GB2215525A - Waveguide polarisation mode separator - Google Patents

Abstract

A device for separating two orthogonally polarised modes in a waveguide comprises two openings 5, 6 in the wall of the waveguide 1 spaced apart along the length of the waveguide, first means such as a row of rods 2, between the openings to transmit one plane of polarisation E2 and to reflect the orthogonal plane E1 into a transverse waveguide 7, and second means beyond the openings, which may consist of a conductor 3 at 45 degrees and a short circuit 4, for retro- reflecting the transmitted plane of polarisation E2 and for rotating its plane of polarisation through 90 degrees into the plane E1, in order that it may be deflected into transverse waveguide 8. The device may be used as the feed of a reflector antenna, one of the branches 7, 8 being used for transmitted radiation and the other for received radiation, and the device presents minimum blockage of the reflector, because both branches 7, 8 present their narrower dimension to the incoming radiation. <IMAGE>

Description

WAVEGUIDES This invention relates to the waveguides, and especially to devices separating two orthogonally polarised modes (orthomode transducers).

Conventionally this is done by a vane which bifurcates a square or circular waveguide. Waves with electric vectors parallel to the vane now see two waveguides each of half the width of the waveguide upstream of the vane and, since the width of the waveguide determines the guide wavelength which can be propagated, the mode upstream cannot propagate along the two halfwidth sections, each of which has a cut-off at a wavelength shorter than that of the mode upstream. Waves with electric vectors perpendicular to the vane are substantially unaffected by it. Consequently, the latter waves continue along the waveguide whereas those with electric vectors parallel to the vane are reflected by the vane, and may propagate through a port in the wall of the waveguide into a transverse rectangular waveguide branch communicating with that port.The main waveguide may taper into a rectangular waveguide beyond the vane to accommodate the other plane of polarisation.

Alternatively, the main waveguide could in theory include a second vane perpendicular to and beyond the first, which would reflect the waves incident on it since their electric vector would be parallel to it, and the waves could therefore be deflected into a second port in the wall of the waveguide again communicating with a transverse rectangular waveguide branch.

Such a waveguide with orthomode transducer may be used as the feed waveguide for a reflector antenna, the separation of orthogonally polarised modes being used to separate the transmitted and received signals.

However, in the case of a small reflector, the blockage and scattering caused by the transverse branches could raise the side lobes above the level permitted by satellite operators with the attendant risk of interferring with other satellites.

Even if the two transverse branches appeared one behind the other as seen from the statelite, one of them would present its wider dimension to the incoming radiation. One could overcome this by adopting the other configuration referred to, namely, one transverse branch with the waveguide tapering into a rectangular section, and then twisting this latter section to lie behind the first transverse branch with both presenting their narrower dimension to the incoming radiation. However, the bends necessary to achieve this would still block the face of the antenna.

The invention provides a device for separating two orthogonally polarised modes in a waveguide, which comprises two openings in the wall of the waveguide spaced apart along the length of the waveguide, first means between the openings to transmit one plane of polarisation and to reflect the orthogonal plane, and second means on one side of both of the openings for retro-reflecting the said transmitted plane of polarisation and for rotating its plane of polarisation through 90 degrees into the orthogonal plane.

Because the second means rotates the plane of polarisation of the radiation transmitted by the first means and reflects it back towards the first means, the first means deflects both of the original orthogonally polarised modes through the openings. The opening may communicate with transverse rectangular waveguide branches arranged one behind the other and presenting their narrower dimensions to radiation travelling parallel to the axis of the waveguide. If the waveguide forms the feed of a reflector antenna, the transverse waveguide branches will present the minimum possible obstacle to the incoming radiation. The orthomode transducer is able to seperate the two modes whether both enter the open end of the waveguide, whether they both enter transverse branches, or whether they travel in counter-directions.

Advantageously, the first means comprises a vane, which may comprise of a row of rods, extending parallel to the axis of the waveguide, for example diametrically.

Advantageously, the second means includes a short circuit such as a closed end of the waveguide for retroreflecting the transmitted wave: it may include a conductor in the form of a wire or a short vane extending across the waveguide at 45 degrees to the planes of the orthogonally polarised modes, or it may include a linear to circular polariser.

The waveguide may be of circular or square crosssection, and is suitable for electro-magnetic waves in the micro-wave region of the spectrum.

A waveguide including an orthomode transducer (OMT) constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section through the waveguide; Figure 2 is axial cross-section through the waveguide; Figure 3 shows the electric vectors of the electromagnetic waves; Figure 4 is a longitudinal cross-section of a modified form of part of the transducer; Figure 5 is an axial cross-section of the part shown in Figure 4; and Figure 6 is a schematic diagram illustrating the polarisation of waves through the parts shown in Figures 5 and 6.

Referring to Figures 1 and 2, a waveguide 1 of circular cross-section has an integral orthomode transducer comprising first means for transmitting one plane of polarisation and for reflecting the orthogonal plane in the form of a vane 2 and second means for retroreflecting the said transmitted plane and for rotating it plane of polarisation through the 90 degrees into the orthogonal plane, in the form of a conductor 3 and a short circuit 4. The waveguide is a feed for a satellite dish aerial (not shown), and is arranged at the focus, with the open end of the waveguide 1 pointing towards the reflecter dish.

