GB2062357A - Antenna arrangements - Google Patents

Abstract

An antenna arrangement which is suitable for use in an earth station which forms part of a geostationary satellite communication system has a main reflector 8 mounted so as to be rotatable about an hour angle axis 6 which is fixed relative to the ground, and about a declination axis 10 which is fixed relative to the main reflector. An hour angle axis is substantially parallel to the earth's axis of rotation. An intermediate reflector 19 is mounted so as to be rotatable about the declination axis and it serves to reflect beam waveguided energy between a horn 40 and the main reflector, by way of the axis 6, an aperture 22 in reflector 8, and by way of a sub-reflector 21. <IMAGE>

Description

SPECIFICATION Antenna arrangements This invention relates to antenna arrangements and is particularly applicable to earth stations, which form part of a satellite communication system. In order to provide sufficiently good signal reception at a ground station, the antenna is usually physically very large and can take the form of a dish reflector. The reflector must be firmly mounted to enable it to withstand high wind speeds and in certain designs it has proved necessary to suspend normal operation of the earth station during gales so that the antenna can be moved to a rest position in which it can be securely clamped down.

The present invention seeks to provide an improved antenna arrangement, which is of a particularly robust form but which does not impair the electrical function which it has to perform.

According to this invention, an antenna arrangement includes a main reflector surface mounted so as to be rotatable about two mutually perpendicular axes, one axis being approximately an hour angle axis which is fixed relative to the ground, and the other axis being a declination axis which is fixed relative to said reflector; and an intermediate reflector mounted so as to be rotatable about said declination axis, said intermediate reflector forming part of a beam waveguide signal path from the main reflector to a signal receiving-transmitting means located on said hour angle axis.

An hour axis is one which is substantially parallel with the earth's axis of rotation. A distant object such as a star can be tracked by rotating a telescope (optical or radio) about only a single axis to allow for the rotation of the earth about its own axis. If the distant object is sufficiently far from the earth as to be effectively at infinity, the required axis of rotation is exactly parallel to the earth's axis of rotation. However, in the case of a communications satellite which is in orbit about the earth's equator, the said one axis will be angularly off-set very slightly from the earth's axis to minimise the declination motion required to enable the antenna to point to other satellites in synchronous orbit.

In the present invention the source of signals which feeds the antenna (and the means which receives signals when the antenna is operating in a receiving mode) can be static and mounted rigidly to the ground. In such a case the beam wave-guide path to the means for receiving the signals is aligned with the hour angle axis over part of its length. The angle which said intermediate reflector makes with said hour angle axis is a function of the angular position of the main reflector about said declination axis.

The invention is further described by way of example with reference to the accompanying drawings in which, Figure 1 is an explanatory diagram, Figure 2 illustrates an antenna arrangement in accordance with the present invention and, Figures 3 and 4 illustrate in greater detail portions of the antenna arrangement.

Referring to Figure 1, a geostationary satellite 1 is depicted in synchronous orbit around the equator 2 of the earth 3 with the earth's axis of rotation being indicated by the line 4. The antenna arrangement 5 is located at a latitude which is displaced from the equator and consequently possesses an hour axis 6, which is angularly offset from the axis 4 by a very small angle 6. For a given satellite 1 the movement of the antenna arrangement about the axis 6 will be very small provided that the satellite is truly synchronous.

Similarly, the variation in the declination angle v will also be very small. However, if the same antenna arrangement is to be used from time to time in conjunction with a different synchronous satellite located at a different position around the equatorial orbit, it is merely necessary to rotate the antenna arrangement about its hour axis 6 and to make small adjustments to the declination angle 97. Since the declination movement needed can be achieved by means of a short jack the construction of the antenna arrangement may be particularly robust and simple.

One embodiment of the antenna arrangement in accordance with the present invention is illustrated diagrammatically in Figures 2, 3 and 4.

A concave main reflector 8 is mounted by means of a pair of bearing stubs 9 so as to be rotatable about the declination axis 10 (which is perpendicular to the plane of the paper in Figure 2). The main reflector 8 is rotated over a small angle about the declination axis 10 by means of a declination jack 1 which controls the distance between the attachment point 12 on the main reflector 8 and a point 13 on the frame 1 6 on which the main reflector 8 is supported. The transmitting and receiving circuits of the antenna arrangement are located in a housing 1 5, which is secured in a non-movable manner to the ground or frame 14 whith the housing being below an hour axis 6.A microwave horn 40 is mounted rigidly to the frame 1 6 so as to lie exactly on the axis 6 and the horn is coupled via a rotatable microwave joint (not shown) to the housing 1 5.

