary display of principal lobes and certain side lobes of beams associated with an exemplary four-beam antenna where feeds ?3~j are not located on the foresight axis and feeds aim beams at separate satellites FIG. 2 is a block diagram of an exemplary four-beam antenna feed arrangement with a side lobe reducing network in accordance with one aspect of the present invention which couples to a feed output of interest a ; separate weighted and phase shifted signal from each of tic other feed outputs; and FIG. 3 is a block diagram of an exemplary four-beam antenna feed arrangement including a side lobe reducing network in accordance with the present invention which cross-couples to each feed of interest, a weighted and phase shifted signal from only one adjacent feed where each feed of interest has a side lobe above a predetermined level in the direction served by the adjacent feed.
Detailed Description The present invention is described hereinafter with regard to an exemplary four-beam antenna feed arrangement to cause reduced side lobe interference at each of the four feeds. It is to be understood that such arrangement is described herein for purposes of exposition only and not for purposes of limitation and that the multlbeam feed arrangement can comprise any arrangement of two or more feeds and still fall within the spirit and scope of the present invention. The present multi beam antenna feed arrangement is designed for use with either front-fed or subreflector-included antennas. In such antennas, each feed is positioned on the focal surface of the antenna to receive signals either from a separate one of a plurality of satellites positioned near each other on the Geosynchronous Equatorial Arc GUY) or from a separate one of a plurality of terrestrial transmitters within the field of view of the antenna.
As was described herein before, in multi beam antennas, interfering signals are characterized by low antenna gain. However, each signal is also available with high antenna gain at the principal feed of each beam. or example ! in FIG. 1, an interfering signal from satellite 12 can enter feed 1 via the third side lobe 10 of the beam associated with feed 1. Simultaneously, a relatively large signal from satellite 12 is available at feed 2. In a similar manner an interfering signal from satellites 13 and 14 can also enter feed 1, but because tune side lobe Lyle of the beam associated with feed 1 is very low at feeds 3 and 4, these interfering signals will be very low compared to the interfering signal from satellite 12. A similar explanation can be provided for the interfering signals at feeds 2-4 from all satellites 11-14 except the one which is the principal source of signals for each feed of interest.
In accord with the present invention, the above-described interference at each feed can be substantially reduced by the arrangements shown in Figs 2 and 3. In FIG. 2, feeds 1-4 are connected at their outputs, via separate wave guide means 6-9, respectively, to respective optional low-noise amplifiers 25-28 which, in turn, are connected to the respective receivers associated with feeds 1-4. disposed between feeds 1-4 and optional amplifiers 25-28 is a network arrangement according to the present invention for substantially reducing interfering signals from other than the principal satellite 11, 12, 13 or 14 of each respective feed 1, I 3 or 4 of interest via an associated side lobe exceeding a predetermined level in each direction served by another feed.
In Figs 2 and 3, it is desired, inter alias to receive in the output path of feed 1 only the signal from satellite 11. As described hereinabove, interference signals are also picked up by feed 1 from transmissions from satellites 12-14 via sîdelobes of the beam associated with feed 1, as shown in FIG. 1. The arrangement of FIG.

2 is designed to substantially reduce at each feed J
interference from all satellites which are not the primary satellite of interest for each feed. However, only the network means for substantially reducing side lobe interference at feed 1 is shown for simplicity. It is to f43~

