GB2188493A

GB2188493A - Orthogonal mode transducer

Info

Publication number: GB2188493A
Application number: GB08607663A
Authority: GB
Inventors: Barry Kenneth Watson
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1986-03-27
Filing date: 1986-03-27
Publication date: 1987-09-30
Also published as: GB8607663D0

Abstract

An orthogonal mode transducer for an antenna feed intended for dual polarised operation in at least two different frequency bands includes a primary waveguide (4) and a first coupling section (5) comprising a side-arm waveguide (7,8) coupled to the primary waveguide for coupling signals in the higher of two different frequency bands supported by the primary waveguide and having a predetermined hand of polarisation. A frequency and polarisation selective filter (11,12) is disposed in the primary waveguide, downstream from the side-arm waveguide in the receive direction. The filter reflects signals with the predetermined hand of polarisation in the higher frequency band but will pass orthogonally-polarised signals in the same frequency band and all signals in the lower of the two bands. A second coupling section (6) is similar to the first section, but its side-arm waveguide (13,14) and its filter (16,17) are displaced angularly about the axis of the primary waveguide by 90 DEG with respect to those of the first section. The second section thereby couples the orthogonally-polarised signals in the higher frequency band. <IMAGE>

Description

SPECIFICATION Orthogonal mode transducer This invention relates to orthogonal mode transducers for use in antenna feeds intended for dual polarised operation (i.e. using both orthogonal hands of linear or circular polarisation at the same frequency) in at least two different frequency bands. This is a common requirement in a number of antenna systems, especially satellite communications systems employing different frequency bands for the up and down communication channels and frequency reuse by dual polarisation to double the capacity of the channels.

An orthogonal mode transducer, known as an OMT, is a device for inputting and extracting from the antenna feed, separately from each other, the orthogonally polarised signals in the different frequency bands, and invariably has a different port for each hand of polarisation in each frequency band. It is important that the device should maintain sufficient electrical isolation between the two ports of each frequency band, and also between the ports of the different bands, otherwise cross channel interference will result in degradation of the antenna performance. In general the isolation should be greater than 30 dB between the ports of the same frequency band, and greater than 60 dB, preferably approximately 130 dB, between the ports of different bands.

Several different forms of OMT are known, but none which are capable of maintaining the desired high isolation performance when the different operating frequency bands are relatively widely spaced and unwanted higher order modes propagate at the higher frequencies, while nevertheless remaining relatively simple to manufacture.

According to the present invention, an OMT for an antenna feed intended for dual polarised operation in at least two different frequency bands includes a primary waveguide, a first coupling section comprising a side-arm waveguide coupled to the primary waveguide for coupling signals in the higher of two different frequency bands supported by the primary waveguide and having a predetermined hand of polarisation, and a frequency and polarisation selective filter disposed in the primary waveguide downstream from the side-arm waveguide in the receive direction which will reflect signals with the predetermined hand of polarisation in the higher frequency band but will pass orthogonally polarised signals in the same frequency band and all signals in the lower of the two bands, and a second coupling section which is similar to the first section but in which its side-arm waveguide and filter are displaced angularly about the axis of the primary wave guide by 90 with respect to those of the first section so that the second section couples the orthogonally polarised signals in the higher frequency band.

It may be possible to locate the first and second coupling sections together in the primary waveguide, but probably it will be necessary for the second section to be displaced axially from the first section.

In operation the OMT in accordance with the invention will input or extract the orthogo nally polarised higher frequency band signals in the first and second coupling sections, and the associated frequency and polarisation selective filters will prevent these signals from propagating downstream in the receive direction to interfere with the lower frequency band ports while permitting the lower frequency sig nals to pass unaffected. In the case of a dual band system, the OMT will simply be provided with a further pair of coupling sections for the orthogonally polarised signals in the lower frequency band, and these sections may be of conventional construction comprising a side-arm waveguide coupled to the primary waveguide and a septum plate suitably positioned in the primary waveguide.As will be appreciated, however, an OMT in accordance with the invention can be constructed which is capable of operation in several different frequency bands, simply by providing the primary waveguide with the appropriate number of further pairs of coupling sections axially displaced from the first pair and having their filters appropriately tuned. In principle the number of different frequency bands which can be catered for is limited only by the permissible transmission loss in the lowest band.

