GB2108326A

GB2108326A - Antennas

Info

Publication number: GB2108326A
Application number: GB08132134A
Authority: GB
Inventors: Arthur Charles Studd
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1981-10-24
Filing date: 1981-10-24
Publication date: 1983-05-11

Abstract

In an antenna, such as a Cassegrain antenna, in which a reflector 2 blocks some of radiation reflected from primary dish 7, a second emitter 4 is provided and the radiation from this is reflected at 5 to supplement the main radiation in the blocked region. The phase and amplitude of the radiation from emitter 4 is adjusted to that of the main emitter 3 by components 8 and 9. <IMAGE>

Description

SPECIFICATION Antennas This invention relates to antennas in which the transmitted or received radiation is partially blocked by physical obstruction. For example, where an antenna assembly has an antenna member which accepts radiation from feed means for transmission or which directs received radiation onto receiving means, the feed/receiving means may physically obstruct the radiation path.

It is emphasised that the invention relates to both transmitting and receiving antennas. However, for ease of description, this specification hereafter makes reference only to transmitting antennas.

Where such blocking occurs, the pattern of the transmitted radiation is disadvantageously affected and, moreover, there may be an attendant reduction in antenna gain and/or undesirably increased power in any side lobes in the transmitted radiation. The latter, in particular, makes an antenna unacceptable for use in many embodiments of which communication satellites are but one example.

This situation can be remedied to some extent by offsetting the feed means but the resultantasym- metry can prove undesirable.

Examples of antennas to which the invention can particularly, but not exclusively, relate are prime focus fed symmetric paraboloidal reflector types, and also those operating on the principle of the Cassegrain optical telescope.

It is an object of the present invention to at least reduce the undesirable effects of such blocking.

According to one aspect of the present invention, an antenna which provides a first radiation output, which output has blocked regions, includes feed means arranged to provide a second radiation output in these blocked regions.

Since the radiation paths within an antenna for the first and second radiation outputs may be different, preferably phase changing means are provided to at least reduce any phase discrepancies between the two outputs.

Additionally, or alternatively, amplitude changing means may be provided.

By this arrangement, the complementary radiation outputs provide an output from which the effects of blocking are at least reduced.

By way of example only, one specific embodiment of this invention will now be described in detail, reference being made to the accompanying Figure which is a cross section of a fronted symmetric Cassegrain antenna embodying the invention.

An antenna comprises a primary dish 1 of para boioidal form centered on the primary axis P of radiation transmission, a sub-dish 2 of hyperboloidal form again centered on the primary axis P, and a primary radiation emitter 3 arranged to emit radiation on to sub-dish 2 which reflects the radiation on to the primary dish 1 whence output radiation is directed along axis P. Radiation from emitter will not be emitted across the whole of the effective aperture of the primary dish 1 on account of the blockage formed by the sub-dish 2. In this example, the sub-dish 2 and the emitter 3 together form a feed means. In the alternative prime focus fed symmetric paraboloidal type, naturally the sub-dish 2 is not present but blockage is caused by the emitter 3.

To overcome this blockage, further radiation feed means are provided which comprise a complementary radiation emitter 4 arranged to direct radiation on to complementary dish 5 of paraboloidal form centered on axis P. Output radiation reflected by complementary dish 5 is directed along axis P. In this embodiment, surface 4 is contiguous with the reflector of the sub-dish 2.

A signal to the primary emitter 3 is fed along primary feed line 6. Power is tapped from line 6 via directional coupler 7 and is fed to the complementary radiation emitter 4 after adjustment of phase at8 and amplitude at 9.

The emitter 4 illuminates the reflecting surface of the complementary dish 5 and provides an in-phase additional radiation output at the effective aperture plane AA' to the output radiation from the primary dish 1. The output at the aperture plane AA' therefore is the resultant of two separate inputs, one from the primary dish 1 and one from the complementary dish 5.

The undesirable effects of blocking are very much reduced by the combined output. Because there is an independent control of the amplitude of the signal fed to the complementary radiation emitter 4, it is found that selective side lobe suppression of the combined output is possible. Increase of power to the complementary radiation emitter 4 beyond the level necessary to 'unblock' the primary dish 1 aperture appears to cause a redistribution of the energy in the side lobes and this redistribution may be used to advantage when a designer attempts to provide an output with a particular radiation profile.

Moreover, by the described arrangement a generally uniformly phased radiation output is provided.

Some residual blockage will, however, remain because of the feed means and its associated support members, but this can be relieved, for example, by further complementary feed means.

