US3838362A

US3838362A - Diplexing coupler for microwave system

Info

Publication number: US3838362A
Application number: US00374981A
Authority: US
Inventors: L Kurtz
Original assignee: Emerson Electric Co
Current assignee: RANTEC MICROWAVE & ELECTRONICS Inc
Priority date: 1973-06-29
Filing date: 1973-06-29
Publication date: 1974-09-24
Anticipated expiration: 1991-09-24
Also published as: GB1470545A; CA986595A; FR2235500B1; JPS5034441A; FR2235500A1; DE2431289A1; IT1016203B

Abstract

A coupler is described for coupling two microwave transmitters or receivers in one frequency band and two microwave receivers or transmitters in a second frequency band to a common central wave guide. The device provides high isolation between the four ports. Energy in a first band of frequencies is coupled to feed wave guides which are dimensioned to transmit this first band of frequencies. These feed wave guides are arranged to handle a pair of separate channels for the first band of frequencies, the signals in one of the channels having the same frequencies but a polarization which is orthogonal to that of the signals of the other channel. A pair of channels in the second band of frequencies, the signals in one channel having the same frequencies but a polarization orthogonal to that in the other channel, may be fed directly to the central wave guide. A branch wave guide is interposed between each of the feed wave guides and the central wave guide for coupling the energy in the first band of frequencies to the central waveguide, the branch wave guides being dimensioned to operate as a cutoff filter at the second band of frequencies (thus preventing such signals from entering the feed wave guides) and a high pass filter at the first band of frequencies.

Description

nited States Patent 1191 Kurtz 1 1 Sept. 24, 1974 DIPLEXING COUPLER FOR MICROWAVE SYSTEM [75] Inventor: Louis A. Kurtz, Woodland Hills,

Calif.

[73] Assignee: Emerson Electric Co., St. Louis, Mo.

22 Filed: June 29, 1973 [21] Appl. No.: 374,981

[52] US. Cl 333/9, 333/11, 333/21 A, 333/98 TN [51] Int. Cl 1101p 1/06, H01pl/16,H01p 5/12 [58] Field of Search 333/1, 6, 9, 10,11, 21 A,

333/73 W, 98 TN; 343/756, 175, 176; 325/23-25 References Cited [5 7] ABSTRACT A coupler is described for coupling two microwave transmitters or receivers in one frequency band and two microwave receivers or transmitters in a second frequency band to a common central wave guide. The device provides high isolation between the four ports. Energy in a first band of frequencies is coupled to feed wave guides which are dimensioned to transmit this first band of frequencies. These feed wave guides are arranged to handle a pair of separate channels for the first band of frequencies, the signals in one of the channels having the same frequencies but a polariza tion which is orthogonal to that of the signals of the other channel. A pair of channels in the second band of frequencies, the signals in one channel having the same frequencies but a polarization orthogonal to that in the other channel, may be fed directly to the central UNITED STATES PATENTS wave guide. A branch wave guide is interposed be- 3 150 333 9 1964 Bowman 333/9 twee each of the feed wave guides and the Central 3,265,993 8/1966 Davidson et al.. 333 1 Wave guide for Coupling the energy in the first band of 3,668,567 5/1972 Rosen 343/756 x frequencies to the central waveguide, the branch av 3,731,235 5/1973 Ditullio et a1 333/6 guides being dimensioned to operate as a cutoff filter 3,731,236 5/1973 DiTullio 333/9 at the second band of frequencies (thus preventing such signals from entering the feed wave guides) and a Primary Examiner-James W. Lawrence high pass filter at the first band of frequencies. Assistant ExaminerMarvin Nussbaum Attorney, Agent, or Firm-Edward A. Sokolski 1 C ims, 6 raw lg Flgllles 2o 23 l 26 50 I8 I 40 1 '1 I RJ Ru RJ RJ OMJ MJ 1 I MJ 1256/ TRANS. REC/TRANS. R121; ITRANS. REG/TRANS. *1 I *2 a *4 (1 FREQ. (1 FREQ. 12 FREQ. (2' FREQ.

BAND) BAND) BAND 1 BAND) mdE DOV

DIPLEXING CQUPLER FOR MICROWAVE SYSTEM This invention relates to a coupler for coupling microwave energy to a transmission or reception wave guide, and more particularly to such a device for coupling a plurality of microwave channels in two bands of frequencies to a common wave guide.

