GB2079538A

GB2079538A - Beam waveguide feed for antenna

Info

Publication number: GB2079538A
Application number: GB8022088A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1980-07-04
Filing date: 1980-07-04
Publication date: 1982-01-20
Also published as: JPS5748805A; EP0043689A3; EP0043689A2

Abstract

In microwave communication systems it is common practice to use a beam waveguide to transmit energy between a waveguide (2) connected to transmitting and receiving equipment located at ground level and a main antenna reflector mounted above ground level. Hitherto the waveguide (2) has terminated in a very large, accurately machined, horn which is necessary to produce a beam having a small angle of divergance suitable for entry into the beam waveguide (6). The present invention proposes that this large (and very expensive) horn be replaced by a relatively small horn (5A) which produces a beam having a relatively wide angle of divergence. By using a concave reflector (22) and a sub-reflector (21) in Cassegrain configuration this widely diverging beam is converted into a more nearly parallel (or a totally parallel) beam which enters the beam waveguide (6) as illustrated. <IMAGE>

Description

1

GB2079538A

1

SPECIFICATION

Beam waveguide feed for antenna

* 5 This invention relates to apparatus for feeding electromagnetic signals, particularly in the microwave and radio frequency areas of the spectrum between, on the one hand a reflector belonging to an antenna and, on the other 10 hand, transmitting and/or receiving equipment for delivering signals to or receiving them from the antenna.

Particularly when large reflectors are required, such as is normal for satellite commu-1 5 nication systems, it is necessary for practical reasons that the transmitting and/or receiving equipment be at ground level whilst the reflector be raised above ground level. An ordinary waveguide provided with rotating 20 joints can be used for feeding energy between the equipment on the ground and the reflector but such waveguides are inefficient in that much of the energy is lost during transmission along them. It is therefore generally preferred 25 to use what is known as a "beam waveguide" which is defined for the purpose of this Specification as an arrangement of reflectors designed to direct radiation along a desired path.

30 A typical known apparatus of the type described above is shown very schematically in Fig. 1 of the accompanying drawings. Referring to Fig. 1 there is shown a building 1 which houses microwave signal detecting and 35 generating equipment 1A. Microwave energy passes from 1A along a short waveguide 2 which terminates in a large corrugated radiating horn 3. This emits a beam 4 of radiation which diverges from point 5 inside the mouth 40 of the horn with an angle of divergence (a).

The diverging beam 4 enters a beam waveguide 6 mounted on a support structure 7 which has wheels 8 riding on a circular track 9. The support structure 7 and the beam 45 waveguide 6 can thus be rotated about a vertical axis X-X. The means for effecting such rotation is not illustrated in the drawing.

The beam waveguide 6 has a lower tubular portion containing a flat reflector 10 from 50 which the diverging beam 4 is reflected to a parabolic reflector 11. The latter has its focus at point 5 and therefore produces a parallel beam which is directed to another parabolic reflector 12. The reflector 1 2 brings the paral-j&5 lei beam to a focus at point 1 3 after reflection \ from a flat reflector 14. The beam, now diverging again, is reflected from a sub-reflec-tor 1 5 to a main parabolic reflector 1 6 from where the signals are transmitted into the 60 atmosphere or into space if the apparatus is located in an extra terrestrial position.

The main reflector 16 has support arms 1 7 pivotted about a horizontal axis Y-Y to the support structure 7. The beam waveguide 6 is 65 divided as indicated at 6A into upper and lower parts so as to enable the upper part to rotate with the reflector 16 about the axis Y-Y whilst the lower part of the beam waveguide remains 70 stationary. The means for rotating the reflector 16 about the horizontal axis Y-Y is not shown in the drawing. By selecting the appropriate positions of rotation about the axes X-X and Y-Y the reflector 16 can be made to point in 75 any desired direction.

In arrangements such as shown in Fig. 1 the horn 3 needs to radiate most of the energy within a narrow angle (a). Otherwise not all the energy would be incident on the 80 reflector 10. In order to achieve a small angle (a) the horn must be very large, possibly several metres high. The horn is therefore very expensive, particularly if, as is usually the case, the horn is internally corrugated along 85 its entire length.

This invention provides apparatus for transmitting or receiving electromagnetic signals comprising a path for the signals extending: from a signal generator or detector to a mem-90 ber located in front of a concave reflector;

from the said member to the concave reflector; from the concave reflector, past opposite sides of the said member to a beam waveguide; and from the beam waveguide to a main 95 reflector.

The concave reflector provided by this invention enables a widely diverging beam,

from for example a small horn, to be converted into a beam having a smaller angle of 100 divergance, such as would be obtained from a large horn. The cost of the conventional large horn is thus substantially eliminated at the expense of a relatively inexpensive concave reflector. The concave reflector can be de-105 signed to give a beam having as small an angle of divergance as may be required or even a parallel beam or a converging beam. The invention therefore makes it possible in some cases to eliminate the need for concave 110 reflectors in the beam waveguide thus further reducing the cost of the system.

