GB2165998A

GB2165998A - A dual reflector antenna

Info

Publication number: GB2165998A
Application number: GB08524398A
Authority: GB
Inventors: David Garrood; Roger Crawshaw; Robert Ian Henderson; Frank Allwood Dutton
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-02-09
Filing date: 1985-10-03
Publication date: 1986-04-23
Also published as: GB2165998B; GB8524398D0

Abstract

A dual-reflector antenna has an annular shielding device 9 positioned between the feed horn 8 and sub-reflector 10 to reduce spillover. A main reflector 1 is provided. The arrangement is more fully described in G.B. Application 2154067A. <IMAGE>

Description

SPECIFICATION A dual reflector antenna This invention relates to a dual reflector antenna and arose in the design of an earth terminal for satellite communication systems.

To avoid interference with other communication systems employing for example an adjacent satellite in the geostationary orbit it may be required that an earth terminal transmit a very low amount of radiation in directions other than the specified main lobe of the antenna; and the invention arose in an endeavour to meet this requirement.

The invention provides a dual-reflector antenna comprising a feed, a sub-reflector arranged to be illuminated by the feed and a main reflector arranged to receive the radiation after reflection from the sub-reflector, characterised by a shielding device defining an annular region of shielding between the feed and the sub-reflector so as to obstruct radiation from the feed which would otherwise miss the sub-reflector.

This technique is applicable to any dualreflector antenna (i.e., Cassegrain or Gregorian) whether or not forming part of a satellite communications system. The technique can achieve a substantial reduction in 'spill-over" i.e., radiation missing the sub-reflector, thereby reducing the amount of radiation emitted in directions other than that required.

The shield also preferably has the effect of reducing the intensity of radiation in the edge regions of the main reflector thus reducing the amount of radiation which misses the latter.

One way in which the invention may be performed will now be described with reference to the accompanying illustrations in which: Figure 1 is a schematic perspective view of a road transportable antenna forming part of a satellite communication system for any form of satellite communication; and Figure 2 illustrates schematically the relationship of the beam shape of the antenna shown in Fig. 1 with the locus of movement of the satellite.

Referring to Fig. 1 of the drawings there is illustrated a road-trailer-mounted offset Gregorian antenna with an elliptical main reflector 1 having a first maximum dimension d1 in the horizontal plane and a second minimum dimension d2 in an orthogonal plane. The reflector 1 has lugs one of which is shown at 2 by which it is pivotted about a horizontal axis on a turntable 3 which can be rotated about an orthogonal vertical axis on a frame 4 which forms part of a road trailer. The trailer carries a television transceiver 5 from which energy to be transmitted is fed along a flexible waveguide 6 to a feed horn 8. From the horn 8 the energy is directed through a shielding device 9 onto an offset concave sub-reflector 10 and then to the main reflector 1.The feed horn 8, shielding device 9 and sub-reflector 10 are mounted on a framework 11 which is pivotted, about a horizontal axis, on lugs 12 fixed to the reflector 1. The framework is held at the illustrated position by removable stays 13 each secured at one end to framework 11 and at the other end to a lug 14 also fixed to the reflector. The feed horn 8, shielding device 9 and sub-reflector 10 are designed so as to illuminate substantially the whole of the main reflector 1. The larger diameter d1 results in a narrower beamwidth in azimuth than is achieved in elevation by the smaller diameter d2.The sub-reflector 10 is designed to spread the energy arriving from the horn 8 across the axes d1 and d2 of the reflector 1 in such a way that the energy is tapered from the centre of the reflector to the edges to a greater extent in the dimension d, than in the dimension d2. It is desirable to accomplish this because the greater taper in direction d, will result in a relatively lower level of sidelobes, while the lesser taper in dimension d2, whilst resulting in higher sidelobes, asists in maintaining the highest possible directionality from the complete aperture.

