GB2234858A

GB2234858A - Cassegrain antenna

Info

Publication number: GB2234858A
Application number: GB8918803A
Authority: GB
Inventors: David John Price; John Thraves
Original assignee: Thorn EMI Electronics Ltd
Current assignee: EMI Group Electronics Ltd
Priority date: 1988-09-02
Filing date: 1989-08-17
Publication date: 1991-02-13
Also published as: DE3930497A1; GB8918803D0; FR2661562A1

Abstract

A Cassegrain antenna 10 comprises a parabolic main reflector 11, an antenna feed 12, and a sub-reflector 13 which is partially transmissive of the radiation to be transmitted or received, irrespective of the polarisation of the radiation. A convex-plano lens 14 is provided, which may support the sub-reflector 13, together with a dielectric layer 15 for causing phase alignment of the radiation passing through the sub reflector 13 with that reflected by the main reflector 11. The sub reflector 13 may comprise an etched layer or thin film of metallic material, such as copper, or a curved plate with an aperture formed at its vertex. <IMAGE>

Description

CASSEGRAIN ANTENNA This invention relates to a Cassegrain antenna and it

relates particularly, though not exclusively, to a Cassegrain antenna operating at radio frequency# especially in the range from 1GEz to 100GHz.

A known Cassegrain antenna, illustrated diagrammatically in Figure 1, comprises a feed horn 1 at the vertex of a main, parabolic reflector 2. Radiation from feed horn 1 is reflected at a hyperbolic, sub-reflector 3 back onto the main reflector, to form a substantially parallel output beam 4. A problem associated with this kind of antenna is that the sub-reflector can present a significant obstruction to the reflected beam creating, in effect, a Oshadow 5 at the centre of the antenna aperture. This shadow causes both a reduction in the gain available and an increase in sidelobe levels.

is In the case of a feed horn producing horizontally polarised radiation, the afore-mentioned shadowing effect has been alleviated by provision of a sub-reflector in the form of a grating, consisting of horizontal wires, which passes vertically polarised radiation but reflects horizontally polarised radiation. In addition, the main reflector is provided with a so-called twist reflector which effects rotation, through 90 0 p of the plane of polarisation of incident radiation. Horizontally polarised radiation from the feed horn is reflected at the sub-reflector and is reflected again, with a 90 0 change in orientation, at the main reflector. In this way, horizontally polarised radiation is transformed into vertically polarised radiation which passes through the sub-reflector without reflection. An arrangement of that kind proves to be inconvenient in that operation of the antenna is confined to a particular plane of polarisation.

It is an objective of the present invention to provide a Cassegrain antenna which at least alleviates the afore-mentioned problems.

Accordingly there is provided a Cassegrain antenna for transmitting and or receiving radiation, the antenna comprising a curved main reflector, an antenna feed at or near the vertex of the main reflector, a sub-reflector facing the antenna feed and the main reflector, wherein the sub-reflector is partially transmissive of the radiation irrespective of the polarisation thereof, and a lens arrangement effective to collimate radiation transmitted through the sub-reflector for causing phase alignment of the radiation passing through the sub-reflector with radiation reflected at the main reflector.

The sub-reflector, being partially transmissive, presents a reduced obstruction and so significantly alleviates the Ishadowingu effect which is an undesirable characteristic of hitherto known Cassegrain antennas.

Preferably, the main reflector is parabolic and the sub-reflector is hyperbolic, and the sub-reflector may comprise a partially transmissive layer at a surface of the lens arrangement. This layer may comprise a metal coating provided with a distribution of holes, and, in order to prevent diffraction of incident radiation, the spacing of holes should be less than)\ /2, where is the wavelength of the radiation.

In another embodiment, the sub-reflector comprises a metal plate incorporating a waveguide structure. The waveguide structure may comprise an array of holes formed in the plate, the transmissivity of the structure being dependent on the depth and transverse dimensions of the holes.

In another embodiment, the sub reflector is constructed using a very thin metallic film which may be formed on a surface of the lens, which may be of hyperbolic profile, the thickness of the film chosen so that it has the desired reflection transmission properties.

In a yet further embodiment of the invention, the sub-reflector comprises a hyperbolic plate or coating having a single aperture at the vertex, the aperture presenting, in effect, a secondary source of radiation.