The aforementioned comments with regard to the angle of divergance of the beam refer to the system when operating as a transmitter. 115 Similar comments of course apply to the apparatus when operating as a receiver in which case the aforementioned angle of divergance will be the angle of convergance.

The said"member" can be a relatively small 120 horn positioned in front of the concave reflector and directed towards the latter. However, such a horn reguires a waveguide feed which, because of its necessary cross-sectional area, obstructs to a significant extent the path be-1 25 tween the concave reflector and the beam waveguide. It is therefore preferred that the said "member" be a sub-reflector positioned axially in front of the concave reflector and arranged so that the said path passes through 1 30 an aperture in the centre of the concave

2

GB2 079 538A 2

reflector between the sub-reflector and a horn or equivalent member located behind the concave reflector.

A sub-reflector as described above can be 5 supported on relatively slender supports which do not present any significant obstacle to the passage of radiation along the aforementioned path.

An example of how the invention may be 10 performed will now be described with reference to Figs. 2,3 and 4 of the accompanying drawings in which:-

Figure 2 is a schematic vertical cross-section through apparatus constructed in accor-1 5 dance with the invention;

Figure 3 is similar to Fig. 2 but shows a modification; and

Figure 4 is a more detailed view of part of the apparatus shown in Fig. 3.

20 Referring firstly to Fig. 2, this apparatus has components

1,1 A,2,6,7,8,9,10,11,12,14,15,16 and 17 identical to those shown in Fig. 1. Of these only components 10 and 11 and part of 25 component 6 are shown in Fig. 2. The large horn 3 of Fig. 1 is replaced, in Fig. 2, by a small horn 18 which emits, or receives, a beam 19 whose angle of divergance, or con-• vergance, is denoted by the letter (b). This 30 angle (b) is much greater than the corresponding angle (a) of Fig. 1 because of the smaller horn size. The beam 19 passes through a central aperture in a concave reflector 20 behind which the horn 18 is situated. The 35 beam is then reflected from a sub-reflector 21 onto the front of the concave reflector 20.

The concave reflector 20 is eliptical having one focus at the point 5, which is the same as the point 5 shown in Fig. 1; and its other 40 focus at a point 5A inside the mouth of the horn 1 8. Accordingly, the beam reflected from the reflector 20 is brought to a focus at point 5. The angle of convergance and divergance at point 5 is chosed to be equal to the 45 angle (a) of Fig. 1 so that after passing point ' 5 the beam behaves exactly as described with reference to Fig. 1 The apparatus depicted in Fig. 2 can thus be used as a direct replacement for the large horn 3 of Fig. 1 without 50 the need arising for any other modifications to the equipment.

Fig. 3 shows an apparatus similar to that of Fig. 2 but modified by the replacement of the eliptical reflector 20 by a parabolic reflector 55 22 having its focus at the point 5A. This produces a parallel beam which is directed into the beam waveguide 6. Since the beam is parallel there is no need for convex reflectors in the beam waveguide. Thus the reflec-60 tors 11 and 1 2 of Fig. 1 can be replaced by planar reflectors such as shown at 11 A.

In order to avoid any reduction in efficiency of the system by obstruction of the beam by the sub-reflector 21, the latter is preferably 65 shaped so that it directs no radiation, or a relatively low intensity of radiation to the central portion of the reflector 22. This is illustrated in Fig. 4 where regions of the beam having maximum intensity values are shown 70 by the shaded areas. From Fig. 4 it is apparent that the sub-reflector 21 is effective to shape the beam 4 into an annular configuration which is incident on the part 23 of the reflector 22 but not to a substantial extent on 75 the central part of this reflector. As a consequence the sub-reflector 21 does not, to a substantial extent, obstruct energy flowing between the concave reflector 22 and the beam waveguide.

80

Claims

CLAIMS:

1. Apparatus for transmitting or receiving electromagnetic signals comprising a path for the signals extending: from a signal generator

85 or detector to a member located in front of a concave reflector; from the said member to the concave reflector and past opposite sides of the said member to a beam waveguide; and from the beam waveguide to a main 90 reflector.

2. Apparatus according to claim 1 in which the said member is a sub-reflector and in which that part of the path which extends from the transmitter or receiver to the sub-

95 reflector passes through an aperture in the concave reflector to a sub-reflector and thence to the said concave reflector.

3. Apparatus according to claim 2 in which the said part of the path includes a

100 horn located behind the concave reflector and pointing through the aperture towards the sub-reflector.

4. Apparatus according to claim 2 or 3 in which the sub-reflector is shaped so that the

105 said path extends past opposite sides of the sub-reflector but is not to a substantial extent intercepted thereby .

5. Apparatus according to claim 1 and substantially as described with reference to

110 Fig. 2.

6. Apparatus according to claim 1 and substantially as described with reference to Fig. 3.

7. Apparatus according to claim 5 and 6

115 and substantially as described with reference to Fig. 4.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.—1982.

Published at The Patent Office, 25 Southampton Buildings,

London, WC2A 1AY, from which copies may be obtained.