Preferably the frequency and polarisation selective filter of each coupling section is a band stop filter formed by a pair of opposed rows of transverse corrugation slots in the wall of the primary waveguide. The desired filter characteristic, i.e. band stop over the whole of the higher frequency band, is determined by the width, spacing and depth of the corrugation slots, and its sensitivity to only one hand of two orthogonal hands of polarisation is determined by the transverse length of the slots and the orientation of the median plane bisecting the rows of slots relative to the polarisation directions.

Preferably each coupling section comprises two side-arm waveguides coupled to the primary waveguide directly opposite each other on opposite sides of the waveguide axis, forming what is known as double feed coupling, and the two side-arm waveguides are joined, preferably by a magic tee junction, to provide a single input/output port. Double feed coupling maintains symmetry in the primary waveguide which serves to avoid, or at least reduce, undesirable disturbances in the higher order modes which propagate in the primary waveguide at the higher frequencies.

This arrangement also helps to avoid coupling of the lower frequency band signals to the side-arm waveguides of the first and second coupling sections, although further means for preventing such coupling may, if desired, be provided in order to improve the isolation between the frequency bands in the low to high direction. Such further means may comprise a frequency sensitive surface in the primary waveguide in the region of the coupling aperture to each side-arm waveguide, or a high pass or band pass filter in the side-arm waveguide itself. Any suitable waveguide filter may be used.

Each side-arm waveguide of the first and second coupling sections will usually be a rectangular waveguide and is preferably coupled longitudinally to the primary waveguide to achieve H-plane coupling of signals polarised in the direction perpendicular to the axis of the side-arm waveguide. Alternatively, each side-arm waveguide may be coupled transversely to the primary waveguide for Eplane coupling of signals polarised in a direction parallel to the axis of the side-arm waveguide.

The invention provides an OMT which can be constructed relatively easily by serially connecting together a series of relatively easily manufactured components and which affords a high degree of isolation between the different ports, especially ports in different frequency bands. The OMT may have a circular or square primary waveguide for inclusion in a circular or square antenna feed, and may be designed for ground station or satellite use.

One example of an OMT in accordance with the invention will now be described with reference to the accompanying drawing which is a perspective view of part of an antenna feed incorporating the OMT.

The antenna feed illustrated is designed for the ground station antenna of a satellite communications system operating with a 6 GHz up link frequency band from 5.925 to 6.425 GHz and a 4 GHz down link frequency band from 3.7 to 4.2 GHz. The ground station antenna therefore transmits high power signals Tx in the higher frequency 6 GHz band and receives low power signals Rx in the lower frequency 4 GHz band.

The antenna feed shown comprises an OMT 1 constructed in accordance with the invention and designed to input separately for simultaneous transmission signals in the 6 GHz band having opposite hands of circular polarisation, and to extract separately circularly polarised signals of opposite hand received simultaneously in the 4 GHz band. The feed further comprises a circular waveguide corrugated polariser 2 connected to the OMT 1, and a coni cal corrugated horn (indicated only partly at 3) connected to the polariser.

The OMT 1 comprises a circular primary waveguide 4 connected to the polariser 2 and provided with first and second coupling sections 5, 6 positioned one after the other along the axis of the primary waveguide 4 for respectively inputting the right and lefthand circularly polarised transmit signals Tx. The first coupling section 5 comprises a pair of rectangular side-arm waveguides 7, 8 coupled longitudinally to the primary waveguide 4 diametrically opposite each other to provide symmetrical double feed coupling, and the two sidearm waveguides 7, 8 are joined by a magic tee junction 9 to provide a single input port 10. The first coupling section 5 also comprises two opposed rows of transverse corrugation slots 11, 12 opening into the primary waveguide 4 diametrically opposite each other.

The rows 11, 12 extend along the primary waveguide 4 downstream from the side-arm waveguides 7, 8 in the receive direction, and are positioned so that the diametral plane through the axis of the primary waveguide 4 bisects the slots in the rows. The individual slots of the rows extend transversely to the axis of the primary waveguide 4, and are arranged so that the two rows form a band stop filter which will stop propagation in the receive direction of righthand circularly polarised high band Tx signals coupled to the primary waveguide by the side-arm waveguides 7 and 8, but will allow the lefthand circularly polarised high band Tx signals coupled to the primary waveguide by the second coupling section 6 to propagate in the transmit direction and will also allow all low band Rx signals to propagate in the receive direction.As indicated in the drawing, the slots at each end of each row 11, 12 progressively decrease in depth and transverse length towards the end of the row for matching purposes.