In microwave links such as utilized in communications applications, it is desirable to optimize the utilization of each link by providing a maximum number of communications channels thereon. With increased de mands on the use of the available frequency spectrum, such optimum employment of communications links has taken on considerable importance. It is further to be noted that maximum utilization of each communications link is also desirable from the point of view of enabling the optimum employment of the communications equipment which of course makes for greater economy and lessens the equipment requirement for handling any given amount of information.

One technique which has been developed to further the above indicated objectives is known as Frequency Re-use, and involves the transmission and/or reception of sets of microwave channels at the same frequencies but with mutually orthogonal polarizations. The orthogonal components can thus be transmitted (sendlink) together and then separated at the receiving end by filtering out the separately polarized signals. The same technique can be applied on reception (receivelink) to double the number of information channels in the assigned region of the frequency spectrum.

Wave guide structure for achieving the above mentioned combination of channels includes the use of separate orthomode junctions, these junctions being appropriately rotated to receive or transmit energy in the proper polarization. Such orthomode junctions are used in conjunction with rotary or choked motional joints to obtain the orthogonal components of the signals received by each orthomode junction. This prior art design approach has a number of disadvantages. First, it involves a considerable number of components which not only increases the size and expense of the installation, but also contributes to signal losses. Further, each orthomode junction design is limited to use for a particular frequency band. Thus, a separate design effort is required for each operating frequency. Further, in such devices of the prior art, the equalization of the phase versus frequency behavior of the signals through the differing kinds of wave guide bends in the two paths between the orthomode junctions presents a difficult task.

The device of the present invention overcomes the aforementioned shortcomings of the prior art, providing a simpler configuration involving less components and higher efficiency. Further, the device of the present invention presents significantly less problems in equalizing the phase vs. frequency behavior of the signals. This improvement is achieved in the device of this invention by utilizing a diplexing O-db coupler orthomode junction which efficiently couples the energy in one range of the frequency spectrum into a central waveguide without disturbing energy passing through that central waveguide in a second range of the frequency spectrum.

It is therefore the principal object of this invention to provide an improved microwave coupler device for coupling a plurality of microwave channels to a common waveguide.

Other objects of this invention will become apparent as the description proceeds in connection with the accompanying drawings, of which:

FIG. 1 is a schematic drawing indicating a first embodiment of the invention;

FIG. 2 is a schematic drawing of a second embodiment of the invention;

FIG. 3 is a side elevational view of an implementation of the diplexing O-db coupler orthomode junction of the invention;

FIG. 4 is a cross sectional view taken along the plane indicated by 44 in FIG. 3;

FIG. 5 is a cross sectional view taken along the plane indicated by 55 in FIG. 3; and

FIG. 6 is a top plan view with partial section cutaway of the feed waveguide, branch waveguide and central waveguide portions of the device of FIG. 3.

Briefly described, the device of the invention is as follows:

A centrally positioned waveguide is used to form a common channel for handling all of the signals. A pair of microwave transmitters or receivers in-a first band of frequencies are coupled to feed waveguide means, dimensioned to transmit this first band of frequencies. In one embodiment of the invention, the feed waveguide means is formed by two pairs of waveguide sections, each such pair of waveguide sections being positioned parallel to a pair of respective opposite walls of a central waveguide. The energy coupled to one of said pair of feed waveguide sections passes to the central waveguide with a polarization which is orthogonal to that of the energy coupled to the other pair of feed waveguide sections. A branch waveguide is interposed between each feed waveguide and the central waveguide for coupling microwave energy therebetween. The branch waveguides are dimensioned so that they operate as high pass filters at the first band of frequencies, and as cutoff filters at a second band of frequencies. Signals from a second pair of channels in the second band of frequencies are coupled directly to the central waveguide with the polarizations of the signals intended for one receiver (or coming from one transmitter) being made orthogonal to that of the signals intended for the other receiver (or coming from the other transmitter) by means of a second orthomode junction. In this manner, four separate sets of receive and/or transmit channels, pairs of such channel sets being in different frequency bands, are coupled to a single waveguide leading to a single antenna horn to enable reception and/or transmission thereon. I

