Antennae

Abstract: An antenna (10) comprises feed lines (11,12), a conducting plate (13) and an overlying conducting element (14). The plate (13) defines a space (16) and dipole structures (17) which project into the space. The dipole structure (17) are arranged in orthoginal sets and define a common gap (21) between their ends.

Description: This invention relates to a dual polarisation antennae. In these days of satellite broadcasting and large mobile phone usage, there is an ever-increasing need for antennae which radiate and receive dual polarised radiation and which have a simplicity of manufacture and a discreet appearance. Considerable work has been done, particularly in the field of so-called slot antennae, but almost all designs have required a significant number of layers of components or they have had other disadvantages such as a peculiar lack of symmetry or limited band widths. EP-A-0243289 (ETAT FRANCAIS MINISTRE DES PTT) describes an antenna utilising two thick folded dipoles fed by a three plate signal feeder including a reflector to radiate at a particular design wavelength. From one aspect the present invention consists in a dual polarisation antenna including a non-conducting space surrounded by a ground plane, two angularly offset sets of dipole structures penetrating into or overlying the space, each set having an orientation and comprising a pair of aligned short circuit elongate dipole structures extending from the ground plane into or over the space from diametrically opposed directions to terminate in respective free ends such that their free ends are adjacent but spaced from each other to define a gap between them; separate means for exciting each set, or dipole structure within a set, individually and a radiating element overlying the dipole structures such that the dipole structures couple, in use, with the radiating elements, causing said radiating elements to radiate polarisations determined by the orientation of each of said set of dipole structures As is well known antennae which transmit also receive in a reciprocal manner and any terminology in this specification which implies or requires transmission is to be understood as including the corresponding receiving function. The dipole structure may be constituted by a short-circuit dipole. Alternatively, each dipole structure may comprise a conducting element extending from the ground plane and a pair of parallel monopoles extending from the free end back along respective sides of the conducting element. In that case the conducting element may be connected to the ground plane at a voltage node. Preferably the gap between the dipole structures is common to each set. The dipole structures may be continuous with that ground plane. Thus, for example,the ground plane and dipole structures may be in the form of a deposited metallic conducting layer on the surface of an insulating support, which can be planar, and the space may be an aperture in that layer which can conveniently be formed by etching. In certain arrangements it may be desirable to have the dipole structures in a separate plane from the ground plane so that they overlie, rather than penetrate, the space. In this and other context the word "overlie" is intended to cover the circumstances where one thing is either above or below the other and the term is not affected by the particular orientation. It is particularly preferable that the dipole structures are symmetrically disposed within the space and indeed that the space, radiating element and dipole structures are symmetrical about the intended planes of polarisation. Thus conveniently the space and/or the radiating element may be circular, square or polygonal. In this arrangement the radiation phase centres of the sets of dipole structures should be coincident, but any other configuration which achieves this coincidence is also desirable. For most purposes it is expected that the sets of dipole structures will be orthogonal. It is envisaged that the dipole structures will act at one quarter wave resonance,or multiples thereof, and hence may consist of a narrower strip about a one quarter wave length long, at the central desirable operating frequency. It will be excited by applying a voltage from the free end either to the ground plane or to the opposite similar dipole structure in the set. For the short circuit dipoles, the free end will be a voltage antinode, in these circumstances, whilst the grounded end will be a voltage node. In transmission mode, the dipole structures can be excited in a number of ways for example at least one exciting means may comprise a feed line extending along, but spaced from, a first of the dipole structures in its set, across the gap and along, but spaced from, a part of the second dipole structure to form an open circuit stub. In many arrangements this feed line will be in a different plane to the dipole structures, but in at least one configuration the feed line may be co-planar with the dipole structures, in which case each dipole structure may be in the form of parallel probes and the feed line may extend between them to form a co-planar wave guide feed arrangement. The open circuit stub may be tuned to be short circuit at the intended operating frequency and the feed line may be connected to one or both dipole structures by a probe. Conveniently the feed line can be microstrip or stripline in many embodiments. One alternative is a coaxial feed whose outer conductor is connected to a first of the dipole structures in its set and whose inner conductor is connected to the second dipole structure in that set. Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above or in the following description. The invention may be performed in various ways and specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic exploded view of an antenna according to the invention; Figures 2 to 7 show a view from above at a and a sectional view at b of a number of different ways of exciting the antenna of Figure 1 (a single polarisation excitation means is shown, for clarity, in each case, the other corresponds); Figure 8 is a view from above illustrating a further means of excitation; and Figure 9 is a view from above of an alternate form of an antenna. Referring to Figure 1 an antenna 10 comprises feed lines 11, 12 which are fed from frequency sources (not shown) A and B; a conducting plate 13 mounted on a planar non-conducting element (not shown) and an overlying radiating patch or element 14. The conducting plate is etched away at a central portion 15 so that it effectively defines a non-conducting rectangular space 16 into which project dipoles 17. The dipoles structures 17, which are constituted by short circuit dipoles 17a, are arranged in generally orthogonal sets 18, 19, each of which comprises a pair of dipoles 17a which extend into the space 16 from diametrically opposed directions such that their free ends 20 are adjacent, but spaced from each other, to define a gap 21 between them. It will be seen that the arrangement of space 16, dipoles 17a and patch or radiating element 14 is symmetrical about the longitudinal axes of the dipoles 17a, which, as will be seen below, correspond with the plane of polarisation of the dipoles. Thus the feed lines 11, 12 extend along, but are spaced from, a first of the dipoles in each set 18, 19, across the gap 21 to terminate adjacent the far end of the other dipole 17a in the set 18, 19 so that the feed lines form open circuit stubs tuned to short circuit at the intended operating frequency of the antenna. It will also be noted that the dipoles 17a are each connected to the main body of the conducting plate 13 which is earthed to form a ground plane. It is preferable that the dipoles are a one quarter wave length long, at the operating frequency. When the feed lines 11, 12 receive respective signals an exciting voltage is induced across the free ends of the dipoles in the respective set so that the free end is a voltage anti-node whilst the ground end is a node. Each set of dipoles 18, 19 couples with the patch to cause dual polarised radiation as indicated at 22. As has been mentioned previously it is desirable that the space 16, the dipoles 17a and the patch 14 are symmetrical about the polarisation planes and hence the space and patch are conveniently symmetrical geometrical shapes such as squares, circles etc. Turning to Figures 2 to 7, each illustrates a different way of exciting the antenna of Figure 1 but essentially using the principles outlined above. For clarity only one polarisation is illustrated. Thus Figure 2 indicates more clearly the arrangement of Figure 1 and shows the feed line 11 being mounted on one side of a dielectric plate 23 with the ground plane and dipoles formed on the other side. In this case the feed line 11 is microstrip. In Figure 3 a stripline feed extends between a pair of ground planes which are earthed together. The conducting plate 13 may be a sheet of metal, a metal clad laminate or a flexible circuit. Dielectric foam may be used to space the components apart. Figure 4 illustrates a coaxial feed 24 whilst Figure 5 shows how the arrangement of Figure 1 can be almost entirely co-planar, other than the jumper leads 25, by using co-planar wave guide feeds. Figure 6 shows an arrangement in which the dipoles 17a are stepped away from the ground plane and this may be particularly convenient for generating a locally high impedance for matching purposes. Figure 7 illustrates how the dipoles 17a may be fed directly using a probe 26 from a microstrip feedline 11. Finally Figure 8 illustrates a method of feeding both dipoles in a set with oppositely directed feed lines 27, 28 connected in parallel to the feed line 11 in such a way that one of the feed lines 26 is one quarter of a wave length longer than the other creating an effective half wave length delay to give a 4:1 impedance transform enabling the antenna to be matched directly to low impedance feeds. It will be understood that when used as a receiving aerial the antenna operates in exactly the reciprocal manner. Figure 9 shows an analogous form of antenna using open-circuit dipoles. Thus the dipole structures 17a comprises monopoles 29 which extend back along respective sides of a conducting element 31, which is connected to the ground plane 30. This antenna may be fed and manufactured in the manners previously described.

