GB2155694A

GB2155694A - Antenna for circularly polarised radiation

Info

Publication number: GB2155694A
Application number: GB08405837A
Authority: GB
Inventors: William Benjamin Walter Alison
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1984-03-06
Filing date: 1984-03-06
Publication date: 1985-09-25
Also published as: EP0156549B1; DE3571175D1; EP0156549A1; GB2155694B; WO1985004051A1; EP0174329A1

Description

1

SPECIFICATION

Antenna for circularly polarised radiation The present invention relates to an antenna for circularly polarised radiation. Circular polarisation of RF radiation is known to be useful in various applications. For example, it is well known that 70 circular polarisation can be helpful in radar systems in discriminating desired targets from clutter, especially rain clutter. Additionally, when a circularly polarised signal is reflected from a substantially smooth sur face, such as the sea on a relatively calm day, the reflected signal becomes circularly polarised in the opposite hand. Thus, provision of an antenna system which discriminates in favour of circularly polarised signals in a particular hand can be useful in discrimi nating againstsuch signals which have been reflected from the sea. Thisfeature has particular applications in position fixing systems in which the distance of a station of which the position is to befound from transponders at known fixed positions is determined by interrogating the transponders and measuring the time delay of receipt at the interrogating station of response signals from the transponders. It can be appreciated thatthese time delay measurements can be substantially corrupted if the transponder, orthe interrogating station respondsto signals reflected from sea.

Antennas generally produce radiation which is linearly polarised on a particular axis. Circular polaris ers are known for converting these linear polarised emissions to circularly polarised signals. In general, such circular polarisers are arranged as anisotropic regions, that is regions which have differing capacita tive and inductive effects on thetransmitted radiation in each of two orthogonal directions transverse to the direction of transmission. The orthogonal directions of the anisotropic region can be described as the orthogonal axes of the region. In one known example of circular polariser, parallel plates are used inclined at 45'to the linear polarisation axis of the radiation from the antenna which isto be circularly polarised. The components of the linearly polarised radiation having electricvectors parallel and orthogonal to the parallel plates of the polariser are changed in phase dif ferentially as they pass through the polariser. Careful design of the spacing and depth (in the transmission direction) of the plates can arrange forthis phase change to be 90', thereby resulting in circular polarisa tion. It can be appreciated that the parallel plate polariser described above is essentially suitable only for signals transmitted with a substantially planar wave front in a predetermined direction. The system is not suitable for use in a compact arrangement on a wide beam antenna.

The prior art also includes a number of printed polarisers in which anisotropic arrangements of conductors are provided on insulating supporting sheets. However, with such arrangements it has hitherto been found necessary to use at least two such anisotropic sheets or layers to minimise reflections GB 2 155 694 A 1 fromthese layers resulting in serious mis-match atthe antenna. However, anisotropic layer polarisers of this kind can be shaped to correspond to the wave frontof signals from the antenna and therefore used in wide beam applications.

There is a clear needfora more compactcircular polarising arrangement providing good polarisation discrimination in compact antenna whilst affording reasonable or good matching at the antenna without excessive voltage standing wave ratio (VSWR).

According to the present invention, an antenna for circularly polarised radiation comprises a lineararray of primary radiating elements having a common linear polarisation, and means to supporting a single anisotropic layer shaped to be substantially parallel to the wave fronts of radiation emitted by the array, the orthogonal axes of the anisotropic layer being at acute angles to the linear polarisation of the primary radiating elements, the geometry and structure of the anisotropic sheet and the radial spacing of the sheet f rom the array being selected such that repeated reflections between the sheet and the primary radiat- ing elements produce a radiated wave from the antenna which is circularly polarised.

The above invention is based primarily on the somewhat surprising discovery that a satisfactory antenna with circular polarisation can be produced using a single anisotropic layer. By carefully designing the anisotropic layer itself and careful spacing of the anisotropic layerfrom the primary radiating elements, mis-matching of the antenna can be minimised and good circular polarisation achieved.

Itwill be appreciated that, as normal in antenna systems, the present antenna can be equally suitable for receiving radio frequency radiation as fortransmitting and references throughout this specification to radiations and emissionsfrom the antenna are included solely for convenience in describing the antenna and should not be construed as limiting the antenna to transmission applications.

Conveniently said supporting means of the antenna comprises a radome of dielectric material carrying the anisotropic layer. The anisotropic layer may comprise parallel spaced conductors extending in one of the orthogonal axes of the layer. The spaced conductors maybe at4Wto the common linear polarisation of the radiating elements.

In one arrangementthe lineararray provides omnidirectional radiation in planes perpendicularto the array and said anisotropic layerforms a complete cylinder surrounding the array.

Examples of the present invention will now be described in greater detail with reference to the accompanying drawings in which:

Figure 1 is a partial view of an antenna for circularly polarised radiation with a portion of the radome and anisotropic layer broken awayforclarity and Figure 2 is an alternative embodiment of antenna employing a different form of primary radiating element.