The lens arrangement may comprise a collimating lens, made of a suitable dielectric material, and may be a convex/planar lens, and, in addition to the collimating lens, there may be provided means to effect a shift of phase; for example, a layer of a dielectric material applied to the plano surface of said lens.

It will be appreciated that a Cassegrain antenna in accordance with the present invention satisfies the principle of reciprocity and may be utilised to transmit and or receive radiation.

In order that the invention may be carried readily into effect specific embodiments thereof are now described, by way of example only, by reference to the accompanying drawings in which Figure 1 illustrates a known Cassegrain antenna, Figure 2 illustrates a Cassegrain antenna in accordance with the present invention, Figure 3 shows a fragmentary view of waveguide structure used as a phase shifter, Figure 4 illustrates another embodiment of a Cassegrain antenna in accordance with the present invention and Figures Sa and 5b illustrate the distribution of antenna gain respectively obtained by a conventional Cassegrain antenna and a Cassegrain antenna in accordance with the present invention.

Referring now to Figure 2 of the drawings, Cassegrain antenna 10 comprises a parabolic, main reflector 11, typically made of metal (e.g. aluminium or copper), a feed horn 12 at the vertex of the main reflector and a hyperbolic sub-reflector 13 positioned opposite the main reflector and the feed horn.

In this embodiment, the antenna operates in the transmission mode, the feed horn 12 providing a source of r.f.

radiation and, as is illustrated by the exemplary ray FABC, radiation from the feed horn is reflected at the sub-reflector, back towards the main reflector where the radiation is reflected again to produce a parallel, out-going beam.

In contrast to conventional Cassegrain antennas, and in accordance with the present invention, sub-reflector 13 is partially transmissive of the r.f. radiation incident thereon and, as is illustrated by the exemplary ray FAD, the divergent beam which passes the sub-reflector is collimated by a convex/planar dielectric lens 14. Being partially transmissive, the sub-reflector presents a reduced obstruction to incident radiation and significantly alleviates the afore-mentioned shadowing effect which is an undesirable characteristic of hitherto known Cassegrain antenna.

It will be apparent that in Figure 2, the path length of the reflected ray FABC exceeds that of the transmitted ray PAD. Nevertheless, in many instances, it is possible to select a 10 geometry and a lens material such that the resulting phase difference is a multiple of 2n, the phases being in substantial alignment across the entire beam front at the antenna aperture (represented in Figure 2 by the broken line through C and D). if necessary, phase alignment can be achieved by means of a layer 15 of a suitable dielectric material which introduces an additional phase shift. Alternatively, layer 15 may comprise a lattice of substantially box-like waveguides W, of which a fragmenting view is shown in Figure 3. Each box-like waveguide is square in cross-section, having sides (a) which are between 20 and X /2 long, and wall thickness (s) which is very much less than, where is the wavelength of the r.f. radiation. This arrangement is advantageous in that it is effective over a relatively wide frequency band.

In order to achieve effective collimation, the eccentricity 25 (e) of the sub-reflector 13 should be matched to the dielectric constant() of the lens material and, preferably, the eccentricity should be tailored to match a selected one of a p number of high quality, commercially available dielectric materials. Typically, a dielectric constant of around 2.5 is used.

In a typical case, the sub-reflector 13 has a reflection coefficient /p of about 0.95, and ideally P should be commensurate with the eccentricity (e) of the sub-reflector. It can be shown that the optimum value of.P is related to the eccentricity (e) by the expression P - - 1 1-2 -1) / 1 +( ú2el and Table 1 lists, by way of example, the optimum values of for three different eccentricities.

TABLE 1

P e 0.92 - 0.95 3 0.97 2 1 In an example of the present invention sub-reflector 13 consists of a coating of a suitable partially transmissive material (e.g. :opper) which is deposited at the convex surface of collimating lens 14. Alternatively, a metal coating having a pattern of holes, formed by etching, could be used, and in order to prevent diffraction of incident radiation the distribution of holes is such that their spacing is less than > /2.

In another example, the sub-reflector may consist of a metal plate incorporating a waveguide structure, similar to that 1 0 : 7:

shown In Figure 3. In this case, each waveguide comprises a respective hole formed in the plate, the reflection coefficient f of the plate being dependent both on the depth and on the transverse dimension of the holes. Again, the hole spacing should be less thanX /2.

