GB1305117A - - Google Patents

Abstract

1305117 Aerials BRITISH AIRCRAFT CORP Ltd 12 July 1971 [13 July 1970] 33940/70 Heading H4A An aerial comprises a feed horn, a main reflector and a sub-reflector which comprises a grating of parallel conductors embedded in a dielectric medium and sandwiched between a pair of polarizers and which reflects circularly polarized radiation of one hand but transmits circularly polarized radiation of the opposite hand. Radiation from the horn which has circular polarization of the said one hand is reflected by the sub-reflector and the main reflector, after which it has circular polarization of the said opposite hand, and is able to pass through the sub-reflector. As described, a hyperbolic sub-reflector 15 consists of a pair of quarter-wave plates 2, 6 spaced apart by a dielectric such as a glass reinforced plastics material which contains a grating 4 of horizontally disposed wires 3. A feed horn 1 directs radiation with R.H. circular polarization towards the sub-reflector 15. The quarter-wave plate 2 converts the polarization to horizontally linear, so that the radiation is reflected back from the grating 4. As it passes through the quarter-wave plate 2, the polarization is reconverted to R.H. circular, and the radiation proceeds to a metal main reflector 5 of parabolic form, where collimation and a change of polarization to L.H. circular take place. That part of the emergent radiation which falls upon the sub-reflector 15 has its polarization changed to vertically linear by the quarter-wave plate 2, so that it passes through the grating 4. The polarization is re-converted to L.H. circular by the quarter-wave plate 6. The construction of the quarter-wave plates may be as described in Specification 1,240,529 in which a pair of dielectric skins are spaced apart by a layer of lower dielectric constant and each contain a wire grating with wires which are orientated at 45 degrees to the wires of the grating 4.