GB2165999A - A transportable antenna - Google Patents

Abstract

An antenna comprises a feed 8, a sub-reflector 10, a main reflector/and a supporting structure 4. The main reflector 1 is mounted so that it pivots about horizontal and vertical axes relative to the supporting structure 4 and, for transportation purposes, the feed 8 and sub-reflector 10 are mounted on a frame 11 which pivots about axis X2 relative to the main reflector 1. The whole assembly can thus be collapsed to fit for example in a standard container or in an aircraft fuselage; or to assist in transportation by road. <IMAGE>

Description

SPECIFICATION A transportable antenna This invention relates to a transportable antenna for satellite communication systems.

The invention arose in the design of a road transportable terminal. A previous design had incorporated a circular antenna reflector of three metres diameter which, whilst large for the purposes of road transport, presented no insurmountable problems in this respect. However, with the increasing number of communication satellites it has now become necessary to use more highly directional antennas to prevent interference between different satellite communication systems.

In order to meet the requirement for improved directionality a four metre diameter reflector was initially considered necessary but it soon became apparent that this could not be transported by road because of height limitations imposed by bridges and other overhead obstructions. Similar difficulties arise with air transport where a height limitation is imposed by the shape of the aircraft fuselage.

By employing this invention it is possible to provide a compact arrangement to improve the ease with which the antenna can be transported e.g. by road if the whole assembly is formed as part of a road trailer or vehicle, or in a standard container, or in an aircraft fuselage.

Thus, according to the invention there is provided a transportable antenna comprising a supporting structure, a main antenna pivotted relative to the supporting structure about orthogonal axis, a sub-reflector, a feed, and a supporting frame carrying the sub-reflector and pivotted relative to the reflector so as to enable the sub-reflector to be pivotted from an operational position where it is spaced from the main reflector to a position for transportation where it is located relatively close to the main reflector.

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 d, 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 d, 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 axis 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 d1 than in the dimension d2.

It is desirable to accomplish this because the greater taper in direction d1 will result in a relatively lower level of sidelobes, while the lesser taper in dimension d2, whilst resulting in higher sidelobes, assists in maintaining the highest possible directionality from the compiete aperture.

The purpose of the shielding device 9, supported between the horn 8 and reflector 10 on struts 1 1A forming part of the framework 11, it 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 subreflector 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 17 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 11B 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 tranceiver 5 via line 5A.

Because of the highly directional nature of the transmitted beam in azimuth coupled with 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 permanently 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 (5)

1. A transportable antenna comprising a supporting structure, a main antenna pivotted relative to the supporting structure anout orthogonal axis, a sub-reflector, a feed and a supporting frame carrying the sub-reflector and pivotted relative to the reflector so as to enable the sub-reflector to be pivotted from an operational position where it is spaced from the main reflector to a position for transportation where it is located relatively close to the main reflector.

2. An antenna according to Claim 1 in which the feed is also carried on the frame.

3. An antenna according to Claim 1 or 2 in which a shielding device is mounted on the frame between the feed and the sub-reflector so as to obstruct stray radiation from a feed which would otherwise miss the sub-reflector.

4. An antenna according to Claim 3 in which the shielding device includes a frustoconical surface tapering towards the subreflector.

5. An antenna according to any preceding Claim comprising a flexible waveguide connected to the feed.