GB973583A - Improvements in or relating to microwave aerials - Google Patents

Abstract

973,583. Aerials. POSTMASTER GENERAL. April 11, 1963 [April 11, 1962], No. 14057/62. Heading H4A. In an aerial comprising a major reflector 1, Fig. 2, a minor reflector 2, and a feed device such as the conical horn 5 arranged substantially on the axis of symmetry of the aerial, the major reflector surface is generated by rotating a section of a parabola AB, Fig. 1, about the axis of symmetry XY of the aerial, which is parallel (but not coincident) with the axis m, n of the parabold, and the minor reflector surface is defined by the rotation about the aerial axis XY of a regular plane curve; this surface being so shaped and located as to produce a beam from the aerial which is of annular cross-section. The minor reflector surface may be a section of a hyperbola CD having one focus F2 coincident with that of the parabold and its second focus F1 lying on the aerial axis XY coincident with the phase centre of the feed device. With this arrangement the virtual source for the major reflector 1 is the circle formed by rotation of the focus F2 so that a parallel annular cross-section beam is reflected. Other suitable curves for generating the minor reflector surface are (i) an ellipse whose two foci are located similarly to those of the hyperbola CD, and (ii) the perpendicular bisector of a line joining the phase centre of the feed device to the focus F2 of the parabola. The minor reflector 2 may be supported from either the major reflector by a sheet or rods of dielectric material or from the feed device 5 by means such as the dielectric block 6. In the latter case the reflector 2 may be formed by spraying a metallic coating on to the end of the block 6. Various shapes of blocks are described with reference to Fig. 2 and Figs. 4, 7 and 8 (not shown) to provide emission of energy to the major reflector 1 as an equiphase front after reflection by the minor reflector 2, and waveabsorbing means 9 surrounding the reflector 2 may be used. Scanning in either a conical or spiral pattern, or arcuate movement of the beam in more than one plane, may be effected by movement (relative to the major reflector) of the feed device, the minor reflector, or both.