GB2132026A

GB2132026A - Antenna systems

Info

Publication number: GB2132026A
Application number: GB08331965A
Authority: GB
Inventors: Granville John Cooper; Ronald William Elston
Original assignee: Mcmichael Ltd
Current assignee: Mcmichael Ltd
Priority date: 1982-12-03
Filing date: 1983-11-30
Publication date: 1984-06-27
Also published as: GB8331965D0

Abstract

A microwave antenna system comprises a primary antenna (12) and a reflector formed by a plurality of paraboloidal reflecting surfaces (S1 to S9) on the major face of a rectangular plate (11), the reflecting surfaces lying within one another. When the plate (11) is mounted against a vertical surface, the arrangement enables the antenna system to receive radio waves from a direction inclined to the normal to that surface. <IMAGE>

Description

SPECIFICATION Antenna systems This invention relates to antenna systems. It is more particularly, but not exclusively, concerned with antenna systems for use at microwave frequencies which, for the purpose of this specification is to be taken as wavelengths less than 100 millimetres.

For microwave reception, it is well known to use a reflector having a paraboloidal reflecting surface which serves to reflect incident electromagnetic waves propagated parallel to the reflector axis to a primary antenna located on that axis at the focus.

However such reflectors usually have an axial dimension in the region of 30 to 40% of their radial dimensions and one object of the present invention is to provide a new construction of antenna system in which the axial dimension is reduced.

According to the present invention, an antenna system comprises a primary antenna and a reflector having a plurality of reflecting surfaces each of which has the shape of part of a paraboloid and which are nested within one another, the said surfaces all having a common focus at which the primary antenna is located and those surfaces being arranged so that electromagnetic waves of a particu larfrequency received by the antenna system form a predetermined direction during use of the system arrive at the primary antenna in phase after reflection by the individual reflecting surfaces.

Preferably the reflector is of generally plate-like form having the said plurality of reflecting surfaces on one major face thereof. In this case, the said predetermined direction may be at an acute angle to the general plane of the reflector.

According to a feature of the present invention, an antenna reflector of generally plate-like form has on one major face thereof a plurality of reflecting surfaces each of which has the shape of part of a paraboloid and which are nested within one another, the said surfaces all having a common focus and being arranged so that, during use, electromagnetic waves of a particular frequency and from a predetermined direction inclined at an acute angle to the general plane of the reflector are in phase at the focus after reflection by the individual reflecting surfaces.

One example of an antenna system in accordance with the invention will now be described with reference to the three Figures of the accompanying drawing in which: Figure 1 is an explanatory diagram, Figure 2 shows part of the complete antenna system, and Figure 3 is a further explanatory diagram.

The idea behind the invention will be apparent from Figure 1. Referring now to this Figure, it shows a cross-section through a reflector 1, having three annular reflecting surfaces 2,3 and 4 which nest within one another and each of which is in the shape of art of a paraboloid. The surfaces 2,3 and 4 all have a common focus 5 and lie between two relatively closely spaced planes 6 and 7. It follows therefore that incident waves 8 falling on the reflector 1 are all directed to the focus 5 after reflection by the surfaces 2,3 and 4.The reflecting surfaces 2,3 and 4 are arranged so that the total lengths of the different wave-paths to the focus 5 differ by integer numbers of wavelengths with the result that the waves after reflection by the surfaces 2,3 and 4 all arrive at the focus 5 in phase.

In Figure 1 the focus 5 lies on the reflector axis 9 which is perpendicular to the planes 6 and 7.

However by modification of the surfaces 2,3 and 4, the focus of those paraboloidal surfaces can be offset from the axis 9 as is the case of the embodiment of the antenna system incorporating the invention now to be described.

Referring to Figure 2 of the accompanying drawing, the antenna system comprises a rectangular plate 11 (only part of which is shown) having nine reflecting surfaces S1 to S9 on a major face thereof and a primary antenna 12 which may be in the form of a waveguide horn. The primary antenna 12 is carried on a stalk 13 projecting fom the plate 11 and this stalk 13 may contain a waveguide or other transmission line for passing received signals to a suitable radio receiver or amplifier.

