GB776258A - Improvements in or relating to reflector aerials - Google Patents

Abstract

776,258. Radio reflectors. MARCONI'S WIRELESS TELEGRAPH. CO., Ltd. May 27, 1955 [March 2, 1954], No. 6161/54. Class 40(7) An aerial structure is in the form of a surface of revolution with reflector elements distributed round it. Each of the elements extends in a direction substantially at right angles to the diametrically opposite element, so that plane polarized waves incident radially upon a given element and totally reflected by it will pass through the diametrically opposite part of the surface. The structure can be used in an all round scanning aerial system or in a reflector system. Where it is not necessary to cover the full 360 degrees the structure need not extend all round. In one embodiment the structure comprises a number of lengths of conductive reflector elements arranged helically at an angle of 45 degrees on a barrel shaped surface of parabolic profile; only a few of these elements are shown in Fig. 1. A feeder wave guide 2 is mounted along the barrel axis and terminates at rotating joint 3 from which guide 4 extends to primary element horn 5 tilted as indicated and half way along'the barrel radius. The horn may be arranged to transmit a beam, as indicated by the chain lines, polarized at 45 degrees so that the reflector elements illuminated reflect a parallel beam diametrically across the barrel and out through the far side. Rotation of the horn over the circular locus 6 causes the emergent beam to scan over 360 degrees. In the embodiment the primary element is offset so as not to be in the path of the beam across the structure, this is not essential however and the horn may rotate in the centre plane of the structure and be directed radially outwards (Figs. 2 and 3, not shown). If it is desired to provide a beam narrow in azimuth but broad in elevation the sides of the structure may be parallel with the axis (Figs. 4 and 5, not shown). If it is desired to scan in azimuth as well as elevation the primary element 5 (Fig. 7) may be moved vertically between limiting vertical positions such as those shown, as well as rotated, to move the emergent beam between the single and double chain dotted positions shown. A reflector structure in the shape of a right cylinder is simpler to construct than a barrel shaped one and may be used combined with a surrounding lens cylinder, plano-convex in vertical cross-section, to give a pencil beam instead of a beam fan shaped in a vertical plane (Figs. 8 and 9. not shown):. In a modification a stepped plano-concave lens cylinder is placed within a right cylindrical reflector structure for the same purpose (Figs. 10 and 11. not shown). A wide angle search beam may be provided in addition to the narrow beam of any of the above embodiments by providing a second horn primary element radiating waves polarized at right angles to those from the first so, that they pass directly through the reflecting structure. The second horn may rotate with, but be angularly separated from, the first. An aerial system according to the invention can be used for all round transmission or reception by replacing. the moving primary element by a number of primary elements distributed along the focal circle each associated with its own transmitter and/or receiver. A series of primary elements can also be switched into use in turn for step by step scanning. Instead of using wires or strips to form the aerial structure this may comprise a suitably shaped metal plate having helically arranged lines of end on slots cut in it at an angle of 45 degrees. If the slots are close enough-together the remaining metallic structure can be replaced by the equivalent wire mesh or expanded metal (Figs. 18 and 19 not shown). The wesh-work structure is very selective as to the frequency of the signals with which it will operate and the selectivity may be increased still further by providing two or more mesh structures one inside the other. Polarization converting means, such as sets of vertical or horizontal quarter wave plates, may be attached to the outside of the aerial structure. For maximum mechanical rigidity the flat surfaces of the aerial structures may be slightly corrugated and the reflector elements embedded in low loss concrete. Passive structures. Reflector beacons capable of returning incident beams irrespective of the azimuthal direction of its arrival can be constructed by using an aerial structure according to the invention. In a simple embodiment the primary element and feed is replaced by a ring shaped reflector (or series of reflectors) lying round the locus 6 and focused on the inside of the reflector structure. A more complex embodiment may include means for modulating the returned beam; this may comprise spaced holes in the central ring shaped reflector and means to rotate it about its axis or the arrangement shown in Fig. 16 which has no moving parts. This construction has a central reflector comprising a plurality of polyrod elements 10 each connected to a waveguide 11 in which is embedded a ferrite rod 13. The end of the guide beyond the ferrite rod is short circuited. Series connected coils 14 are wound to influence the reluctance of rods 13 and are fed from a modulating source 15. Specification 776,259 is referred to.