GB2171257A

GB2171257A - A dipole array

Info

Publication number: GB2171257A
Application number: GB08432186A
Authority: GB
Inventors: Richard Garwood Wash
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1984-12-20
Filing date: 1984-12-20
Publication date: 1986-08-20
Also published as: GB2174251A; GB2174251B; GB8523076D0; GB8531268D0; GB8432186D0

Abstract

An antenna formed by an array of dipole elements 7 fed by a triplate or stripline system is provided with earthed posts 14 between the dipoles. These posts act as shields preventing radiation from one dipole being received by the paths 9, 10, 11, 12, 13 of the feed lines feeding the adjacent dipoles. The effect is to improve significantly the efficiency of the antenna. A mode suppressor 6D couples a coaxial feed to the triplate and improves impedance matching. <IMAGE>

Description

SPECIFICATION A dipole array This invention relates to an antenna comprising an array of dipoles.

The purpose of forming an antenna from such an array is to enable energy to be distributed to the individual elements so as to achieve required distributions of phase and amplitude over the aperture of the antenna thereby obtaining a required beam shape and direction.

A problem with known dipole arrays is that 'mutual coupling' occurs between the elements. Mutual coupling is the direct transmission of energy from one element to an adjacent element. It has the undesirable effect of increasing the reflection co-efficient of each element i.e., reducing the amount of radiated energy and increasing the amount reflected back along a transmission line leading to the element.

This invention provides an antenna comprising an array of dipoles characterised by conductive shielding means positioned between them to reduce coupling therebetween.

The invention is considered to be of particular application to antenna structures of the type in which the dipoles are formed on the ends of projections extending from and distributed along one edge of a stripline or triplate structure for feeding energy to the dipoles. In such an arrangement the shielding means can conveniently be formed by protrusions from the said edge and preferably from a conductive layer or layers forming part of the stripline or triplate structure. The projections and the dipoles can similarly be formed from further extensions of the same conductive layer or layers. In one arrangement the dipoles form the arms of T shaped extensions of which the legs are formed by the aforementioned projections.

The length of the protrusions is of some significance. They must not be so long as to interfere with radiation from the dipoles in the desired directions i.e., away from the antenna. They must however be sufficiently long to reduce substantially the amount of radiation received by one dipole from adjacent dipoles. Experiments have shown that, ideally the protrusions terminate at positions approximately directly between i.e., in line with the dipoles.

One way in which the invention may be performed will now be described with reference to the accompanying drawings in which: Figure 1 is a side view of one of a number of vertical triplate systems constructed in accordance with the invention and shown with one of its earth planes and one of its dielectric sheets removed to reveal the central conductors; and Figure 2 is a cross-section through the line XX of Figure 1.

Referring to the drawings, the illustrated triplate structure comprises two identical earthed conductive sheets 1 and 2 forming the earth planes, one of these being removed in the case of Figure 1. Between the earth planes 1 and 2 are conductive strips 3 separated from the sheets 1 and 2 by insulating layers 4 and 5 of foam plastics material. Layers 1, 2, 4 and 5 are connected together by bolts, (one of which is shown at 6E) arranged to establish electrical contact between the earth planes 1 and 2.

Energy to be transmitted is fed from a co-axial line (not shown) to a co-axial socket 6 shown in Figure 2. This socket comprises an earthed outer bush 6A in electrical contact with the earth plane 1 and a central pin 6B connected electrically to the conductive strips 3 through a circular hole 6C in the earth plane 1. In the illustrated arrangement it is notable that the hole 6C is in fact defined by a rebated portion of the socket 6A which fits into the sheet 1 and therefore forms part of the earth plane.

