IL125513A - Multifrequency dual mode rf antenna feed - Google Patents

Abstract

A RF feed, for receiving orthogonally polarized signals of a first frequency and transmitting signals of at least one second frequency higher than the first frequency, comprising: (a) a waveguide (52) having a cross section, relative to a longitudinal axis (54), of at least fourfold rotational symmetry, and including a first axial port (56), a second axial port (60), and at least one pair of opposed lateral ports (64); (b) a filter (66), directly coupled to said first axial port, for passing the signals of the first frequency and rejecting signals of the at least one second frequency; (c) a mechanism (72) for coupling the orthogonally polarized signals of the first frequency to said first axial port via said filter; and (d) for each of said at least one pair of opposed lateral ports, a mechanism (80) for coupling a signal of one of the at least one said second frequency to said each at least one pair of opposed lateral port

Description

u til m sptan RF iiiWN now MULTIFREQUENCY DUAL MODE RF ANTENNA FEED MULTIFREQUENCY DUAL MODE RF ANTENNA FEED FIELD AND BACKGROUND OF THE INVENTION The present invention relates to RF satellite communications and, more particularly, to a RF antenna feed for simultaneous reception and transmission.

Figure 1 shows, schematically, an antenna 10 of a satellite communications ground station. Antenna 10 includes a parabolic dish 12 and an RF feed 14 at the focus of dish 12. Transmissions (downlink) from a satellite to the ground station are intercepted by dish 12 and reflected by dish 12 to RF feed 14. Transmissions (uplink) from the ground station to the satellite are transmitted by RF feed 14 to dish 12 and reflected by dish 12 towards the satellite.

Figure 2 shows, in schematic cross section, a prior art RF feed 14. The heart of RF feed 14 is an ortho-mode transducer (OMT) 16, which is a waveguide that supports propagation of two orthogonal, linearly polarized RF signals as described below. OMT 16 includes a front port 18 that faces dish 12, a rear port 22 and a top port 26. At front port 18, OMT 16 is coupled to a conical RF feed horn 30. At rear port 22, OMT 16 is coupled to a transmit (Tx) waveguide 34. At top port 26, OMT 16 is coupled to a receive (Rx) waveguide 38. OMT 16 is attached mechanically to RF feed horn 30 by means of a flange 20 on OMT 16 and a matching flange 32 on RF feed hom 30. OMT 16 is attached mechanically to Tx waveguide 34 by means of a flange 24 on OMT 16 and a matching flange 36 on Tx waveguide 34. OMT 16 is attached mechanically to Rx waveguide 38 by means of a flange 26 on OMT 16 and a matching flange 40 on Rx waveguide 38.

RF signals to be transmitted to the satellite are sent to RF feed 14 via Tx waveguide 34. The electric field vector of these signals is polarized in the plane of Figure 2. These signals propagate from right to left through OMT 16. The RF signals received by RF feed 14 from the satellite are those with their electric field vector oriented perpendicular to the electric field vector of the transmitted signals, i.e., perpendicular to the plane of Figure 2. Rx waveguide 38 is oriented so that these signals couple into Rx waveguide 38. A conductor such as a wire 42 stretched across OMT 16 near port 22 in the direction perpendicular to the plane of Figure 2 serves to short out the electric field of the received signals to prevent the coupling of the received signals to Tx waveguide 34. In other words, RF feed 14 separates transmitted and received signals based on their polarizations. Additional separation is provided by using different frequencies for transmission and reception, and by coupling appropriate bandpass filters (not shown) to waveguides 34 and 38. A typical example of the frequencies used is 10.7-12.75 GHz for reception and 14-14.5 GHz for transmission.

OMT 16 has a circular cross section 44 on the left, where OMT 16 is coupled to RF feed horn 30, a square cross section 46 in the middle, where OMT 16 is coupled to Rx waveguide 38, and a rectangular cross section 48 on the right, where OMT 16 is coupled to Tx waveguide 34. Rectangular cross section 48 matches the cross section of Tx waveguide 34 and supports the propagation of TE10-mode signals from Tx waveguide 34. Square cross section 46 supports the propagation of TE10-mode signals from Tx waveguide 34 and to Rx waveguide 38. Circular cross section 44 supports the propagation of TEn-mode signals to and from RF feed hom 30.

