GB2249872A - Antenna arrangement - Google Patents

Abstract

A millimetre wave antenna arrangement includes lens components (11, 12, 13) configured as an afocal telescope and an array 20 of antenna elements mounted at an exit surface of the telescope. The array presents an aperture at the entrance surface of the telescope which is larger than the region occupied by the array at the exit surface. <IMAGE>

Description

ANTENNA ARRANGEMENT This invention relates to an antenna arrangement suitable for receiving and/or transmitting millimetre wavelength electromagnetic radiation, and particularly, although not exclusively, radiation of a wavelength in the range from 3 mm to 10 mm.

A known antenna arrangement includes an array of discrete dipole elements which may be phased so as to synthesise a highly directional response characteristic. Moreover, by appropriately controlling the relative phasing, the response characteristic can be scanned across a scene in accordance with a desired scanning format.

To achieve adequate resolution and signal-to-noise ratio it is desirable that a significant dimension of the array be at least 10 > , where > is the wavelength of radiation used.

Since > may typically be about 8 mm the array may be difficult to construct reliably; moreover, the array size may be prohibitive, especially in applications in which space is limited.

It is an object of the present invention to provide an antenna arrangement whereby the above-described problem is at least alleviated.

Accordingly there is provided an antenna arrangement suitable for receiving and/or transmitting millimetre wavelength electromagnetic radiation comprising, an afocal telescope including lens components, transmissive of said radiation and an array of discrete antenna elements occupying a region at the exit surface of the telescope, wherein the telescope is configured so that, in effect, the array presents an aperture which is substantially larger than said region.

It will be appreciated that this arrangement can offer a receiving and/or transmitting aperture commensurate with a desired resolution and signal-to-noise ratio, even though the physical dimensions of the array itself may be relatively small.

In a particularly beneficial arrangement, of compact construction, different points on the entrance surface of the telescope are imaged at substantially the same respective positions on the exit surface thereof, independently of the direction of received and/or transmitted radiation.

In alternative arrangements a telescope having a magnification in the range from 2 to 5 may prove beneficial.

In order that the invention may be carried readily into effect two embodiments thereof are now described, by way of example only, by reference to Figures 1 and 2 of the drawings which show respective longitudinal cross-sectional views through two different antenna arrangements.

The embodiment described hereinafter by reference to Figure 1 concerns an antenna arrangement used specifically as a receiver; it will be appreciated, however, that the antenna arrangement could be used alternatively as a transmitter or as a transmitter/receiver.

Referring to Figure 1 the antenna arrangement includes an afocal telescope, shown generally at 10, and a planar array 20 of discrete antenna elements 21 formed at the exit surface of the telescope.

In this example, a square array, of 25 mm side, is used; however, the telescope is configured so that the effective aperture presented by the array to received radiation is considerably larger. In effect, the antenna arrangement functions as though the array is located at, and substantially fills, the entrance aperture A of the telescope. It will be appreciated that since, in this example, the entrance aperture has a diameter of about 100 mm satisfactory resolution and signal-to-noise ratio can be achieved even though the array itself is relatively small.

The telescope comprises a lens system in the form of an objective lens 11, having respectively convex paraboloid and concave hyperboloid front and rear surfaces, a field lens 12 having a planar front surface and a convex spherical rear surface and an "eye-piece" 13 having a convex paraboloid front surface and a planar rear surface. The lenses are of alumina, having a refractive index of 3.082, and are provided with anti-reflective coatings C, typically 1.5 mm thick and having a refractive index of 2.30. Clearly other suitable dielectric materials, having refractive indices typically in the range from 2 to 4 and having dielectric constants typically in the range from 8 to 12 could be used. Suitable materials include barium nonotitanate (Ba2TigO20) or polystyrene loaded with titanium dioxide.

The drawing shows a scaled representation having rotational symmetry about the longitudinal axis XX' of the telescope. The curvatures, thicknesses and relative positions of the lenses are chosen, in this example, so that different points on the entrance surface of the telescope are imaged at substantially the same relative points on the array, irrespective of the direction of incident radiation. In particular, the point P at the pole of the entrance surface is focussed at P' at the centre of the exit surface. It will be appreciated that this lens configuration proves to be beneficial in that a relatively compact construction can be achieved, even in applications requiring scanning of the antenna response characteristic.