be understood that similar additional network means are needed for each of feeds 2-4 to substantially reduce interference at these feeds. In FIG. 2, interference in the output of feed 1 from satellite 12 is substantially reduced by means of a passive cross-coupling path comprising an amplitude and phase adjusting means 16, a directional coupler 17 disposed in wave guide means 7 associated with feed 2, and a directional coupler 18 disposed in wave guide means 6 associated with feed 1.
Directional couplers 17 and 18 are disposed to transmit a portion of the signal propagating in wave guide means 7 received by feed 2 through the amplitude and phase adjusting means 16 and into wave guide means 6 associated with feed 1. For purposes of illustration, the amplitude and phase adjusting sections of means 16 have been labeled Aye ~21 to indicate amplitude adjustment (A) and phase adjustment (~) from the output of feed 2 to the output of feed 1.
Three essential steps are: (a) part of the signal originating at satellite 12 and received with high earth-station antenna gain at feed 2 is coupled out of wave guide means 7 of feed 2 via directional coupler 17 (b) amplitude and phase of the coupled signal are adjusted by, for example, attenuator Aye and phase shifter ~21 in means 16, and (c) the adjusted signal is coupled into wave guide means 6 of feed 1 via coupler 18. Reduction of interference of the signal from satellite 12 in the wave guide means 6 associated with feed 1 is achieved by initially adjusting the amplitude to be somewhat less than, and the phase to be 180 degrees out of phase with the interfering signal from satellite 12 arriving at feed 1 via the pertinent side lobe of the beam associated with feed 1, which in the exemplary illustration of FIX 1 is the third side lobe 10. Similarly. an unlink signal propagating in wave guide means 6 associated with feed 1 can be reduced in the direction of satellite I However if unlink and down link frequencies are substantially different, a second Aye ~21 network that is in parallel with the first network, and which has different Aye ~21 settings, is needed to accommodate the somewhat different side lobe of an unlink beam.
Extension of this technique allows other interfering signals due to side lobes of the beam associated with feed 1 pointing toward satellites 13 and 14 to also be reduced by the addition of a directional coupler 20 in wave guide means 8 associated with feed 3 which is connected to an amplitude AYE) and phase adjusting (~31) means 21 r and a directional coupler 23 disposed in wave guide means 9 associated with feed 4 which is connected to an amplitude (Aye) and phase adjusting (~41) means 24. The outputs from amplitude and phase adjusting means 21 and 24 are introduced into wave guide means 6, associated with feed 1, by either directional coupler 18 or by separate directional couplers (not shown) disposed in wave guide means 6 associated with feed 1. Again side lobes associated with the beam of feed 1 pointing towards satellites 13 and 14 can be reduced by proper adjustment of the amplitude and phase of a cross-coupled signal coupled out of means 20 and 23, respectively, and introduced into wavegulde means 6. Such adjustments may require a minor readjustment of the amplitude and phase in means 16 to maintain a given level of reduced interference from satellite 12.
The network portion shown in FIG. 2 reduces interfering signals only received in the output of feed 1.
A similar network is needed to reduce interference in each of the other outputs of feeds 2-4 caused by side lobes from each of the associated beams Networks such as the one shown in FIG. 2 can be implemented in different ways. Fur example, the wave guide means 6 9 from feeds 1-4, respectively, and the associated directional couplers, can be short in length and made of low-loss wave guide. This minimizes the noise temperature of each receiver. However, a somewhat larger loss can be tolerated in pathways that -I