The second coupling section 6 also comprises a pair of diametrically opposite rectangular side-arm waveguides 13, 14 coupled longitudinally to the primary waveguide 4 and connected by a magic tee junction (not shown) to provide a second input port 15, and a pair of opposed rows of transverse corrugation slots 16, 17 forming a band stop filter. The construction of the second coupling section 6 is in fact identical to that of the first coupling section except that its side-arm waveguides 13, 14 and its rows of corrugation slots 16, 1 7 are turned through 90" abut the axis of the primary waveguide 4 with respect to those of the first section 5. In the case of the second coupling secton 6, the band stop filter 16, 17 serves to stop propagation in the receive direction of lefthand circularly polarised high band Tx signals coupled to the primary waveguide by the side-arm waveguides 13, 14 for transmission, but will allow propagation of all low band Rx signals in the receive direction.

Downstream from the second coupling section 6 in the receive direction the primary waveguide 4 leads through a differential phase shift equaliser 18 to a pair of serially connected coupling sections 19, 20 designed to extract right and lefthand circularly polarised Rx signals respectively in the lower frequency 4 GHz band. These coupling sections 19, 20 are of conventional construction having single rectangular side-arm waveguides 21, 22 coupled longitudinally to the primary waveguide 4 and providing the OMT with third and fourth ports 23, 24.

As will be appreciated, the OMT 1 will also function in reverse, using the ports 23 and 24 to input for transmission orthogonally polarised signals in the lower frequency 4 GHz band, and the ports 10 and 12 to extract orthogonally polarised signals received in the higher frequency 6 GHz band.

Claims

1. An orthogonal mode transducer for an antenna feed intended for dual polarised operation in at least two different frequency bands, comprising a primary waveguide; a first coupling section comprising a side-arm waveguide coupled to the primary waveguide for coupling signals in the higher of two different frequency bands supported by the primary waveguide and having a predetermined hand of polarisation, and a frequency and polarisation selective filter disposed in the primary waveguide downstream from the side-arm waveguide in the receive direction which will reflect signals with the predetermined hand of polarisation in the higher frequency band but will pass orthogonally-polarised signals in the same frequency band and all signals in the lower of the two bands; and a second coupling section which is similar to the first section but in which its side-arm waveguide and filter are displaced angularly about the axis of the primary wave guide by 90O with respect to those of the first section so that the second section couples the orthogonally-polarised signals in the higher frequency band.

2. A transducer as claimed in claim 1, wherein the frequency and polarisation selective filter of each coupling section is a band stop filter formed by a pair of opposed rows of transverse corrugation slots in the wall of the primary waveguide.

3. A transducer as claimed in claim 1 or claim 2, wherein each coupling section comprises two side-arm waveguides coupled to the primary waveguide directly opposite each other on opposite sides of the waveguide axis.

4. A transducer as claimed in claim 3, wherein the two side-arm waveguides are joined to provide a single input/output port.

5. A trandsucer as claimed in claim 4, wherein the two side-arm waveguides are joined by a magic tee junction.

6. A transducer as claimed in any preceding claim, including a frequency-sensitive surface in the primary waveguide in the region of the coupling aperture to each side-arm waveguide to reduce coupling of the lower frequency band to the side-arm waveguides.

7. A transducer as claimed in any one of claims 1-6, including a filter in each side-arm waveguide to reduce the passage of the lower frequency band through the side-arm waveguide.

8. A transducer as claimed in any preceding claim, wherein each side-arm waveguide is a rectangular waveguide.

9. A transducer as claimed in claim 8, wherein each side-arm waveguide is coupled longitudinally to the primary waveguide to achieve H-plane coupling of signals polarised in the direction perpendicular to the axis of the side-arm waveguide.

10. A transducer as claimed in claim 8, wherein each side-arm waveguide is coupled transversely to the primary waveguide for Eplane coupling of signals polarised in a direction parallel to the axis of the side-arm waveguide.

11. A transducer as claimed in any preceding claim, including a further pair of coupling sections for the orthogonally-polarised signals in the lower frequency band.

12. An orthogonal mode transducer substantially as hereinbefore described with reference to the accompanying drawing.