Referring now to FIG. 1, one embodiment of the invention is schematically illustrated. A first receiver/- transmitter 11 providing a first set of microwave channels is coupled through waveguide rotary joint 12 to feed waveguides l4 and 15 which are symmetrically arranged along opposite walls of central waveguide 20. The terminology receiver/transmitter is used herein to indicate a receiver or transmitter in the alternative. The energy fed to feed

waveguides

14 and 15 has a fixed polarization in relation to the position of

waveguides

14 and 15, but that polarization may be rotated by rotating the entire device consisting of

waveguides

14, 15, 20 and 25. This rotation is accomplished with-- out also rotating CM] 40, the horn 18, or the receiver/- transmitter 11 in view of the

motional joints

45 and 50,

and the rotary joint 12. Similarly, a second set of microwave channels is provided from receiver/transmitter 21 through rotary joint 23 to feed waveguide 25 and a paired feed waveguide (not shown) on the opposite side of central waveguide 20. The polarization of the energy from the second receiver/transmitter 21 is always orthogonal to the energy from the first receiver/- transmitter 11 regardless of the rotational position of that section of central waveguide labeled 20. As is well known in the art, it is important that the polarizations be orthogonal to avoid crosscoupling between two channels. However. there is no requirement that the polarizations be horizontal and vertical respectively, as long as they bear an orthogonal relationship to each other.

The energy in feed waveguide 14 is coupled through branch waveguide 26 to central waveguide 20, this end result being achieved by means of slots formed in the adjacent walls of the feed and central waveguides, as to be explained further on in the specification. These slots are arranged to maintain the desired signal polarization (i.e., orthogonal to that of its paired signal). Similarly, the energy in feed waveguide 25 and its paired opposite is coupled to central waveguide through branch waveguides (not shown) for feed waveguide and its paired opposite. The branch waveguides, as to be more fully described further on in the specification, each comprise a plurality of waveguide sections running between associated feed waveguides and the central waveguide, these branch waveguide sections being dimensioned so that they operate as high pass filters at the operating frequencies of receivers/transmitters 11 and 21, and as cutoff filters at the operating frequencies ofreceivers/transmitters 33 and 34.1n this manner, the orthogonally polarized signals are efficiently coupled to and from (as the case may be) the feed waveguide and the central waveguide. with no coupling to the feed waveguide of energy in the frequency band of receivers/

transmitters

33 and 34. The use of branch waveguides as filters is well known in the art and is described, for example, in an article entitled Synchronous Branch Guide Directional Couplers for Low and High Power Applications" by Leo Young, which appeared in the [RE Transactions on Microwave Theory and Techniques (page 459) for November, 1962.

The microwave channels for receivers/

transmitters

33 and 34 are coupled to orthomode joint 40 by means of

rotary joints

41 and 42 respectively. Rotary joints 4] and 42 and motional joint 45 are used to adjust the polarizations for optimum operation of the link as is well known in the art, and are arranged to couple their respective channels to the correct receiver or transmitter, despite the fact that the same frequencies are being used by both receivers/transmitters. The two channels having orthogonally related polarizations are coupled from orthogonal mode joint 40 to central waveguide 20 through motional joint 45.

It is to be noted that either circular or rectangular wave guides may be utilized in implementing the device of the invention. It is further to be noted that the frequency band for

receivers

33 and 34 is lower than that for transmitters 11 and 21, to enable the design of the branch wave guide for the desired filtering action. Further, it is desirable to utilize balanced feed from each of receivers/transmitters 11 and 2| to the central waveguide, this being implemented by means of pairs of balanced feed waveguides, as to be explained further on in the specification. With this type of an arrangement in conjunction with feed waveguide elements dimensioned to operate in the TE modes, only the TE and TE modes are created in the central waveguide section for the signals at the frequencies of receivers/transmitters 11 and 21. It is to be noted along these lines that the central waveguide is dimensioned to efficiently handle signals at the lower frequencies of receivers/-

transmitters

33 and 34 and thus could readily propagate unwanted higher order modes at the frequencies of the signals of receivers/transmitters 11 and 21. The two pairs of signals, each such pair being in a separate frequency band, and the signals of each pair having orthogonally related polarizations, are coupled through a motional joint 50 to antenna 18, or coupled from an- .tenna 18 through the motional joint to the central waveguide, as the case may be. Motional joints and are provided to enable the rotational adjustment of central waveguide 20 for proper signal polarizations for optimum operation of the link. As has been indicated, the device of the invention can be utilized for handling either four transmitting channels, four receiving channels, or any combination of four receiving and transmitting channels.