In general, the effective aperture of the array, as presented at the entrance aperture of the telescope, is given by mD, where m (about 4 in this example) is the magnification and D (about 25 mm in this example) is a corresponding dimension of the array.

In this example a square array is used, and comprises a plurality of discrete antenna elements which may be crossed, or alternatively, linear dipoles formed as an evaporation at the exit surface of the telescope i.e. on the rear surface of the "eye-piece" 13. Typically the antenna elements are spaced apart by > /2.

Alternatively, an array of monolithic structure could be used; this could be fabricated using known IC techniques, as described, for example, in "Monolithic Integration of a Dielectric Millimetre Wave Antenna and Mixer Diode - An Embryonic Millimetre Wave IC" by Yao and Schwarz, IEEE Trans on Microwave Theory and Techniques, Vol. MTT, 30, No. 8, August 1982, p. 1241-1246. In these circumstances it may be desirable to provide a suitable interfacing material between the array and the exit surface of the telescope.

Alternatively the array may be embedded in the exit surface; moreover a circular or a linear array could be used.

Figure 2 shows an alternative configuration 30 in which respective lens components 31-36 are arranged in abutting relationship. This arrangement eliminates air gaps between adjacent lens components, permitting imaging over a relatively wide range of incident angle, typically about 400 as compared with 200 using the configuration of Figure 1. In this example the components labelled A and C are made of polystyrene loaded with titanium dioxide and have refractive indices respectively of 2.94 and 3.28, whereas components labelled B are made of barium nanotitanate (Ba2TigO20) and have a refractive index of 6.16. Each abutting lens surface is provided with an anti-reflection coating having a refractive index equal to the square root of the product of the refractive indices of the adjoining components.

It will be appreciated that by suitably controlling the relative phase (or frequency) of radiation received at respective elements in the array 20 it is feasible to synthesise a response characteristic which can be scanned across a scene in accordance with a desired scanning format. In effect, the characteristic is caused to swing about a "pivot" point P at the pole of the entrance aperture.

Control may be effected by providing suitable phase shifting circuits between respective pairs of antenna elements, as described for example, in "Introduction to Radar Systems" Skolnik, International Student Edition, McGraw Hill Sec. 7.7 p.

294-320.

It will be understood that the present invention is not limited to the particular antenna arrangements described in the above example; persons skilled in the art will appreciate that other arrangements may be designed resulting in an effective array aperture substantially larger that the actual array itself.

Claims (6)

1. An antenna arrangement suitable for receiving and/or transmitting millimetre wavelength electromagnetic radiation comprising, an afocal telescope including lens components transmissive of said radiation and an array of discrete antenna elements occupying a region at the exit surface of the telescope, wherein the telescope is configured so that, in effect, the array presents an aperture which is substantially larger than said region.

2. An antenna arrangement according to Claim 1 wherein different points on the entrance surface of the telescope are imaged at substantially the same respective points on the exit surface thereof, independently of the direction of received and/or transmitted radiation.

3. An antenna arrangement according to Claim 1 or Claim 2 wherein the telescope has a magnification in the range from 2 to 5.

4. An antenna arrangement including means for scanning the response characteristic of the array electronically.

5. An antenna arrangement substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS 1. An antenna arrangement suitable for receiving and/or transmitting millimetre wavelength electromagnetic radiation comprising an afocal telescope including lens components transmissive of said radiation and an array of discrete antenna elements occupying a region at the exit surface of the telescope, wherein the telescope is so configured that, in effect, said array presents an aperture which is substantially larger than said region, each point at the entrance aperture of the telescope being imaged at a respective point at the exit surface of the telescope whereby rays incident at, or emergent from, any point at the entrance surface of the telescope are emergent from,. or incident at, a respective point at the exit surface of the telescope independently of the direction of received or transmitted radiation.

2. An antenna arrangement according to Claim 1 wherein said telescope has a magnification in the range from 2 to 5.

3. An antenna arrangement according to Claim 1 or Claim 2 wherein said lens components comprise an objective, a field lens and an eyepiece, there being a respective air gap between adjacent ones of the components.

4. An antenna arrangement according to Claim 1 or Claim 2 wherein said lens components are in abutting relationship, there being no air gap between adjacent ones of the components.

5. An antenna arrangement according to any one of Claims 1 to 4 including means for scanning a response characteristic of the array electronically.

6. An antenna arrangement substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.