The wall of the waveguide has two iris openings 5, 6 which communicate with transverse rectangular waveguides 7, 8 which are arranged so their narrower dimension is presented when viewing in the axial direction of the waveguide 1. These waveguides 7, 8 are connected to a feed and a receiver, respectively, arranged at the rear of the reflector. A plane polarised electromagnetic signal to be transmitted by the reflector is fed into the waveguide 7 and a signal received by the reflector is fed to the receiver from waveguide 8. In the waveguide, the waves are orthogonally polarised with electric vectors in directions El, E2.

The vane 2 consists of a row of conducting rods diametrically arranged across the waveguide parallel to the direction El. It is known that such a vane will substantially not affect a mode with an electric vector in the direction E2 but, since it defines two waveguides each with a cut-off below the wavelength propagating upstream, will reflect a mode with an electric vector El parallel to its plane. If radiation enters the open end of the waveguide 1 with an electric vector parallel to El, it will accordingly be reflected through iris 5 which is sized to obtain the best energy transfer into the waveguide 7, with the electric vector remaining perpendicular to the plane of the drawing as seen in Figure 1. Consequently, feed radiation in the waveguide 7 with axis of polarisation in direction El is reflected out of the open end of the waveguide 1 towards the reflector.

Waves polarised with electric vector parallel to E2 are incident on the wire 3 arranged diametrically across the waveguide and at 45 degrees to the plane of the vane 2. Only the component of E2 (Ep) parallel to the wire 3 (which sees a large inductance shunted across the waveguide) is reflected while the component of E2 (En) normal to the wire 3 (which sees only a small capacsitance) is substantially unaffected. The latter component is reflected by the short circuit formed by the closed end 4 of the waveguide.

The wire 3 is spaced from the short circuit 4 by a distance approximately equal (the exact distance being determined empirically) to a quarter of the wavelength of the radiation, with the result that when the reflected components Ep and En recombine, En is 180 degrees phase shifted compared with Ep. It follows that the plane of polarisation of E2 has been rotated through 90 degrees on recombination. This can be seen from Figure 3 where the reflected wave En is shown dotted and the reflected resultant E2 is also shown dotted.

Now that the plane of polarisation of the reflected wave is parallel to El, the wave is reflected by the other end of the vane 2 and is accordingly deflected via a matching iris 6 into the rectangular waveguide 8. (Equally radiation entering waveguide 8 with its plane of polarisation parallel to the direction El would have its plane of polarisation rotated by the wire 3 and short circuit 4, so that it would be transmitted down the waveguide 1 and out its open end).

As an alternative, the wire 4 could be replaced by a short vane extending at the same angle, and the vane may be attached to the short circuit plane.

As another alternative, instead of the wire or short vane, a circular waveguide section 9 (Figures 4 and 5) with a series of short rods 10 could be used, the rods 10 extending through the wall and each rod being inclined at 45 degrees to the El and E2 directions and being in a diametrical plane, the row extending along the length of the waveguide section. Such a device is a known converter of linearly polarised radiation to one hand of circularly polarised radiation.

Referring to Figure 6, radiation entering with one direction of linear polarisation is converted, say, to left hand circular polarisation. This becomes right hand circular polarisation on reflection at the short circuit, which in turn becomes linear polarisation at right angles to the incident plane of polarisation after passage: through the waveguide section 9.

As an example, one port of the orthomode transducer could operate between the 10.95 and 12.75 GHz, while the differently polarised port could operate from 14 to 14.5 GHz. The diameter of the waveguide could be of the order of 19 mm. However different dimensions could be used for operation in different parts of the micro-wave region of the electro-magnetic spectrum.

Instead of the waveguide sections 1 and 9 being circular, they could be square in the cross-section.

Although the orthomode transducer has been described in relation to a feed and receiver for use with a satellite dish, the orthomode transducer is applicable to the transmission of two orthogonally polarised components or the reception of two orthogonally polarised components.

Equally, the orthomode transducer could be used in any situation where seperation of two orthogonally polarised modes is required in any circumstances.

Claims (12)

1. A device for deparating two orthogonally polarised modes in a waveguide, which comprises two openings in the wall of the waveguide spaced apart along the length of the waveguide, first means between the openings to transmit one plane of polarisation and to reflect the orthogonal plane, and second means on one side of both of the openings for retro-reflecting the said transmitted plane of polarisation and for rotating its plane of polarisation through 90 degrees into the orthogonal plane.

2. A device as claimed in claim 1, in which the first means comprises a vane extending parallel to the axis of the waveguide.

3. A device as claimed in claim 2, in which the vane comprises a row of rods.

4. A device as claimed in any one of claims 1 to 3, in which the second means includes a short circuit for retroreflecting the said transmitted wave.

5. A device as claimed in claim 4, in which the second means includes a conductor extending across the waveguide at 45 degrees to the planes of the said two orthogonally polarised modes

6. A device as claimed in claim 4 or claim 5, in which the conductor comprises a length of wire.

7. A device as claimed in claim 4 or claim 5, in which the device comprises a vane.

8. A device as claimed in claim 4 or claim 5, in which the conductor is a spaced from the short circuit by a distance in the region of one quarter of the wave length of the said transmitted component.

9. A device as claimed in any one of claims 4 to 8, in which the short circuit consists of a closed end of the waveguide.

10. A device as claimed in claim 4, in which the second menas includes a linear to circular polariser.

11. A device for seperating two orthogonally polarised modes in a waveguide substantially as herein described with reference to the accompanying drawings.

12. A reflector antenna which is fed by a waveguide incorporated in a device for separating two orthogonally polarised modes as claimed in any one of claims 1 to 11.