RF feed 14 is designed for communication with a satellite that transmits and receives linearly polarized RF transmissions that are orthogonally polarized with respect to each other. There also are satellites that transmit in two channels, at the same carrier frequency, that are distinguished by being orthogonally polarized with respect to each other. For example, such transmissions are commonly used in downlinks of television programs. It would be useful for a ground station to be able to transmit to a satellite while receiving such transmissions, using a single antenna with a single parabolic dish and a single RF feed.

More generally, there is thus a widely recognized need for, and it would be highly advantageous to have, an RF feed that can transmit a wide band signal at one polarization plane, either horizontal or vertical, while receiving a wide band signal at another frequency and at any polarization plane that is determined by the satellite downlink. The two signals are tightly coupled in the sense that both signals are aligned to the locally optimized satellite orientation of the polarization plane.

European Patent Application EP 0 812 029 Al, which is incorporated by reference for all purposes as if fully set forth herein, teaches an RF feed which, in principle, can be used for this application. The RF feed of EP 0 812 029 Al is similar to RF feed 14, but a waveguide of circular cross section intervenes between an OMT similar to OMT 16 and an RF feed horn similar to RF feed horn 30. The circular waveguide includes rows of apertures along which are placed multiple coupling waveguides. The OMT is used to couple with two orthogonal RF signals at a first frequency. The coupling waveguides are used to couple with two orthogonal RF signals at a second frequency. The coupling waveguides include layers of dielectric material. The internal and external dimensions of the coupling waveguides, and the aperture spacings, are chosen to ensure that only the signals of the second frequency, and not the signals of the first frequency, couple to the coupling waveguides. The RF feed of EP 0 812 029 Al is intended for simultaneous transmission or reception by a satellite at two different frequencies, in both polarizations, using the same parabolic dish antenna. In principle, the RF feed of EP 0 812 029 Al could transmit at one frequency while receiving at the other frequency. Nevertheless, it would be highly advantageous to have a simpler and less expensive RF feed for this application, for use in a ground station.

SUMMARY OF THE INVENTION According to the present invention there is provided a RF feed, including: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a filter, directly coupled to the first axial port, for passing signals of a first frequency and rejecting signals of at least one second frequency higher than the first frequency; (c) a mechanism for coupling orthogonally polarized signals of the first frequency to the first axial port via the filter; and (d) for each of the at least one pair of opposed lateral ports, a mechanism for coupling a signal of one of the at least one the second frequency to the each at least one pair of opposed lateral ports.

According to the present invention there is provided a RF feed, including: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a mechanism for coupling orthogonally polarized signals of a first frequency to the first axial port; and (c) for each of the at least one pair of opposed lateral ports, a magic-tee power divider for coupling a signal of a second frequency directly to the each at least one pair of opposed lateral ports.

According to the present invention there is provided a RF feed, including: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a mechanism for coupling orthogonally polarized signals of a first frequency to the first axial port; (c) for each of the at least one pair of opposed lateral ports, a mechanism for coupling a signal of a second frequency directly to the each at least one pair of opposed lateral ports; and (d) a mechanism for rotating the waveguide about the longitudinal axis.

According to the present invention there is provided a RF feed, including: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and a single pair of opposed lateral ports; (b) a mechanism for coupling orthogonally polarized signals of a first frequency to the first axial port; and (c) a mechanism for coupling a signal of a second frequency higher than the first frequency to the pair of opposed lateral ports.

The RF feed of the present invention is based on a waveguide of circular cross section. More generally, the cross section of the central waveguide of the present invention has at least fourfold rotational symmetry. So, for example, the cross section could be square or octagonal. The waveguide has two axial ports and at least one pair of opposed lateral ports. As in the prior art RF feeds, one axial port is coupled to a conical RF feed horn. The other axial port is coupled directly to a filter which, in turn, is coupled to a low noise block (LNB) for selecting either of two orthogonally polarized RF signals of a first frequency to be received via the RF feed of the present invention. Transmissions at a second frequency are coupled into the waveguide via each pair of opposed lateral ports, using a magic-tee power divider coupled to each pair of opposed lateral ports. The filter passes signals of the first frequency and reflects signals of the second f equency, and so isolates the LNB from the transmitted signals. Typically, the filter is a bandpass filter. In the usual case of the first frequency being lower than the second frequency, the filter preferably is a low pass filter.