cross-couple energy between wave guide mean 6-9.
Consequently, adjustable attenuators (A), adjustable phase shifters (~) and connecting lines between the associated directional couplers and amplitude and phase shifting means can be coaxial, thereby resulting in a network which is more compact ! and easier to assemble, than a network composed entirely of wave guide parts.
In FIG. 2, cross-coupled energy can be increased, to allow for further reduction of interference, by increasing the coupling of each directional coupler.
However, such increase of coupling can degrade receiver noise temperatures significantly. This problem can be avoided by inserting an identical low-noise amplifier 25-28 at the output of each feed as shown in the arrangement of FIG. 3 rather than as shown in the arrangement of FIG. 2.
Modest losses in subsequent feed lines Jo and from the transmitter and receiver can then be tolerated. Although the herein before description has been directed to radio frequency (RF) interference reduction, it is to be understood that similar results can be achieved at intermediate frequencies IF) if etch low-noise amplifier 25-28 in the arrangement of FIG. 2 is disposed directly at the output of each feed 1-4 as shown in FIX. 3, rather than after the network arrangement as shown in FIG. 2, and the amplifiers are equipped with an RF-to-IF converter. It is to be understood that with such RF-to-IF conversion, the cross-coupling network portions should also be designed to operate at IF.
Side lobes shown in FIG. 1 assume that beam blockage is negligible. Actually, blockage can be significant resulting in an additional component of the antenna pattern known as a "blockage pattern". In comparison to each main Lowe each blockage pattern (not shown) is (a) very wide, by low in E-field amplitude, I
opposite in E-field sign, and Ed) centered about each main lobe. ~ur~hermore, even-numbered side lobes of each beam have the same R-field sign as the main lobe, odd-numbered side lobes have the opposite Elude sign Adding such E-fields algebraically odd-numbered nearside lobes are larger and even-numbered side lobes smaller than the ones shown in FIG. 1. Adjacent side lobes that are near each main lobe are opposite in sign because they are larger in amplitude than the blockage antenna pattern.
Conversely, adjacent side lobes that are far from each main lobe (not shown) have the same sign because they are smaller than the blockage pattern.
In FIG. 2, coupling of the beam associated with feed 2 into the beam associated with feed 1 via directional couplers 17 and 18 and amplitude and phase adjusting means 16 is also constant in sign. If the phase, ~21~ of the signal coupled out of wave guide means 7 is adjusted such that the main lobe of the beam associated with feed 2 is opposite in sign to the third side lobe 10 of the beam associated with feed 1, then adjustment of the amplitude Aye in means 16 is the equivalent to subtracting a variably attenuated portion of the pattern of the beam associated with feed 2 prom that of the beam associated with feed 1.
Simultaneously, sides of the main lobe of the beam associated with feed 2 add to the patterns of the second and fourth side lobes of the beam of feed 1 because both of these side lobes have the opposite sign. Effects on other side lobes of the beam associated with feed 1 are negligible because the gain of the beam associated with feed in those side lobe directions is essentially zero.
Consequently, even when nearside lobes are enlarged by aberrations and beam blockage, the side lobe showing the greatest interference, e.g., the third side lobe 10 in FIG. 1, can be reduced at least I dub.
For largest cancellation bandwidth, the principal and cross-coupling path lengths associated with each satellite are chosen to be substantially equal.
Moreover, if satellite 12 in FIG. 1 is located between near side lobes, then Aye and ~21 can be I