Referring to Figure 1, the antenna comprises an array of three dipole radiators 10 stacked end to end The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

2 along a common axis 11. The detailed construction of the dipole radiators 10 of the array is not essential to the understanding of the present invention and nothing more will be included herein. It can be seen that the dipoles 10 of the array have a common linear polarisation producing, in the absence of any circular polariser, radiation with an electric vector parallel to axis 11. Furthermore, it can be seen that the array of dipoles 10 can produce radiation in all directions in a plane parallel to the axis 11. Thus,with the axis 11 vertical, the antenna is omnidirectional in azimuth.

The array of dipoles 10 is enclosed in a cylindrical radome of which a lower part only is shown at 12. The upper part of the radome is broken away for clarity so asto reveal the dipole array. The cylindrical radome 12 is arranged to surround the dipole array so that the axis 11 of the arrayis on the axis ofthe cylinderof the radome. The radome is made of a dielectric material and is arranged to provide substantial weather protection forthe antenna. For example the antenna mayform the antenna of a transponder unit in a navigation or position fixing system and maytherefore be located in an unattended and exposed position for example on the coastline.

In this example of the present invention,the radorne 90 12 also supports an anisotropic layerformed of parallel helical wires or conducting paths 13. The wires 13 are either embedded in the thickness of the dialectric material of the radome 12 or supported on the inside wall of the radome. In an alternative arrangementthe wires or paths 13 may be contained between an inner and an outer layer of the radome. For examplethe paths may be supported on the outer cylindrical surface of a rigid plastics cylinderforming an innerlayerof the radome and providing the mechanical strength of the radome. The conducting paths 13 of the anisotropic layer are then encapsulated by an outer layer of the radome which may for example be formed of a heat shrinkable plastics shrunk onto the inner layerof the radome to coverthe 105 conducting paths.

The helical wires or conducting paths 13 are arranged with a helical pitch of 450so thatthey are always at4Wto the axis 11 of the dipole array.

ftwill be appreciated bythose experienced in this fieldthatthe components of the radiation emitted by the dipole arraywhich are perpendicular and parallel respectivelyto the paths orwires 13 will be affected differently bythe anisotropic layer.The component which is perpendicular to the wires or paths 13 will see 115 the anisotropic layer as more capacitative than the component parallel to the wires, which latter will in turn seethe layer as more inductive. [twill be appreciated also that there will be ref iections from the anisotropic layer back towards the array of dipoles and return reflections from the dipoles.

It has been discovered that careful selection of the design of the anisotropic layer and the material and thickness of the radome, togetherwith careful spacing of the radome and anisotropic layerfrom the dipole array can result in producing effective circular polarisation of radiation emitted bythe arraywith reasonable orgood matching of the antenna.

Theselection of the various parameters of the antenna is largely, though not entirely, and empirical GB 2 155 694 A 2 process and of course these parameters will be different for different applications and, in particular, different frequencies.

In one example, an antenna as shown in Figure 1 was made to operate at a wave length of about7 cm with a spacing between the helical wires 13 of about 1.5 cm and a radiusfrom the anisotropic layerto the axis of the dipole array of about4.5 cm.

Referring nowto Figure 2, an alternative arrange- ment is shown which is essentially similarto that of Figure 1 exceptthat the primary radiating elements are slots 14 in a wave guide 15. Again the slotted wave guide is enclosed in a substantially cylindrical radome carrying helical wires or conducting paths to form a

Claims

single anisetropic layer. In other respects, the arrangement of Figure 2 maybe substantially similar to that of Figure 1. CLAIMS

1. An antenna for circularly polarised radiation comprising a linear array of primary radiating elements having a common linear polarisation, and means supporting a single anisotropic layershaped to be substantially parallel to the wave fronts of radiation emitted bythe array, the orthogonal axes of the anisotropic layer being at acute angles to the linear poi-A.risation of the primary radiating elements, the geometry and structure of the anisotropic sheet and theradial spacing of the sheet f rom the array being sel.,>cted such that repeated reflections between the shpet anc; the primary radiating elements produce a radiated wave from the antenna, which is circularly polarised.

2. An antenna asciaimed in Claim 1 wherein said supporting means comprises a radome of dielectric material carrying the an isotropic layer.

3%. An antenna as claimed in either of Claims 1 or2, wherein the anisotropic layer comprises parallel spaced conductors extending in one of the orthogonal axes of the layer.

4. An antenna as claimed in Claim 3, wherein said spaced conductors are at4Wto the common linear polarisation of the radiating elements.

5. An antenna as claimed in any preceding claim wherein said linear array provides omnidirectional radiation in planes perpendicular to the array and said anisotropic layerforms a complete cylinder surrounding the array.

6. An antenna for circularly polarised radiation substantially as hereinbefore described with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, 8818935, 9185, 18996. Published at the Patent Office, 25 Southampton Buildings. London WC2A lAY, from which copies may he obtained.