The simplest embodiment of the present invention has only one pair of opposed lateral ports. The transmitted signal is polarized parallel to one of the reception channels and perpendicular to the other reception channel. The target satellite may be configured to receive uplink transmissions polarized in one of two orthogonal directions: either parallel or perpendicular to the polarization direction of the satellite's downlink transmissions. If the target satellite is configured to receive uplink transmissions polarized parallel to the downlink transmissions, then the LNB of the present invention is set to receive signals polarized parallel to the signals transmitted by the present invention. If the target satellite is configured to receive uplink transmissions polarized perpendicular to the downlink transmissions, then the LNB of the present invention is set to receive signals polarized perpendicular to the signals transmitted by the present invention. In either case, the present invention is oriented for transmission and reception by mechanically rotating the waveguide around the longitudinal axis of symmetry thereof until received power is maximized.

A more elaborate embodiment of the present invention has two pairs of opposed lateral ports, oriented 90° apart, each with its own magic-tee power divider. The downlink polarization direction is determined electronically, by finding the amplitude mixing ratio of the two reception channels that maximizes received power.

The corresponding uplink transmission polarization is obtained by applying the transmitted signal to both pairs of opposed lateral ports, with the proper mutual amplitude ratio. More generally, the present invention is used to communicate with a satellite that uses any orthogonal polarization scheme, for example opposite circular polarizations, to distinguish between downlink channels, by adjusting the relative amplitudes and phases of the two reception channels to maximize received power and by adjusting the relative amplitudes and phases of the signals supplied to the magic-tee power dividers accordingly.

Within the scope of the present invention, the bandpass filter can be coupled to any mechanism for receiving or transmitting signals of the first frequency in either or both of two orthogonally polarized channels. Similarly, two pairs of opposed lateral ports can be coupled to any mechanism for receiving or transmitting signals of the second f equency in either or both of two orthogonally polarized channels, and a single pair of opposed lateral ports can be coupled to any mechanism for receiving or transmitting signals of the second frequency in one linearly polarized channel.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a schematic depiction of a prior art ground station antenna; FIG. 2 shows a prior art RF feed; FIG. 3 is a perspective view of a RF feed of the present invention; FIGs. 4A and 4B are cross sections of two variants of the waveguide at the opposed lateral ports; FIG. 5 is a schematic depiction of a ground station antenna including the RF feed of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a simple RF feed for dual-frequency communication from a ground station to a satellite.

The principles and operation of an RF feed according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring again to the drawings, Figure 3 is a perspective view of a RF feed 50 of the present invention, for use in a parabolic dish antenna of a satellite ground station. A waveguide 52 of circular cross section has a first axial port 56, a second axial port 60, and two opposed lateral ports 64, one of which is shown in Figure 3. Geometrically, waveguide 52 is a right circular cylinder with a longitudinal rotational axis of symmetry 54. As a right circular cylinder, waveguide 52 has a cross sectional area, transverse to axis 54, that is uniform along axis 54. Coupled to waveguide 52 at first axial port 56, and coaxial with axis 54, is a bandpass filter 66. Coupled to bandpass filter 66 at the other end thereof is a LNB 72. Coupled to waveguide 52 at second axial port 60, and coaxial with axis 54, is a conical RF feed horn 76. Waveguide 52 is attached mechanically to bandpass filter 66 by means of a flange 58 on waveguide 52 and a matching flange 68 on bandpass filter 66. Waveguide 52 is attached mechanically to RF feed horn 76 by means of a flange 62 on waveguide 52 and a matching flange 78 on RF feed horn 76. LNB 72 is attached mechanically to bandpass filter 66 by means of a flange 74 on LNB 72 and a matching flange 70 on bandpass filter 66. A magic-tee power divider 80 is coupled to both lateral ports 64. Both LNB 72 and magic-tee power divider 80 are coupled electronically to the circuitry of the satellite ground station.