adjusted to achieve a deep null. The null, however, moves toward the main lobe as frequency is increased This, in turn, increases interference from satellite 12, and illustrates that interference cancellation is bandwidth limited even when principal and cross-coupling pain lengths are substantially equal. Nonetheless, worst-case interference at the edges of, for example, the 3.7-4.2 Go band, can still be reduced about 5 dub it respect to adjacent nearside lobe peaks. Additionally, suppose that feed apertures are small, as needed to accommodate closely spaced beams. Then the null moves more slowly as a function of frequency. Consequently, worst-case interference can probably be reduced another 5 do, thereby giving a total reduction of 10 dub.
In a similar manner, further-out side lobes of the beam associated with feed 1. which point in the same directions as beams associated with feeds 3 and 4, can be reduced by proper adjustment of amplitude and phase adjusting means 21 and 24, respectively, of FIG. 2. If further-out side lobes have the same sign, such adjustments can reduce the amplitudes of adjacent sid~lobes simultaneously. This is equivalent to substantial cancellation of the beam-blockage pattern in directions of beams associated with feeds 3 and 4. resulting side lobes in such directions are approximately equal to those of an antenna without blockage, and are small enough to be neglected.
Worst interference is due to the higher side lobes, which are located near the main lobe on the foresight axis side of each off-boresight beam. These side lobes are higher, relative to those of a foresight beam due to the aberration called coma, and increase in amplitude as a function of off-boresigh~ direction of a beam. Thus, the side lobes of the beam associated with feed 1 in FIG. 1 pointing in the same general direction US the beam associated with feed 2 are coma lobes, and these - 0 side lobes Are larger than similar coma lobes snot shown) of beam 2 associated with feed 2 which point in the same general direction as the beam associated with feed 3 because the beam associated with feed 2 is closer to the foresight axis of the antenna In contrast, coma lobes of beam 1 associated with feed 1 which point in the directions of beams 3 and 4 associated with Leeds 3 and 4, respectively, are relatively small, and in most applications are probably small enough to be neglected.
Similarly, coma lobes ox beam 2 associated with feed 2 pointing in the direction of the beam associated with feed 4 r and side lobes of beam 2 pointing in the direction of the beam associated with feed 1, are probably small enough to be neglected.
Assuming that such side lobes are small enough to be neglected, the entire cross-coupling network can be made relatively simple as, for example, shown in the arrangement of FIG. 3. In such simpler arrangement according to the present invention, only four cross-coupling paths are needed and additionally, nearly all parts can be coaxial, cross-coupling adjustments can be made outside the antenna aperture, noise temperature is not degraded, and additional amplifiers are not needed since amplifiers 25-28 shown are required anyhow for normal signal reception. As shown in FIG. 3, for an exemplary four-beam feed arrangement, directional couplers 17 and 18 and amplitude and phase adjusting means 15 (Aye, ~21j disposed between wave guide means 7 and 6 used in the arrangement of FIG. 2 will still be required in the arrangement of FIG 3.
To reduce side lobe interference at feed 2 from satellite 13 associated with feed 3, a cross-coupling path including directional couplers 30 and 31, disposed in wave guide means 8 and 7, respectively, and an amplitude and phase adjusting means 32 (Aye ~32) introduce an amplitude and phase adjusted version of the signal propagating in wave guide means 8 into wave guide means 7. Similarly, to reduce side lobe interference at feed 3 from satellite 12 associated with feed I, a cross it coupling path including directional couplers 34 and 35, disposed in wave guide means 7 and I respectively, and an amplitude and phase adjusting means 36 (Aye ~23) are used to introduce an amplitude and phase adjusted version of the signal propagating in wave guide means 7 into wave guide means 8. Finally, to -reduce side lobe interference at feed 4 from satellite 13, a cross-coupling path including directional couplers 38 and 39, disposed in wave guide means 8 and 9, respectively, and an amplitude and phase adjusting means 40 (Aye, ~34) introduce an amplitude and phase adjusted version of the signal propagating in wave guide means 8 into wave guide means 9.
It is to be understood that the above-described embodiments are simply illustrative of the principles of the invention. Various other modifications and changes may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof. For example, as was stated herein before, bandwidth is limited by fre~uency-sensitive changes on direction of the side lobes which point toward the largest sources of interference. Nonetheless, bandwidth is sufficient for, for example, 4 GHz receive-only applications provided that the principal and cross-coupling path lengths associated with each satellite readjusted to be substantially equal, e.g., as indicated in FIG. 3 by the use of optional path length 41, or optional path length 42, each associated with satellite 11.
Similarly, unlink interference suffered by adjacent satellites can be also reduced across the 6 GHz band by applying a second cross-coupling network to the same earth-station antenna. In the arrangement of JIG, 3, with N
feeds, where N it an even number only N cross-coupling paths are required. In similar arrangements where the number of feeds is an odd number, only N-1 cross-coupling paths may be needed.

