GB2168854A - Antenna systems - Google Patents

Abstract

An antenna suitable for use with satellite broadcasting comprises a hollow body formed of plastics material in a rotational moulding operation. The body serves as a combined radome 12 and reflector 16. A detector 20 is mounted in the wall of the body directed inwardly of the body and the opposing wall portion 16 is parabolicly shaped. The exterior of the parabolic wall portion is formed with a metallic layer 26 serving as a reflector. <IMAGE>

Description

SPECIFICATION Antenna systems This invention relates to antenna systems and in one example to satellite receiving antenna systems.

It is anticipated that direct broadcast transmissions of television and radio from satellites will become widely used within the next decade. There is therefore a requirement for an antenna system for private users which is efficient but economic in production. Antenna systems have in the past been fabricated from spun steel or aluminium or from fibreglass laminates. All of these constructions are relatively expensive.

It is of course essential for the orientation of an antenna system to be accurately controlled and the rigidity of the dish and the supporting structure must be sufficient to handle expected wind velocities. Alternatively, the dish, together with the detector supported at the focus, may be enclosed within an aerodynamically shaped radome. A dish which is provided with a radome may of course be formed with a less substantial and therefore less expensive construction, but the separate radome itself adds significantly to the total cost.

It is an object of this invention to provide an improved antenna system which can be produced economically.

It is an object of one aspect of this invention to provide an improved antenna system with a radome.

Accordingly, the present invention consists in one aspect in an antenna system adapted to operate at specified frequency band of electromagnetic radiation, comprising a hollow body formed integrally of material transparent in said frequency band; an appropriate source or detector for the frequency band mounted in the body wall and directed inwardly of the body and a layer which is reflective in the frequency band applied to a portion of the body wall opposed to the source or detector, said wall portion being shaped with the focus substantially at the location of the source or detector.

Advantageously, the body is formed of plastics material in a rotational moulding operation.

In another aspect, the present invention consists in an antenna system comprising a rigid support body formed of plastics material transparent to electromagnetic radiation at the relevant frequency, the body defining a reflection surface; a metallic layer provided on said reflection surface and wholly supported by said body; and a source or detector, the support body being disposed between the metallic layer and the source or detector.

The invention will now be described by way of example with reference to the accompanying drawing which is a section through an antenna system according to this invention.

The antenna system shown in the drawing is adapted to operate at 12 GHz and comprises a hollow body 10 which is formed from high density polyethylene in a rotational moulding technique.

This technique is well known in other applications and involves, in one example, rotating a suitably shaped and heated mould about three axes, moulding material being introduced into the mould cavity in powder form. By controlling the amount of moulding material introduced, a typical wall thickness of 8 mm is achieved.

The body comprises a forward wall portion 12 which extends smoothly from an apex 14 to a parabolic rear wall portion 16. At the apex 14 there is provided an integral mounting 18 for a feed or detector 20. Towards the base of the housing, the forward wall portion is provided with an integral mounting flange 22 which enables the antenna system to be secured to a suitable support arrangement.

The exterior surface of the rear wall portion 16 conforms to within t 0.5 mm to a parabolic shape with the focus of the parabola substantially at the location of the detector 20. This accuracy can be achieved without special precautions in a rotational molding technique. The thickness over the rear wall portion 16 is controlled at + 1.0 mm. A metallic layer 26 is applied to the external parabolic surface. This layer may be of fine copper or brass gauze or a thin layer of copper spray or a layer of conductive silver paint. Still further alternatives will occur to the skilled reader.

When used as a satellite television receiving antenna, the described system would be mounted on a roof top or other suitable location with the integral mounting flange enabling the antenna to be secured in a straightforward manner to an appropriate support. The antenna is aligned with the axis of the parabolic wall portion 16 aligned with the satellite. Radiation from the satellite will pass through the wall portion 12, into the interior of the body and through the parabolic portion 16 until it contacts and is reflected by the metallic layer 26.

Reflected radiation will pass back through the parabolic wall portion 16 and be focused on the detector 20. The particular details on the detector form no part of the present invention. In the preferred embodiment, however, the detector will be capable of receiving circular polarisation signals and will match into a standard WG 17 waveguard.

In order to achieve the desired efficiency it is thought desirable that, in the present embodiment, the reflecting layer should conform to a parabolic shape within a tolerance of + 0.5. With the use of a rational moulding process, the advantage is secured that the exterior surface of the body is determined directly by the mould and the desired tolerances present little practical differences. Examination has also been made of the effect of passage of the radiation through the wall portion. The variation of the phase delay on reflection with the thickness of the polythene support is a complex one but it is found that for certain optimum thicknesses the dependence of phase delay on thickness is very slight.It has been determined that the optimum value for the wall thickness is one half the wave length in the dielectric material at the appropriate angle of incidence which has been taken at 15". With a frequency of 12.1 GHz, this calculation gives a wall thickness of 8.3 mm. Further calculations show that a ~ 1 mm change in the thickness of the polythene wall would give rise to phase errors of t 2.5 . As these errors are likely to be random in nature, arising as they do from uncertainties as to the interior surface formed by the moulding operation, the effects of the errors are unlikely to be of significance. With other materials and different frequencies, an alternative optimum thickness would be determined.