LNB 72 is set to select received RF signals, at a first frequency, polarized either vertically or horizontally, as seen in Figure 3, or circularly. Magic-tee power divider 80 supplies transmitted signals to waveguide 52 at a second frequency higher than the first frequency. Waveguide 52 is dimensioned to support only the TEn mode of the received signals. Because waveguide 52 supports several modes of the transmitted signals, magic-tee power divider 80 is used to balance the amplitude and phase of the transmitted signals coupled to waveguide 52 via ports 64 so that only the TEn mode of the transmitted signals is excited in waveguide 52. The transmitted signals are polarized vertically as seen in Figure 3. Preferably, the second frequency is at least about three times the first frequency. In a typical application of the present invention, the first frequency is between 10.7 GHz and 12.75 GHz (Ku band) and the second frequency is between 29 GHz and 30 GHz (Ka band).

Bandpass filter 66 passes signals of the first frequency but blocks signals of the second frequency, to isolate LNB 72 from the transmitted signals. Preferably, bandpass filter 66 is of circular cross section, as shown, with internal baffles positioned so as to pass signals of the first frequency and block signals of the second frequency. Methods of designing appropriate circular bandpass filters are well-known in the art. See, for example, Luciano Accatino, Giorgio Bertin and Mauro Mongiardo, "A four-pole dual mode elliptic filter realized in circular cavity without screws", ΓΕΕΕ Transactions on Microwave Theory and Techniques vol. 44 no. 12 (December 1996) pp. 2680-2686 and the references therein. Alternatively, bandpass filter 66 is of square cross section and an appropriate taper is provided as a transition between waveguide 52 and bandpass filter 66.

Figure 4A is a schematic cross section through RF feed 50 transverse to axis 54 at lateral ports 64, showing the circular cross section of waveguide 52 concentric with axis 54. Opposed lateral ports 64 are on opposite sides of axis 54. Figure 4B is a similar schematic cross section through a variant of RF feed 50 whose waveguide 52' has a square cross section, as well as two pairs of opposed lateral ports 64' and 64". In this variant, axis 54 is an axis of fourfold rotational symmetry. Lateral ports 64' are on opposite sides of axis 54. Lateral ports 64" are on opposite sides of axis 54, displaced 90° relative to lateral ports 64'. A first magic-tee power divider 80' is coupled to lateral ports 64' A second magic-tee power divider 80" is coupled to lateral ports 64". Any desired polarization mode (linear, circular elliptical) and polarization direction of the transmitted signal is obtained by supplying the same transmitted signal to lateral port pairs 64' and 64" with the correct relative amplitudes and phases. In general, the waveguide component of the present invention exhibits at least fourfold rotational symmetry about longitudinal axis 54, in order to support the TEn modes of the received and transmitted signals. Waveguide 52' requires a transition section to a circular cross section at second axial port 60.

It will be appreciated that waveguides 52 and 52' can be provided with several pairs of lateral ports in tandem, with each pair of lateral ports coupled to a magic-tee power divider, for transmitting at several frequencies, analogous to the similar use in EP 0 812 029 Al of coupler assemblies in tandem, as illustrated in Figure 6 thereof.

Figure 5 shows, schematically, RF feed 50 incorporated in a parabolic dish antenna 82. RF feed 50 is mounted at the focus of a parabolic dish 84 of antenna 82.

Note that axis 54 is also the rotational symmetry axis of dish 84. Dish 84 is mounted on a motor 86 which rotates dish 84 about axis 54. Motor 84 is operated by a control system 88. Control system 88 also selects the desired reception orientation and frequency band of LNB 72, and also receives received signals from, and supplies transmitted signals to, transmission/reception electronics 90. Transmission/reception electronics 90 supplies the transmitted signal to magic-tee power divider 80 and receives the received signal from LNB 72. To orient antenna 82 properly about axis 54, so that the transmitted signals are polarized in the correct orientation, control system 88 measures and monitors the power of the received signals and activates motor 86 to rotate antenna 82 about axis 54 until that power is maximized.