The surfaces S1 to S9 are again paraboloidal having a common focus at which the primary antenna 12 is located. The stalk 13 may conveniently lie on the focal axis of the surfaces S1 to S9.

The plate 11 may itself be of metal so as to provide the required reflecting surfaces S1 to S9. Alternatively the plate 11 may be non-metallic, e.g. of a plastics material, with the reflecting surfaces S1 to S9 having a metal coating. The connecting surfaces 14 need not be metallic.

Figure 3 is a diagrammatic cross-section through part of the antenna system of Figure 2 showing a plurality of rays. It will be seen that the incoming waves are all directed to the focus F after reflection by the surfaces S1, S2, etc.

The plate 11 of the antenna system of Figure 2 may be mounted vertically by securing to a flat vertical surface and the system may then be capable of receiving electromagnetic waves from a direction inclined at acute angles of azimuth and elevation to the normal to that surface.

The grooves in the front of the plate 11 formed by the paraboloidal surfaces S1 to S9 and the connecting surfaces 14 may be filled with dielectric material so that the reflector structure is in the form of a rectangular slab with a flat front surface although this may necessitate some slight modification of the contours of the reflecting surfaces S1, S2 etc. The dielectric material serves to weatherproof the reflecting surfaces and slow down deterioration in an exposed environment.

Alternatively the dielectric material referred to in the last paragraph may be extended out to form a solid 'radome' containing the primary antenna 12. In this case the transmission line connected to the antenna 12 may be embedded in the dielectric material or be attached to the surface of the 'radome'.

In an alternative construction of antenna system, the reflecting structure instead of being formed by a plate having paraboloidal reflecting surfaces as previously described may be formed by a combination of a Fresnel lens in front of a plate having a flat reflecting surface. With this arrangement received waves pass through the lens twice before converging on the primary antenna.

Claims

1. An antenna system comprising a primary antenna and a reflector having a plurality of reflecting surfaces each of which has the shape of part of a paraboloid and which are nested within one another the said surfaces all having a common focus at which the primary antenna is located and those surfaces being arranged so that electromagnetic waves of a particularfrequency received by the antenna system from a predetermined direction during use of the system arrive at the primary antenna in phase after reflection by the individual reflecting surfaces.

2. A system as claimed in Claim 1, in which the reflector is of generally plate-like form having the said plurality of reflecting surfaces on one major face thereof.

3. A system as claimed in Claim 2 in which the said predetermined direction is at an acute angle to the general plane of the reflector.

4. An antenna reflector of generally plate-like form having on one major face thereof a plurality of reflecting surfaces each of which has the shape of part of a paraboloid and which are nested within one another, the said surfaces all having a common focus and being arranged so that, during use, electromagnetic waves of a particular frequency and from a predetermined direction inclined at an acute angle to the general plane of the reflector are in phase at the focus after reflection by the individual reflecting surfaces.

5. An antenna system comprising a reflector as claimed in Claim 4 and a primary antenna at said focus.

6. A system as claimed in any one of Claims 2,3 or 5, or a reflector as claimed in Claim 4, in which grooves formed in said one major surface by said reflecting surfaces and surfaces connecting those reflecting surfaces are filled with dielectric material such that the reflector is in the form of a rectangular slab with a flat front surface.

7. A systems as claimed in any one of Claims 2,3 or 5, in which grooves formed in said one major surface by said reflecting surfaces and surfaces connecting those reflecting surfaces are filled with dielectric material, and in which said dielectric material is extended out to form a solid 'radome' containing said primary antenna.

8. An antenna system comprising a primary antenna and a reflector, the reflector being formed by the combination of a Fresnel lens in front of a plate having a flat surface, the arrangement being such that electromagnetic waves of a particular frequency received by the antenna system from a predetermined direction during use of the system pass through the lens twice before converging on the primary antenna where they arrive in phase.

9. An antenna system substantially as herein described with reference to the accompanying drawing.