A further factor that has been noted is that the variation in the angle at which reflected radiation is focused will lead to a variation in the thickness of dielectric material through which the radiation is transmitted and accordingly to a variation in the phase delay. Taking a range of angles from 0 to 30 , this may introduce a phase delay of approximately 18O. The effect this change with angle is to alter slightly the focal length of the reflector. It is proposed that this effect can be overcome by displacing the detector a very small distance from the mathematical focus of the reflector.

The technique of rotational molding lends itself particularly well to this application. By producing what is effectively a combined radome and dish, an antenna system is produced which can withstand normal wind velocities and yet be of relatively lightweight construction. Even if the antenna system is intended for use in a sheltered location and an aerodynamically shaped radome is not thought necessary, the fact that the source or detector is enclosed will significantly reduce problems of weather damage or fouling. Again, the technique of rotational moulding enables the hollow body to be produced integrally and at relatively low cost. The inherent stiffness of a hollow body as compared with, for example, a dish is an important advantage. By shaping the body to provide a mounting for the detector and also the fixing bracket, the overall design is further simplified.

In a modification, the reflective layer is formed on a laminate applied to the exterior of the parabolic wall portion. This laminate takes the form of a thin and very smooth film or layer of acetate or other radio transparent material. A silver layer is deposited to one face of the acetate, the very smooth surface of the acetate thus generating a microscopically smooth conducting surface for reflection. For structural reasons, a protective backing sheet may be provided over the silver layer.

The laminate is bonded to the exterior parabolic surface of the body with suitable adhesives. The acetate is then adjacent the body surface so that the described smooth reflecting surface faces the detector. The laminate may be first vacuum formed to a shape complementary to that of the body surface and then bonded in position. Alternatively, the laminate may be applied in sections to the body surface, the flexibility of the laminate enabling it to conform closely to the body contour. The slight amount of overlapping that is required to ensure electrical continuity between adjacent sections of laminate should not detract substantially from the antenna performance. Heat forming or other application techniques are also possible.

It should be understood that this invention has been described by way of example only and a variety of modifications are possible without departing from the scope of the invention.

The described use of a hollow body providing both the parabolic surface and the drag reducing cowling will be useful in cases where the reflective layer is constructed otherwise than as a laminate.

The hollow body will not always be required-to act as a drag reducing cowling; in certain applications its primary function will be to protect the feed, in others to provide stiffness to the structure. Whilst, rotational moulding enables the hollow body to beproduced integrally at relatively low costs other moulding or casting processes may be employed.

Indeed the body could be produced from two separate halves secured together. In place of the described polyethylene other materials such as PEEK or nylon may be employed. The use of a metallic layer -whether on a laminate, as a gauze or a paint or spray -which is supported on a plastics body lying between the metallic layer and the feed, will offer advantages even where the body is not hollow. The actual reflecting surface, that is to say the metallic surface adjacent the support body, is fully protected. With a support body formed by rotational moulding, there is the further convenience that the surface which defines the shape of the reflecting layer is the one most easily controlled in manufacture. Whilst the major intended use of the antenna system is to receive satellite broadcasts, other applications will exist, in the reception and transmission of other forms of electromagnetic radiation. The source or detector will be selected as appropriate and may of course comprise a wave guide or other transmission element communicating with remote equipment.

Claims (16)

1. An antenna system adopted to operate at a specified frequency band of electromagnetic radiation, comprising a hollow body formed integrally of material transparent in said frequency band; an appropriate source or detector for the frequency band mounted in the body wall and directed inwardly of the body and a layer which is reflective in the frequency band applied to a portion of the body wall opposed to the source or detector, said wall portion being shaped with the focus substantially at the location of the source or detector.

2. An antenna system according to Claim 1, wherein the body is formed of plastics material.

3. An antenna system according to Claim 12, wherein the body is formed in a single moulding or casting operation;

4. An antenna system according to Claim 3, wherein the hollow body is formed in a rotational moulding operation.

5. An antenna system according to any one of the preceding claims wherein the reflective layer comprises a metallic layer applied to said wall portion at the exterior of the body.

6. An antenna system according to Claim 5, wherein a protective layer is provided over said reflecting layer.

7. An antenna system according to any one of the preceding claims, wherein the wall thickness is maintained substantially constant over the body.

8. An antenna system according to any one of the preceding claims, wherein said wall portion is substantially parabolic.

9. An antenna system according to Claim 8, wherein the focus of said parabolic shape is displaced from the location of the source or detector by an amount calculated to compensate for the variation in phase lag on transmission through said wall portion at varying angles of incidence.

10. An antenna system according to any one of the preceding claims, wherein the body is outwardly shaped to reduce wind resistance.

11. An antenna system according to Claim 5, wherein the metallic layer is deposited on a transparent film bonded in turn to the said wall portion with the film disposed between the wall portion and the metallic layer.

12. An antenna system according to Claim 11, wherein said film is preformed to a parabolic shape before bonding to the said wall portion.

13. An antenna system comprising a rigid support body formed of plastics material transparent to electromagnetic radiation at the relevant frequency, the body defining a reflected surface; a metallic layer provided on said reflection surface and wholly supported by said body; and a source or detector, the support body being disposed between the metallic layer and the source or detector.

14. An antenna system according to Claim 13, wherein the support body is formed in a rotational moulding process.

15. An antenna system according to Claim 13 or Claim 15, wherein the thickness of the support body in the region of the reflection surface is selected so that the variation of reflection phase delay with thickness is at a minimum.

16. An antenna system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.