The purpose of the shielding device 9, supported between the horn 8 and reflector 10 on struts 11 A forming part of the framework 11, is to act as an obstruction to radiation from the horn which would otherwise miss the sub-reflector 10. It also reduces the radiation intensity at the edges of the sub-reflector and therefore in the region of the edges of the main reflector, thus reducing the amount of radiation from the sub-reflector which misses the main reflector. The radiation which misses the two reflectors is called "spill-over" and it is desirable to reduce this as much as possible to minimise interference e.g., with other satellite communication systems. The shielding device 9 is, as shown on Fig. 1, formed by a frusto-conical metal surface tapering towards the sub-reflector.This is preferable to an annular surface since it enables a shielding effect to be obtained over a considerable angle without obstructing radiation passing from the sub-reflector to the main reflector.

The main lobe of the transmitted beam is shown schematically by the shaded area 15 on Fig. 2. It's boresight 16 is shown aligned with a satellite 1 7 which moves within a roughly square region 18 centred on a geostationary orbit 19 of the satellite 17.

Before deployment, the reflector 1 lies substantially horizontally on the frame 4, the stays 13 are stowed away, and the framework 11 is folded so as to lie against the reflector. An extension 1 1 B of the framework 11 extends through a hole 1 A of the reflector 1 and is secured thereto by a catch mechanism (not shown) behind the reflector.

When the illustrated transmitter is to be deployed the reflector 1 is tilted in elevation on its lugs 2 by manually operated jacks shown schematically at 20 and is rotated in azimuth using the turntable 3 and a servo mechanism 3A which engages teeth on the edge of the turntable. An accurate inclination sensing instrument 1B is used to enable the boresight 16 to be set at the elevation of the satellite which will usually be as illustrated at approximately the highest point of the orbit 19. The azimuth is then set roughly to the direction of the satellite using a relatively inaccurate compass. Fine adjustment is then effected by an operator until the satellite has been acquired.

Following this the satellite is automatically tracked in azimuth during movements from one side to another of the square 18. The tracking is effected by automatic rotation of turntable 3 by the servo mechanism 3A under the control of the transceiver 5 via line 5A.

Because of the highly directional nature of the transmitted beam in azimuth and because of the lower sidelobes in this plane, interference with other communication systems using other satellites such as that shown at 21 on Fig. 2 is avoided. Deployment of the system is facilitated because of the provision of the azimuth tracking system which provides the necessary mechanical means for the operator to effect the fine adjustment referred to previously and ensures that the beam is correctly aligned in azimuth with the satellite. Finally of course the shape of the antenna enables it, and it's transporter, to travel under most road bridges and overhead obstacles or, in a slightly modified version to be carried by air.

In practice it is envisaged that the antenna will be needed in circumstances when the geostationary orbit 19 makes an angle of no more than 45' with the horizontal in the region 7. In such circumstances little penalty is paid in using an antenna with its major axis pemanently horizontal as in the illustrated example. There may however be circumstances where it is desired to communicate with a satellite in a part of the orbit which appears inclined to the horizontal. In such circumstances the antenna can take advantage of the features already described if the axis d is inclined so that it lies effectively tangential to the position of the satellite in the geostationary arc as viewed from the antenna. Such an inclined mounting arrangement can be achieved on a mobile installation: but is more readily achieved on a permanent stationary installation.

Claims

1. A dual-reflector antenna comprising a feed, a sub-reflector arranged to be illuminated by the feed and a main reflector arranged to receive the radiation after reflection from the sub-reflector. characterised by a shielding device defining an annular region of shielding between the feed and the sub-reflector so as to obstruct radiation from the feed which would otherwise miss the sub-reflector.

2. An antenna according to Claim 1 in which the shielding device is supported by struts on a frame carrying the feed and/or the sub-reflector.

3. An antenna according to Claim 1 or 2 in which the shielding device comprises a frustoconical shielding surface tapered towards the sub-reflector and whose axis is aligned with the optical axis between the feed and the subreflector.