GB2191045A - Dipole antenna - Google Patents

Abstract

In a triplate fed dipole antenna 4, each arm 10, 12 of the dipole 4 is formed by bending one of the ground planes 2, 3 of the triplate 1. The two arms 10, 12 are electrically linked by a connector 14 between the two ground planes 2, 3. A ground reflector 17 is provided to produce a unidirectional radiation pattern. <IMAGE>

Description

SPECIFICATION An antenna This invention relates to an antenna having a triplate feed connected to a dipole.

A known antenna of this type comprises an array of dipoles fed from a common triplate, the two arms of each dipole being formed by precision machining of the edges of the ground planes of the triplate. This machining process can be a major exercise particularly when manufacturing an antenna having an array of many hundreds of dipoles.

A dipole constructed in the manner described above is the subject of patent specification 2048571 and incorporates dipole arms (4) & (5) in the plane of the triplate to radiate or receive radiation polarized in that plane. It is sometimes required that a dipole should be orientated in a different plane perpendicular to the triplate so that it will radiate or receive radiation polarized in that different plane. The structure described in patent specification 204571 incorporates dipoles (16,20) for that purpose so that in co-operation with dipole (4,5) it transmits or receives circularly polarized radiation if fed correctly. The first dipole, parallel to the triplate, is formed by a T shaped projection divided by an accurately machined slot 7 of length approximately A/4.

The second dipole, perpendicular to the triplate, is formed by rods 16,20 which are similarly separated by the slot 7. A 900 relative phase shift between the feeds to the respective dipoles is provided by machining further slots 10 of length A/8. These provide the 90" phase difference between dipoles required for circular polarization. It is notable that the production of the second dipole is more complex than the first dipole since it requires the manufacture and fitting of the rods and of slots 10 in addition to the machining of projections 8 and 9 and slot 7. Another problem with the construction shown in specification 2048571 is the need for the bushes (15, 21) to connect the outer and inner plates. It may be necessary to make and assemble thousands of these which adds significantly to the cost of assembly.Furthermore, for operation at high frequencies the size of the bushes need to be very small making the art of assembly virtually impossible and making it difficult to achieve the required tolerances and to achieve the required electrical connections.

This invention provides an antenna comprising: a triplate feed having a centre conductor and two outer conductors and two extensions secured to or integral with respective outer conductors each extension having a first arm connected to an outer conductor and a second arm which lies in a plane transverse thereto the second arms co-operating to form a dipole, the first arms being connected together at a position spaced from the second arms, and the centre conductor being connected or arranged or feed energy to the dipole.

By employing the invention it is possible to construct a dipole in a particularly simple way at correspondingly low cost e.g. by bent extensions from the two ground planes. In its simplest form this will notably result in a dipole whose arms extend in a plane perpendicular to the triplate, notably meeting the function of the more complex "sound" dipole 16, 20 of specification 2048571. If it is required, in an antenna employing the invention to include dipoles extending also in the plane of the trip late, these can be of conventional form, the dipoles of each type being distributed over an aperture of the antenna in a manner described in our application No.

8612907.

The extensions can be an integral part with the outer conductor being formed by cutting or machining the latter or can be separate conductive members, preferably of sheet material, secured conductively to the ground planes. In the latter case they could be preformed, e.g. cast with the required bend, though it is preferred, for simplicity, that they may be made of flat sheet material which is subsequently shaped by a bending operation.

The "first" arms and the connection between them is intended to provide a balun transformer. This is done without the need to machine accurate slots like those shown at 7 in specification 2048571. The connection between the first arms can be provided by a conductive member which is adjustable so as to vary the length of the balun transformer thereby tuning the dipole to a desired frequency.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a front elevation of a part of an antenna constructed in accordance with the invention, showing one dipole connected to a triplate feed; Figure 2 is a plan view of the part of the antenna shown on Fig. 1; and Figure 3 is a cross-section through the line X-X of Fig. 2.

Referring to the drawings, a triplate 1 comprises two outer conductors 2 and 3 and a number of centre conductors forming a feed network connected to a linear array of dipoles. Only one dipole 4 and its associated centre conductor 5 is shown. The outer conductors are separated from the centre conductor by a low loss dielectric block 6.

The outer conductors 2 and 3 extend to form two extensions 7 and 8 of identical size and shape. Each extension 7 and 8 is bent outwards at ninety degrees so that extension 7 forms a first arm 9 and a second arm 10 while extension 8 forms a first arm 11 and a second arm 12. The second arm 10 and 12 are identical in size and shape, as are the first arms 9 and 11. The two second arms 10 and 12 act as a dipole.

The two second arms 10 and 12 are able to act as a dipole because the centre conductor 5 of the triplate structure is connected to second arm 10 at a joint 13 and because the two second arms 10 and 12 are electrically linked by short-circuiting spacers 14 to form a balun transformer.

Signals are provided to the triplate line through a connector 16.

To provide a unidirectional radiation pattern a ground reflector 17 is positioned below the dipole.

For accurate tuning it is simple to arrange each of the short-circuiting spacers 14 to be moveable along, for example, a pair of slots 18 cut in second arms 8 and 9 so that they can be moved to an optimum position and then fixed in place.

The profile of the arms 10, 12 of the illustrated dipole was selected to give a wide frequency band of operation but it will be understood that any other suitable profiles could be used in other embodiments of the invention.

Claims (5)

1. An antenna comprising: a triplate feed having a centre conductor and two ground planes; and two extensions secured to or integrai with respective ground planes each extension having a first arm connected to a ground plane and a second arm which lies in a plane transverse thereto the second arms co-operating to form a dipole, the first arms being connected together at a position spaced from the second arms, and the centre conductor being connected or arranged to feed energy to the dipole.

2. An antenna as claimed in claim 1 where each extension is formed from a sheet of conductive material formed into said first and second arms by a bending operation.

3. An antenna as claimed in claim 2 where said sheet of conductive material is integral with one of the ground planes.

4. An antenna as claimed in any preceding claim where the first arms are connected together by connecting means that can be moved about the first arms so as to vary the distance between the connecting means and the second arm.

5. An antenna substantially as illustrated in Fig. 2 of the accompanying drawings and substantially as described with reference thereto.