Referring now to FIG. 2, another embodiment of the invention is schematically illustrated. This embodiment differs from the first in that in lieu of the rotary joints, 180 plates and 61 operating in conjunction with motional joints and orthogonal mode joints are utilized to achieve rotation of the orthogonally related polarizations for each of the pairs of signal polarizations. Such independent rotations are necessary to optimum operation of the sending and receiving communications links. The 180 plates, which are also known as halfwave plates, are well known in the art and are described, for example, on pages 1495-1497 of the Proceedings of the Institute of Radio Engineers for December, 1947. Receivers/transmitters l1 and 21 are coupled by means of orthogonal mode joint 65 and motional joint 67 to plate 61. The orthogonal mode joint polarizes the signals so that they are in orthogonal polarization relative to each other. 180 plate 61 is then properly rotated between

motional joints

45 and 67 to adjust the inclination of the pair of orthogonal polarizations for optimum operation of the link. 180 plate 61 is coupled to central waveguide 20 through motional joint 45.

Transmitters

33 and 34 are similarly coupled to 180 plate 60 by means of orthogonal mode joint 71 and motional joint 72. The orthogonally polarized signals are coupled between 180 plate 60 and each of the feed waveguide pairs 14, 15 and 25 and its paired feed waveguide by means of motional joint 73 and orthogonal mode joint 74. 180 plate 60 is rotatably adjusted between

motional joints

72 and 73 in the same manner as described above for plate 61 to achieve control of the inclination of the pair of orthogonal polarizations for the two receivers/transmitters operating at the same frequencies.

The same functional results are achieved in this second embodiment as in the first, but now the diplexing O-db coupler orthomode junction which includes central waveguide 20 is never rotated but rather the 180 plates are rotated to control the transmit or receive polarization angles.

Referring now to FIGS, 3-6, an implementation of the diplexing 0-db coupler orthomode junction of the invention is illustrated. The rotary and motional joints shown in the previous Figures are not shown, in view of the fact that their use and implementation are well known in the art. Signals from receiver/transmitter No. 1 (not shown) in a first band of frequencies are fed from waveguide 90 to balanced waveguide assembly 92. Balanced wavequide assembly 92 comprises

similar legs

92a and 92b.

Waveguide sections

92a and 92b are coupled to feed waveguides l4 and 15 by means of waveguide sections 120.

Waveguide legs

92a and 92b and the waveguide sections 120 connected to each are made identical in their electrical characteristics so as to provide balanced feed of energy to feed

waveguides

14 and 15 respectively.

Similarly, receiver/transmitter No. 2 (not shown) also in the first band of frequencies is coupled by means of waveguide section 95, balanced waveguide assembly 97, and waveguide sections 120 to feed

waveguides

25 and 27 respectively. The ends 126 of the feed waveguides are terminated with a short circuiting metal end cap (not shown).

Feed waveguides

14 and 15 are oriented relative to feed

waveguides

25 and 27 so as to provide signals in

feed waveguides

14 and 15 which have a polarization which is orthogonal to the polarization of the signals in

feed waveguides

25 and 27. Further,

waveguide assemblies

90, 92, 95, 97, and feed

waveguides

14, 15, 25 and 27 are all dimensioned so as to efficiently propagate only the TE rectangular waveguide mode in the first frequency band. Each pair of

balanced waveguide sections

92a, 92b and 97a, 97b are made to have identical electrical characteristics so as to feed signals to their associated paired feed waveguides in balanced relationship. Such balanced feeding assures mode purity of the TB and TE mode in the central waveguide 20. Imperfections in that balance would cause unwanted TE and TM modes to occur in waveguide 20.

lnterposed between each of

feed waveguides

14, 15, 25 and 27 and central waveguide 20 is a

branch waveguide assembly

26, 28, 30 and 31, respectively. Each of the branch waveguide assemblies is formed by a plurality of separate waveguide elements 110 as best seen in FIG. 6, which run between the feed waveguides and the central waveguide 20. Energy is coupled from the feed waveguides to the branch waveguides through slots 1 12 formed in the walls of the feed waveguides. Slots 112 for each of the feed waveguides run parallel to each other and are spaced along the inner walls of the feed waveguides, the slots for feed waveguides l4 and running normal to those for

feed waveguides

25 and 27. Slots 114 are formed along the extent of all four walls of central waveguide to enable the coupling of microwave energy from each branch waveguide assembly to the central waveguide. Slots 114 run parallel to each other for each wall, with the slots for one pair of opposite walls running normal to those for the other pair of opposite walls.