The configuration illustrated in Figure 5 is illustrative, and is not intended to limit the present invention. For example, the parabolic dish antenna may be an offset antenna, and RF feed 50 need not be coaxial with the antenna, but may be mounted at an offset angle. It is necessary to rotate only RF feed 50, and not the entire antenna assembly, about axis 54. This rotation may be effected either mechanically, as illustrated, or manually.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims (55)

12 125513/2 WHAT IS CLAIMED IS:

1. A RF feed, for receiving orthogonally polarized signals of a first frequency and transmitting signals of at least one second frequency higher than the first frequency, comprising: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a filter, directly coupled to said first axial port, for passing the signals of the first frequency and rejecting signals of the at least one second frequency; (c) a mechanism for coupling the orthogonally polarized signals of the first frequency to said first axial port via said filter; and (d) for each of said at least one pair of opposed lateral ports, a mechanism for coupling a signal of one of the at least one said second frequency to said each at least one pair of opposed lateral ports.

2. The RF feed of claim 1, further comprising: (e) a feed horn coupled to said second axial port,

3. The RF feed of claim 1, wherein said cross section is circular.

4. The RF feed of claim 1 , wherein said cross section is square. 13 125513/2

5. The RF feed of claim 1, including a single said waveguide having an axially uniform cross sectional area.

6. The RF feed of claim 1 , wherein said filter is a bandpass filter.

7. The RF feed of claim 1, wherein said filter is a low pass filter.

8. The RF feed of claim 1, wherein said mechanism for coupling said orthogonally polarized signals of said first frequency to said first axial port includes a low noise block.

9. The RF feed of claim 1, wherein said mechanism for coupling said signal of said one of said at least one second frequency to said each at least one pair of opposed lateral ports includes a magic-tee power divider.

10. The RF feed of claim 8, wherein said magic-tee power divider is coupled directly to said each at least one pair of opposed lateral ports.

11. The RF feed of claim 8, wherein said waveguide includes at least two of said pairs of opposed lateral ports in tandem, and wherein said second frequency of said signal that is coupled to a first of said at least two pairs of opposed lateral ports is different from said second frequency of said signal that is coupled into a second of said at least two pairs of opposed lateral ports. 14 125513/2

12. The RF feed of claim 1 , further comprising: (e) a mechanism for rotating said waveguide about said longitudinal axis.

13. The RF feed of claim 1, including only one pair of said opposed lateral ports.

14. A RF feed, for receiving orthogonally polarized signals of a first frequency and transmitting a signal of a second frequency, comprising: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a mechanism for coupling the orthogonally polarized signals of the first frequency to said first axial port; and (c) for each of said at least one pair of opposed lateral ports, a magic-tee power divider for coupling the signal of the second frequency directly to said each at least one pair of opposed lateral ports.

15. The RF feed of claim 14, further comprising: (d) a feed horn coupled to said second axial port.

16. The RF feed of claim 14, wherein said cross section is circular.

17. The RF feed of claim 14, wherein said cross section is square. 15 125513/2

18. The RF feed of claim 14, including a single said waveguide having an axially uniform cross sectional area.

19. The RF feed of claim 14, wherein said mechanism for coupling said orthogonally polarized signals of said first frequency to said first axial port includes a low noise block.

20. The RF feed of claim 14, wherein each of said at least one second frequency is higher than said first frequency.

21. The RF feed of claim 14, wherein said waveguide includes at least two of said pairs of opposed lateral ports in tandem, and wherein said second frequency of said signal that is coupled to a first of said at least two pairs of opposed lateral ports is different from said second frequency of said signal that is coupled into a second of said at least two pairs of opposed lateral ports.

22. The RF feed of claim 14, further comprising: (d) a filter, coupled to said first axial port, for passing signals of said first frequency and rejecting signals of said at least one second frequency.

23. The RF feed of claim 22, wherein said filter is coupled directly to said first axial port.

24. The RF feed of claim 22, wherein said filter is a bandpass filter.

25. The RF feed of claim 22, wherein said filter is a low pass filter. 16 125513/2

26. The RF feed of claim 14, further comprising: (d) a mechanism for rotating said waveguide about said longitudinal axis.