In another example of the invention, shown in Figure 4, the sub-reflector comprises a metal plate 13 having a single hole 16 at the vertex. This hole creates, in effect, a secondary source of radiation, and the size of the hole determines the proportion of incident radiation which can pass the plate.

Figure Sa illustrates the distribution of field G across the aperture of a conventional Cassegrain antenna and shows the effect of aperture blockage, namely the creation of a central shadow region S and prominent diffraction lobes L. By way of contrast, Figure 5b shows the distribution of field achieved by an antenna in accordance with the present invention and demonstrates that the problems associated with aperture blockage have been significantly alleviated. Indeed, the distribution attained by the present invention is close to the theoretical distribution evaluated using a model which assumes that there is no aperture blockage.

The sub-reflector need not be hyberbolic in shape. It is known that the gain of a conventional Cassegrain antenna can be enhanced by increasing the curvature of the sub-reflector, giving a more uniform distribution across the aperture. The present invention is also applicable to this modified form of Cassegrain antenna, however, the collimating lens and the phase shifting layer (if required) will need to be spaced away from the sub- reflector.

The specific embodiments described hereinbefore all relate to antennas operating in the transmission mode, the feed horn 12 providing a source of r.f. radiation. it will be understood, that an antenna in accordance with the present invention may operate in the transmission and or receiving modes.

The present invention, which offers improved antenna gain and lower side lobe levels, finds particular, though not exclusive, application in radar seeker applications where the missile/munition diameter imposes severe constraints upon the antenna aperture.

1

Claims

CLAIKS

1. A Cassegrain antenna for transmitting and or receiving radiation, the antenna comprising a curved main reflector, an antenna feed at or near the vertex of the main reflector, a sub-reflector facing the antenna feed and the main reflector, wherein the sub-reflector is partially transmissive of the radiation irrespective of the polarisation thereof, and a lens arrangement effective to collimate radiation transmitted through the sub-reflector for causing phase alignment of the radiation passing through the sub- reflector with radiation reflected at the main reflector.

2. An antenna according to Claim 1 wherein the main reflector is of parabolic profile.

3. An antenna according to Claim 1 or Claim 2 wherein the sub-reflector comprises a partially transmissive layer of metallic material.

4. An antenna according to Claim 3 wherein the partially transissive layer comprises a coating having a distribution of holes therein.

5. An antenna according to Claim 4 wherein the holes are mutually spaced within the distribution by a distance less than one half the wavelength of the radiation.

6. An antenna according to Claim 3 wherein the partially transmissive layer comprises a relatively thin film of metallic material having a thickness arranged to provide the : 10:

partial transmissivity to the radiation.

7. An antenna according to Claim 1 or Claim 2 wherein the sub-reflector comprises a metal plate incorporating a waveguide structure.

8. An antenna according to Claim 7 wherein the waveguide structure comprises an array of holes formed in the metal plate.

9. An antenna according to Claim 1 or Claim 2 wherein the sub-reflector comprises a metallic plate or coating of curved shape having a single aperture provided at the vertex thereof, the aperture presenting a secondary source of the radiation.

10. An antenna according to any one of the preceding claims wherein the sub- reflector is supported by a surface of the lens arrangement.

11. An antenna according to any one of the preceding claims wherein the sub-reflector is of hyperbolic profile and the lens arrangement comprises a dielectric material having a dielectric constant related to the eccentricity of the hyperbolic profile.

12. An antenna according to any one of the preceding claims wherein the lens arrangement comprises a convex/planar lens.

13. An antenna according to Claim 12 wherein the convex/planar lens has provided on the plano surface thereof a layer of dielectric material.

14. An antenna according to Claim 13 wherein the layer of dielectric material comprises a lattice of waveguides of substantially square cross section and having a width in the range between \^ and X /2 and a wall thickness dimension substantially less thanX, where is the wavelength of the : 11:

4 radiation.

15. An antenna substantially as hereinbefore described with reference to Figures 1 to 4 and Figure 5b of the accompanying drawings.

Published lqq) 21 The Patent Office. State House. 66171 High Holbom. London WC I R 4TP- Fuilber copies maybe obtained fromThe Patent Office.