Branch waveguide elements 110 are dimensioned so as to act as high pass filters in the first band of frequencies, i.e., that of the signals fed to the feed waveguides l4, 15, and 27, and as cutoff filters in the second band of frequencies, i.e., that of the signals fed to central waveguide 20 through orthomode junction 40. Thus, signals at the first band of frequencies are efficiently coupled by means of branch waveguide elements 110 between the feed waveguides and the central waveguide, with signals at the second band of frequencies in the central waveguide being blocked by the lower in frequency than that of the first band, are coupled directly to the central waveguide by means of orthomode junction 40. Orthomode junction 40 has first and second leg portions 40a and 40b. One end portion 400 of the orthomode junction has a dimension matching that of the central waveguide and is coupled thereto. The end of leg portion 40a is coupled to a waveguide (not shown) coupled to one of the receivers/transmitters in the second band of frequencies, while leg portion 40b is coupled to a waveguide section (not shown) coupled to the other receivers/ transmitters in this second band of frequencies. The end portions of legs 40a and 40b respectively are dimensioned and oriented so as to bring the signals from each of the receivers/ transmitters in the second band of frequencies together with orthogonally related polarizations. The design and use of orthomode junctions for achieving this end result is well known in the art and need not be further described herein.

The signals to or from the pair of receivers/transmitters in the second band of frequencies are thus coupled directly from or to the central waveguide 20 in mutually orthogonal relationship. As shown in FIG. 3, waveguide coupling sections are used for convenience of design for interconnecting the feed waveguides with the balanced waveguide assembly.

It is to be noted that while the device of the invention has been described in connection with rectangular and square waveguide members, that the invention can be implemented to equal advantage with circular waveguide members.

While the invention has been described and illustrated in detail it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.

I claim:

1. In a diplexing coupler for coupling at least one mi crowave receiver/transmitter channel in a first band of frequencies and at least one microwave receiver/transmitter channel in a second band of frequencies to a common channel comprising:

a central waveguide forming said common channel,

feed waveguide means dimensioned to transmit said first band of frequencies,

means for coupling the channel at said first band of frequencies to said feed waveguide means, means for coupling the channel at said second band of frequencies to said central waveguide, and

branch waveguide means interposed between said feed waveguide means and said central waveguide for coupling said channel at said first band of frequencies therebetween, said branch waveguide means being dimensioned to operate as a cutoff filter at said second band of frequencies and a high pass filter at said first band of. frequencies.

2. The coupler of claim 1 wherein there are a first pair of channels at said first band of frequencies and a second pair of channels at said second band of frequencies and further including means for polarizing the outputs of said first pair of channels in mutually orthogonal relationship to each other and means for polarizing the outputs of said second pair of channels in mutually orthogonal relationship to each other. means is positioned alongside said central waveguide, said branch waveguide means having coupling slots formed at its interface with the central waveguide.

3. The coupler of claim 1 wherein said branch waveguide means is positioned alongside said central waveguide, said branch waveguide means having coupling slots formed at its interface with the central waveguide.

4. The coupler of claim 2 wherein said feed waveguide means comprises a first and second pair of similar waveguide sections, a portion of each of said paired sections being positioned along an associated pair of walls of the central waveguide to provide a balanced feed of microwave energy to and from said central waveguide.