Claims (9)

Claims

1. A multibeam antenna feed arrangement comprising:
a plurality of N feeds capable of receiving signals from a plurality of N remote, spaced-apart, transmitters and transmitting the received signals along separate associated paths CHARACTERIZED IN THAT
the feed arrangement further comprises:
network means comprising a plurality of at least N cross-coupling paths where N is an even integer, or N-1 cross-coupling paths where N is an odd integer, each cross-coupling path including means for providing a predetermined phase and amplitude adjusted version of a signal propagating in an output path of an associated first one of the plurality of N feeds to an output path of an associated second one of the plurality of N feeds where a sidelobe associated with the second one of the feeds exceeds a predetermined sidelobe level in a beam direction associated with the first one of the feeds for substantially reducing interference from the signal destined for the associated first one of the feeds which is also received by the associated second one of the feeds via the sidelobe associated with the second one of the feeds.

2. A multibeam antenna feed arrangement according to claim 1 wherein the means for providing a predetermined phase and amplitude adjusted version of a signal propagating in an output path of a first one of the plurality of N feeds comprises:
a first directional coupler capable of coupling into the associated cross-coupling path a predetermined portion of a signal propagating in a first direction in the output path of the first one of the plurality of N feeds;
a second directional coupler capable of coupling into the first direction of the output path of a second one of the plurality of M feeds a predetermined portion of the signal propagating in the associated cross-coupling path;

and passive adjusting means capable of (a) adjusting the amplitude of the signal from the first directional coupler such that after coupling by the second directional coupler, the signal is equal to or less than the amplitude of the interference signal received by the second one of the feeds, which interference signal was associated with the beam intended for the first one of the feeds, and also (b) adjusting the phase of the signal from the first directional coupler such that after coupling by the second directional coupler, the signal is substantially 180 degrees out of phase with said interference signal received by the second one of the feeds.

3. A multibeam antenna feed arrangement according to claim 1 wherein the network means comprises cross-coupling paths from each of the plurality of N feeds to each of the other N-1 feeds.

4. A multibeam antenna feed arrangement according to claim 2 wherein the network means comprises cross-coupling paths from each of the plurality of N feeds to each of the other N-1 feeds.

5. A multibeam antenna feed arrangement according to claim 1 wherein for N feeds, the network means comprises N cross-coupling paths where N is an even integer, or N-1 cross-coupling paths where N is an odd integer, each cross-coupling path being disposed between the output path of a first one of the plurality of feeds and the output path of an adjacent second one of the plurality of N feeds where a sidelobe of the adjacent second one of the feeds exceeds the predetermined sidelobe level in the direction served by the first one of the feeds.

6. A multibeam antenna feed arrangement according to claim 2 wherein for N feeds, the network means comprises N cross-coupling paths where N is an even integer, or N-1 cross-coupling paths where N is an odd integer, each cross-coupling path being disposed between the output path of a first one of the plurality of N feeds and the output path of an adjacent second one of the plurality of N feeds where a sidelobe of the adjacent second one of the feeds exceeds the predetermined sidelobe level in the direction served by the first one of the feeds.

7. A multibeam antenna feed arrangement according to claim 1 wherein the feed arrangement further comprises:
a plurality of N low-noise amplifiers, each amplifier being disposed in a separate path associated with the plurality of N feeds.

8. A multibeam antenna feed arrangement according to claim 2 wherein the feed arrangement further comprises:
a plurality of N low-noise amplifiers, each amplifier being disposed in a separate path associated with the plurality of N feeds.

9. A multibeam antenna feed arrangement comprising:
a plurality of N feeds, each feed being capable of receiving signals from a separate one of a plurality of N remote, spaced-apart, transmitters and transmitting the received signals along a separate one of N associated output paths to an associated separate one of a plurality of N receivers; and network means comprising a plurality of at least N cross-coupling paths where N is an even integer, or N-1 cross-coupling paths where N is an odd integer, each cross-coupling path including means for providing a separate predetermined phase and amplitude adjusted version of a signal propagating in an output path of an associated first one of the plurality of N feeds to an output path of an associated second one of the plurality of N feeds where a sidelobe associated with the second one of the feeds exceeds a predetermined sidelobe level in a beam direction associated with the first one of the feeds which is also received by the associated second one of the feeds via the sidelobe associated with the second one of the feeds.