A mode coupler 6D is clamped by bolts such as that shown at 6E between the ground planes 1 and 2 and forms an electrical connection between them. The mode coupler 6D is U shaped and extends around the pin 6D on three sides. Its inner surface has a semi-circular part 6F whose centre of curvature 6G lies on the conductive strip 3 leading directly to the pin 6B and is offset from the position where it joins the pin 6B. This offset, which contrasts with known similar arrangements in which the centre of curvature is coincident with the pin 6B, is such that the curved surface 6F, which is of greater radius than the hole 6E just touches the edge thereof and has a common tangent therewith.

The curved portion 6F of the inner surface of the mode supressor 6D merges into straight portions 6G which extend parallel to the part of the line 3 leading to the pin 6B.

The purpose of the mode suppressor 6D is to create in cooperation with the earth planes 1 and 2 and the central conductor 3, a portion of transmission line which is unable to sustain unwanted modes generated at the transition point between the co-axial line and the triplate. The offset arrangement of the illustrated mode suppressor additionally provides an improved impedance match between these two portions of the path.

From the co-axial socket 6 energy is transmitted along circuitous paths to each of an array of dipole elements 7. The routes to the dipoles are arranged to feed energy so that it arrives at the dipoles with a desired phase and amplitude distribution.

Each dipole is formed by arms (each a quarter of a wavelength long) of two T shapes. The leg of each T shape also approximately a quarter of a wavelength long and is formed by a projection from an edge (e.g., edge 1A) of one of the ground planes 1 or 2. The arms of each T are separated by a slot 9 which extends from its open end to a closed end in the leg 10 of the T shape near where it joins the edge, e.g., 1A, of the ground plane 1 or 2.

The conductive strips 3 forming the feeds, terminate at each T shape in a U shaped portion which has: part Il a quarter wavelength long extending along the leg 10 on one side of the slot 9; a part 12 extending across the slot immediately between the dipoles 10A and 10B formed by the arms of the T; and a part 13 which is also a quarter wavelength long and extends back along the leg 10 on the op posite side of the slot to its free end which is just before the closed end of the slot 9. The U shaped portion of a feed strip 3 in co-operation with the leg of the associated T shape, split by the slot 9, forms a balun whose effect is to feed energy to the dipoles so that current always flows in the same direction in the two halves 10A, 10B of the dipole.

Between each dipole 7 is a post 14 formed by protrusions from the ground planes 1 and 2. The free ends of these protrusions 14 lie directly between the arms 10A and 10B formed by the dipoles. The effect of the protrusions 14 is to prevent a substantial amount of mutual coupling between adjacent dipoles thereby reducing the reflection coefficient and improving efficiency.

It will be appreciated that the illustrated structure has been described only as an example of one way in which the invention can be performed. In an alternative configuration the triplate structure illustrated could be replaced by a stripline energy feeding system or indeed by waveguides or co-axial cables. Another modification would be to replace the posts 14 by plates extending at rightangles to the ground planes. Also, although the length of the protrusions 14 are preferably such that their free ends are in line with the dipoles this is not essential and a substantial advantage can be obtained by making them as little as half the length of the projections 10. Another possibility would be to use two or more protrusions between each pair of dipoles. Where only one protrusion is used it is preferably positioned centrally between the dipoles but this is not essential and an offset configuration could also be used.

Claims

1. An antenna comprising an array of dipoles characterised by conductive shielding members positioned between them to reduce coupling therebetween.

2. An antenna according to claim 1 in which the dipoles are formed on the ends of projections extending from and distributed along one edge of a stripline or triplate structure which feeds energy to the dipoles, the shielding members being formed by protrusions from the said edge.

3. An antenna according to claim 2 in which the shielding members are formed by protrusions from a conductive layer or conductive layers of the stripline or triplate structure.

4. An antenna according to claim 3 in which the projections and dipoles form further extensions of the said conductive layer or layers.

5. An antenna according to claim 4 in which the dipoles form the arms of T shaped extensions of which the legs are formed by the said projections.

6. An antenna according to claim 5 in which the protrusions end at positions approximately in line with the dipoles formed by the arms of the T.

7. An antenna substantially as described with reference to the accompanying drawings and substantially as illustrated therein.