5. A diplexing coupler for coupling a first pair of microwave receiver/transmitter channels in a first band of frequencies and a second pair of microwave receiver/- transmitter channels in a second band of frequencies to a common channel for reception/transmission therein, comprising:

a central waveguide forming said common channel,

first feed waveguide means positioned with a pair of the walls thereof parallel to a first pair of the walls of the central waveguide,

second feed waveguide means positioned with a pair of the walls thereof parallel to a second pair of the walls of the central waveguide,

said first and second feed waveguide means being dimensioned to transmit said first band of frequencies,

means for coupling the outputs of said first pair of channels each to a separate one of said first and second feed waveguide means, the polarization of the output coupled to said first feed waveguide means being orthogonal to that coupled to said second feed waveguide means,

branch waveguide means interposed between each of said feed waveguide means and central waveguide for coupling said channels therebetween, each of said branch waveguide means being positioned alongside a separate wall of the central waveguide and being dimensioned to operate as a cutoff filter at said second band of frequencies and a high pass filter at said first band of frequencies, and

means for coupling the outputs of said second pair of channels to said central waveguide with the polarizations thereof in mutually orthogonal relationship.

6. The coupler of claim 5 wherein the means for coupling the channels between the feed waveguide means and the branch waveguide means and between the branch waveguide means and the central waveguide includes coupling slots at the interface between the feed and branch waveguide means and at the interface between the branch waveguide means and the central waveguide.

7. The coupler of claim 5 wherein the second band of frequencies is lower than the first band of frequencies.

8. The coupler of claim 5 wherein the means for coupling the outputs of said first pair of channels to said feed waveguide means comprises a rotary joint for each of said feed waveguide means, the rotary joint for said first feed waveguide means being oriented orthogonally to that for said second feed waveguide means.

9. The coupler of claim 5 wherein the means for coupling the outputs of said first pair of channels to said feed waveguide means and said second pair of channels to said central waveguide comprise plates.

10. The coupler of claim 5 wherein said first and second feed waveguide means each comprises a pair of similar waveguide sections, a portion of each of said paired sections being positioned along an associated pair of walls of the central waveguide to provide a balanced feed to said central waveguide.

11. The coupler of claim 5 wherein said first feed waveguide means is dimensioned for transmission of the TE mode of the microwave energy and said second feed waveguide means is dimensioned for transmission of the TE mode of the microwave energy.

12. The coupler of claim 5 wherein the means for coupling the outputs of said second pair of channels to said central waveguide comprises an orthomode junc-

Claims

1. In a diplexing coupler for coupling at least one microwave receiver/transmitter channel in a first band of frequencies and at least one microwave receiver/transmitter channel in a second band of frequencies to a common channel comprising: a central waveguide forming said common channel, feed waveguide means dimensioned to transmit said first band of frequencies, means for coupling the channel at said first band of frequencies to said feed waveguide means, means for coupling the channel at said second band of frequencies to said central waveguide, and branch waveguide means interposed between said feed waveguide means and said central waveguide for coupling said channel at said first band of frequencies therebetween, said branch waveguide means being dimensioned to operate as a cutoff filter at said second band of frequencies and a high pass filter at said first band of frequencies.

5. A diplexing coupler for coupling a first pair of microwave receiver/transmitter channels in a first band of frequencies and a second pair of microwave receiver/transmitter channels in a second band of frequencies to a common channel for reception/transmission therein, comprising: a central waveguide forming said common channel, first feed waveguide means positioned with a pair of the walls thereof parallel to a first pair of the walls of the central waveguide, sEcond feed waveguide means positioned with a pair of the walls thereof parallel to a second pair of the walls of the central waveguide, said first and second feed waveguide means being dimensioned to transmit said first band of frequencies, means for coupling the outputs of said first pair of channels each to a separate one of said first and second feed waveguide means, the polarization of the output coupled to said first feed waveguide means being orthogonal to that coupled to said second feed waveguide means, branch waveguide means interposed between each of said feed waveguide means and central waveguide for coupling said channels therebetween, each of said branch waveguide means being positioned alongside a separate wall of the central waveguide and being dimensioned to operate as a cutoff filter at said second band of frequencies and a high pass filter at said first band of frequencies, and means for coupling the outputs of said second pair of channels to said central waveguide with the polarizations thereof in mutually orthogonal relationship.

9. The coupler of claim 5 wherein the means for coupling the outputs of said first pair of channels to said feed waveguide means and said second pair of channels to said central waveguide comprise 180* plates.

11. The coupler of claim 5 wherein said first feed waveguide means is dimensioned for transmission of the TE01 mode of the microwave energy and said second feed waveguide means is dimensioned for transmission of the TE10 mode of the microwave energy.

12. The coupler of claim 5 wherein the means for coupling the outputs of said second pair of channels to said central waveguide comprises an orthomode junction.