Claims: A dual polarisation antenna (10) including a non-conducting space (16) surrounded by a ground plane (13), two angularly offset sets (18, 19) of dipole structures (17) penetrating into or overlying the space (16), each set having an orientation and comprising a pair of aligned short circuit elongate dipole structures (17) extending from the ground plane (13) into or over the space (16) from diametrically opposed directions to terminate in respective free ends such that their free ends are adjacent but spaced from each other to define a gap (21) between them; separate means (11, 12) for exciting each set, or dipole structure within a set, individually and a radiating element (14) overlying the dipole structures (17) such that the dipole structures couple, in use, with the radiating element (14), causing said radiating element (14) to radiate polarisations determined by the orientation of each of said set of dipole structures (18, 19). An antenna as claimed in Claim 1 wherein each dipole structure (17) is constituted by a short-circuit dipole (17a). An antenna as claimed in Claim 1 wherein each dipole structure (17) comprises a conducting element (31) extending from the ground plane to the free end and a pair of parallel open-circuit monopoles (29) extend from the free end back along respective sides of the conducting element (31). An antenna as claimed in Claim 3 wherein the conducting element (31) is connected to the ground plane at a voltage node. An antenna as claimed in any one of the preceding claims wherein the gap (21) between the dipole structures (17) is common to each set. An antenna as claimed in any one of the preceding claims wherein the dipole structures (17) are continuous with the ground plane (13). An antenna as claimed in any one of Claims 1 to 5 wherein the ground plane and dipole structures (17) are in the form of a metallic conducting layer (13) on the surface of an insulating support. An antenna as claimed in Claim 7 wherein the support is planar. An antenna as claimed in any one of the preceding claims wherein the dipole structures (17) are symmetrically disposed within the space (16). An antenna as claimed in Claim 9 wherein the space (16), radiating element (14) and dipole structures (17) are symmetrical about the intended planes of polarisation. An antenna as claimed in any one of the preceding claims wherein the space (16) and/or radiating element may be circular, square or polygonal. An antenna as claimed in any one of the preceding claims wherein at least one exciting means (11, 12) comprising a feed line extending along but spaced from a first of the dipole structures (17) in a first set (18), across the gap and along but spaced from, a part of the second dipole structure (17) on the first set to form an open circuit stub. An antenna as claimed in claim 12 wherein the open circuit stub is tuned to produce at its input a short circuit at the intended operating frequency. An antenna as claimed in Claim 12 or Claim 13 wherein the feed line (11) is connected to one or both dipole structures (17) by a probe (26).

Inventors: DAVIES RICHARD SIMON GREVILLE, DAVIES, RICHARD SIMON GREVILLE
Applicants: ALAN DICK & COMPANY LIMITED
Application year: 1995
Main classification: H01Q9
IPC codes: H01Q9