Patent GB-1094993-A (GB)
Title: Improvements in/or relating to an armoured structure antenna
IPC: H01Q19/195
Filed: 1965-03-18
Abstract: 1,094,993. Waveguides. COMPAGNIE FRANCAISE THOMSON HOUSTON - HOTCHKISS BRANDT. March 18, 1965 [March 20, 1964], No. 11551/65. Heading H1W. [Also in Divisions El and H4] A microwave aerial assembly of the Cassegrain type comprises a main reflector which produces a 90 degree rotation in the plane of polarization of incident waves and an auxiliary reflector which is transparent to waves linearly polarized in a particular plane, but is reflective of orthogonally polarized waves, and is in the form of a thick metallic member bored or punched with holes which contain dielectre rods having protruding ends. The said orthogonally polarized waves are propagated through the rods, their mode of polarization being changed by means provided from linear to circular, and are radiated from the ends thereof. As described, a primary feed electromagnetic horn 1, Fig. 1, is located at the focus of a paraboloidally shaped thick metallic auxiliary reflector 2 towards which it emits spherical horizontally polarized waves which are returned as plane waves to a main reflector 3 comprising means for rotating the plane of polarization by 90 degrees. The resulting vertically polarized waves impinge upon the inner ends 8, Fig. 3, of a plurality of cylindrical dielectric rods 7, which are contained in holes bored or punched in the auxiliary reflector body 6, and project from each side thereof, and the said waves are transmitted by waveguide action in the TE 11  mode. The ends 8 of the rods 7 are fitted with short-circuiting metallic blades 10 which prevent the transmission of the horizontally polarized waves initially emitted by the primary feed 1. Each rod 7 also comprises a section 11 of reduced cross-section formed by parallel cuts at 45 degrees to the horizontal which converts the linear polarization of the transmitted waves to circular polarization. The length of the reduced section 11 is made such that it produces a 90 degree phase difference between the rector components at Œ45 degrees to the plane of the entering vertically polarized wave and it has transition sections 13 at its ends for reducing reflections, any residual energy being absorbed by a piece 12 of suitable material placed behind the blade 10. The dielectric rods 7 may be made of alumina and they may be spaced a half-wavelength apart forming an approximately planar array with minimum side-lobe radiation. The rods also act as highpass filters, not transmitting frequencies higher than their cut-off frequency as waveguides. The polarization plane rotator on the main reflector 3, Fig. 1, may consist of a grid of parallel closely spaced wires 5 disposed at 45 degrees to the horizontal and located an odd number of quarter-wavelengths in front of the surface of the reflector 3. Vertical scanning may be effected by tilting the reflector about a horizontal axis perpendicular to X-X and N, and horizontal scanning by rotation about the axis N. The auxiliary reflector (2), Fig. 6 (not shown), may be extended to cover the primary feed (1) and the main reflector (3) and may be placed pon an enclosure (15) containing a transmitreceive device (14) so providing protection for apparatus and operators. The enclosure (15) may be made of concrete and the reflector (3) may be made of lead, to provide shielding against harmful radiation.