In a further example, two frequencies at the input may be selectively coupled to the complementary radiation emitter 4 by separate paths. Also beam radiation patterns may be varied to provide a zoom effect.

1. An antenna which provides a first radiation output, which output has blocked regions, including feed means arranged to provide a second radiation output in these blocked regions.

2. An antenna according to Claim 1 wherein phase changing means are provided to at least reduce any phase discrepancies between the two outputs.

3. An antenna according to Claim 1 or Claim 2,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Antennas This invention relates to antennas in which the transmitted or received radiation is partially blocked by physical obstruction. For example, where an antenna assembly has an antenna member which accepts radiation from feed means for transmission or which directs received radiation onto receiving means, the feed/receiving means may physically obstruct the radiation path. It is emphasised that the invention relates to both transmitting and receiving antennas. However, for ease of description, this specification hereafter makes reference only to transmitting antennas. Where such blocking occurs, the pattern of the transmitted radiation is disadvantageously affected and, moreover, there may be an attendant reduction in antenna gain and/or undesirably increased power in any side lobes in the transmitted radiation. The latter, in particular, makes an antenna unacceptable for use in many embodiments of which communication satellites are but one example. This situation can be remedied to some extent by offsetting the feed means but the resultantasym- metry can prove undesirable. Examples of antennas to which the invention can particularly, but not exclusively, relate are prime focus fed symmetric paraboloidal reflector types, and also those operating on the principle of the Cassegrain optical telescope. It is an object of the present invention to at least reduce the undesirable effects of such blocking. According to one aspect of the present invention, an antenna which provides a first radiation output, which output has blocked regions, includes feed means arranged to provide a second radiation output in these blocked regions. Since the radiation paths within an antenna for the first and second radiation outputs may be different, preferably phase changing means are provided to at least reduce any phase discrepancies between the two outputs. Additionally, or alternatively, amplitude changing means may be provided. By this arrangement, the complementary radiation outputs provide an output from which the effects of blocking are at least reduced. By way of example only, one specific embodiment of this invention will now be described in detail, reference being made to the accompanying Figure which is a cross section of a fronted symmetric Cassegrain antenna embodying the invention. An antenna comprises a primary dish 1 of para boioidal form centered on the primary axis P of radiation transmission, a sub-dish 2 of hyperboloidal form again centered on the primary axis P, and a primary radiation emitter 3 arranged to emit radiation on to sub-dish 2 which reflects the radiation on to the primary dish 1 whence output radiation is directed along axis P. Radiation from emitter will not be emitted across the whole of the effective aperture of the primary dish 1 on account of the blockage formed by the sub-dish 2. In this example, the sub-dish 2 and the emitter 3 together form a feed means. In the alternative prime focus fed symmetric paraboloidal type, naturally the sub-dish 2 is not present but blockage is caused by the emitter 3. To overcome this blockage, further radiation feed means are provided which comprise a complementary radiation emitter 4 arranged to direct radiation on to complementary dish 5 of paraboloidal form centered on axis P. Output radiation reflected by complementary dish 5 is directed along axis P. In this embodiment, surface 4 is contiguous with the reflector of the sub-dish 2. A signal to the primary emitter 3 is fed along primary feed line 6. Power is tapped from line 6 via directional coupler 7 and is fed to the complementary radiation emitter 4 after adjustment of phase at8 and amplitude at 9. The emitter 4 illuminates the reflecting surface of the complementary dish 5 and provides an in-phase additional radiation output at the effective aperture plane AA' to the output radiation from the primary dish 1. The output at the aperture plane AA' therefore is the resultant of two separate inputs, one from the primary dish 1 and one from the complementary dish 5. The undesirable effects of blocking are very much reduced by the combined output. Because there is an independent control of the amplitude of the signal fed to the complementary radiation emitter 4, it is found that selective side lobe suppression of the combined output is possible. Increase of power to the complementary radiation emitter 4 beyond the level necessary to 'unblock' the primary dish 1 aperture appears to cause a redistribution of the energy in the side lobes and this redistribution may be used to advantage when a designer attempts to provide an output with a particular radiation profile. Moreover, by the described arrangement a generally uniformly phased radiation output is provided. Some residual blockage will, however, remain because of the feed means and its associated support members, but this can be relieved, for example, by further complementary feed means. In a further example, two frequencies at the input may be selectively coupled to the complementary radiation emitter 4 by separate paths. Also beam radiation patterns may be varied to provide a zoom effect. CLAIMS

3. An antenna according to Claim 1 or Claim 2, wherein amplitude changing means are provided.

4. An antenna substantially as described with reference to the accompanying drawing.