The horn 40 transmits and receives a guided beam of microwave energy (termed a beam waveguide) along the hour axis 6. An intermediate reflector 1 9 is mounted so as to be rotatable about the declination axis 10, so that its normal bisects the angle between the hour axis 6 and a line 20 which is the axis of symmetry of the main reflector 8.

The signal which is received by the antenna arrangement is passed from the main reflector 8 to the horn 40 by means of a beam waveguide signal feed, the energy being transmitted as a directional beam of microwave energy. Conversely when a signal is to be transmitted, the microwave beam is sent from the horn 40 along the hour axis 6 to the intermediate reflector 19, at which it is reflected along the normal 20 and via a subreflector 21 to the main reflector 8. The main reflector 8, of course, transmits the microwave beam in the required direction towards the satellite. The sub-reflector 21 may be of a conventional nature and preferably both the sub reflector 21 and the size of the aperture 22 in the reflector 8 through which the microwave beam is transmitted, are as small as possible.

The intermediate reflector 1 9 is mounted so that as the main reflector 8 is rotated about the declination axis 10 under the action of the declination jack 11. its normal always bisects the angle between the hour axis 6 and the main reflector axis of symmetry 20. This aspect is illustrated in greater detail in Figure 4. In the arrangement shown in Figure 4, the intermediate reflector 1 9 is provided with a tiller arm 23 which is rigidly fixed to the reflector 1 9 so as to be always normal to its reflecting surface. The arm 23 is located by a slider 24 attached to two arms 25 and 26, which are pivoted at points 27 and 28, 1 7 and 1 8 respectively.By means of this mechanism the angle of the intermediate reflector 1 9 is automatically corrected as the main reflector 8 is rotated about the declination axis 10 so that a signal is always passed from the horn 40 to the sub-reflector 21 (and vice versa). Any movement about the declination axis 10 is expected to be fairly small in practice.

It will be seen that the frame 14 can be designed in a robust and rigid form and the main reflector 8 can be rotated about the hour axis 6 by means of an hour angle jack 30 attached to a balance weight 31. The balance weight 31 can also be designed with aerodynamic considerations in mind so as to enhance the stability of the structure even in gale force winds.

Although the intermediate reflector 1 9 has been depicted as a flat plane surface, it may be shaped as necessary to enhance the transmission properties of the beam waveguide signal feed. In particular, it may be curved so as to enhance the focusing properties of the microwave beam.

Additionally, the surface of the intermediate reflector 1 9 can be shaped so as to compensate for distortions in the surface of the main reflector 8 or that of the sub-reflector 21.

Although the antenna feed 40 has been described as a horn, alternative sources of radio frequency beam radiation could be used instead.

The actual profile of the reflector 1 9 can be trimmed during a setting up procedure so as to enhance the directional properties of the antenna arrangement.

Claims (9)

CLAIMS:

1. An antenna arrangement including a main reflector surface mounted so as to be rotatable about two mutually perpendicular axes, one axis being approximateiy an hour angle axis which is fixed relative to the ground, and the other axis being a declination axis which is fixed relative to said reflector; and an intermediate reflector mounted so as to be rotatable about said declination axis, said intermediate reflector forming part of a beam waveguide signal path from the main reflector to a signal receivingtransmitting means located on said hour angle axis.

2. An antenna arrangement as claimed in claim 1 and wherein said main reflector is concave and carries a convex sub-reflector mounted above an aperture in the reflecting surface of the main reflector, the aperture being aligned with said subreflector and said intermediate reflector.

3. An antenna arrangement as claimed in claim 1 or 2 and wherein said means comprises a microwave horn.

4. An antenna arrangement as claimed in claim 1,2 or 3 and wherein transmitting and receiving circuits associated with said means are located below said hour angle axis.

5. An antenna arrangement as claimed in any of the preceding claims and wherein said intermediate reflector is positioned at a variable inclination to said hour angle axis which is related to the declination angle of said main reflector.

6. An antenna arrangement as claimed in claim 5 and wherein said hour angle axis intersects said declination axis, with the intermediate reflector being located at the point of intersection.

7. An antenna arrangement as claimed in any of the preceding claims and wherein the reflecting surface of said intermediate reflector is flat.

8. An antenna arrangement as claimed in any of claims 1 to 6 and wherein the reflecting surface of said intermediate reflector is concave.

9. An antenna arrangement substantially as illustrated in and described with reference to Figures 2, 3 and 4 of the accompanying drawings.