27. The RF feed of claim 14, including only one pair of said opposed lateral ports.

28. A RF feed, for receiving orthogonally polarized signals of a first frequency and transmitting a signal of a second frequency, comprising: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and at least one pair of opposed lateral ports; (b) a mechanism for coupling the orthogonally polarized signals of the first frequency to said first axial port; (c) for each of said at least one pair of opposed lateral ports, a mechanism for coupling the signal of the second frequency directly to said each at least one pair of opposed lateral ports; and (d) a mechanism for rotating said waveguide about said longitudinal axis.

29. The RF feed of claim 28, further comprising: (e) a feed horn coupled to said second axial port.

30. The RF feed of claim 28, wherein said cross section is circular.

31. The RF feed of claim 28, wherein said cross section is square. 17 125513/2

32. The RF feed of claim 28, including a single said waveguide having an axially uniform cross sectional area.

33. The RF feed of claim 28, wherein said mechanism for coupling said orthogonally polarized signals of said first frequency to said first axial port includes a low noise block.

34. The RF feed of claim 28, wherein each of said at least one second frequency is higher than said first frequency.

35. The RF feed of claim 28, wherein said waveguide includes at least two of said pairs of opposed lateral ports in tandem, and wherein said second frequency of said signal that is coupled to a first of said at least two pairs of opposed lateral ports is different from said second frequency of said signal that is coupled into a second of said at least two pairs of opposed lateral ports.

36. The RF feed of claim 28, further comprising: (e) a filter, coupled to said first axial port, for passing signals of said first frequency and rejecting signals of said at least one second frequency.

37. The RF feed of claim 36, wherein said filter is coupled directly to said first axial port.

38. The RF feed of claim 36, wherein said filter is a bandpass filter.

39. The RF feed of claim 36, wherein said filter is a low pass filter. 18 125513/2

40. The RF feed of claim 28, wherein said mechanism for coupling said signal of said one of said at least one second frequency to said each of said each at least one pair of opposed lateral ports includes a magic-tee power divider.

41. The RF feed of claim 40, wherein said magic-tee power divider is coupled directly to said each at least one pair of opposed lateral ports.

42. The RF feed of claim 28, including only one pair of said opposed lateral ports.

43. A RF feed, for receiving orthogonally polarized signals of a first frequency and transmitting a signal of a second frequency higher than the first frequency, comprising: (a) a waveguide having a cross section, relative to a longitudinal axis, of at least fourfold rotational symmetry, and including a first axial port, a second axial port, and a single pair of opposed lateral ports; (b) a mechanism for coupling orthogonally polarized signals of the first frequency to said first axial port; and (c) a mechanism for coupling the signal of the second frequency to said pair of opposed lateral ports.

44. The RF feed of claim 43, further comprising: (d) a feed horn coupled to said second axial port.

45. The RF feed of claim 43, wherein said cross section is circular. 19 125513/2

46. The RF feed of claim 43, wherein said cross section is square.

47. The RF feed of claim 43, including a single said waveguide having an axially uniform cross sectional area.

48. The RF feed of claim 43, wherein said mechanism for coupling said orthogonally polarized signals of said first frequency to said first axial port includes a low noise block.

49. The RF feed of claim 43, further comprising: (d) a filter, coupled to said first axial port, for passing signals of said first frequency and rejecting signals of said second frequency.

50. The RF feed of claim 49, wherein said filter is coupled directly to said first axial port.

51. The RF feed of claim 49, wherein said filter is a bandpass filter.

52. The RF feed of claim 49, wherein said filter is a low pass filter.

53. The RF feed of claim 43, wherein said mechanism for coupling said signals of said second frequency to said pair of opposed lateral ports includes a magic-tee power divider. 20 125513/2

54. The RF feed of claim 53, wherein said magic-tee power divider is coupled directly to said pair of opposed lateral ports.

55. The RF feed of claim 43, further comprising: (e) a mechanism for rotating said waveguide about said longitudinal axis. 67897 Tel Aviv