GB1416364A - Generation of scanning radio beans - Google Patents

Abstract

1416364 Aerials; radio navigation COMMONWEALTH SCIENTIFIC & INDUSTRIAL RESEARCH ORGANIZATION 30 Jan 1973 [7 Feb 1972] 4627/73 Headings H4A and H4D An aerial for producing a scanning radio beam comprises:-(a) a substantially cylindrical reflector having a surface the shape of which i's substantially that produced by the partial rotation of a generating curve of finite length about an axis, the reflector being so mounted that said axis is the axis of rotation of said beam, and (b) a plurality of radiating elements disposed about an arc centered on said axis and of radius substantially half the minimum distance of the cylindrical reflector from said axis, each element being arranged to radiate towards the reflector so that a reflected planar fan beam is produced, with the plane of the fan parallel to said axis or rotation. Sequential excitation of said elements results in angular scanning of the beam radiated from the aerial. As described, a reflector 3, Figs. 2a, 2b, has a vertical axis C, and is associated with radiator feed elements R(1), ... R(n) arranged along a horizontal arc of radius about half that of the reflector, and offset to avoid aperture blocking. The shape of the reflector may be such as to provide a cosecant-squared radiation pattern in the vertical plane. Alternatively, the generating curve may be a straight line, so that a right cylindrical surface is developed (Figs. 1a, 1b, not shown). An extension 3A, Fig. 2b, may be provided atop the reflector 3, and an auxiliary reflector 4 (which may have a plane surface) so that some radiation is directed behind the aerial. In another embodiment, the axis of the reflector is horizontal, and the feed elements are arranged along a vertical arc, so that scanning is in a vertical plane (Figs. 3a, 3b, not shown). To reduce aperture blocking, since the feed elements are directly in front of the reflector, the central portion of said reflector is specially shaped. A lens may be inserted to provide broadside expansion of the beams from the feed elements. In application to radio navigation for aircraft an aerial system such as that shown in Figs. 2a, 2b, develops a succession of narrow radio beams which are progressively scanned in azimuth either by exciting each feed element in turn (step-scanning) or by exciting several adjacent elements with a particular distribution and similarly stepping (quasi-continuous scanning). In the latter case, four feed elements may be excited at a time, and the power distribution may be according to a cosine-squared law (Fig. 4, not shown). A power detector may be located in the region of the axis C (through which all beams pass) which will monitor transmitted radiation, and can be coupled to an error corrector. Switching may be effected by ferrite or diode switches within a waveguide feed system (Fig. 5, not shown). Beams of radiation may be distinguished from each other by either exciting them in a known sequence relative to a reference signal from an omni-directional aerial, or by applying a code to each beam by modulation or frequency change. Azimuth information for aircraft overshooting a landing position is provided for in the embodiment shown in Figs. 2a, 2b. Alternatively, two azimuth transmitting aerials may be installed, one at each end of a runway, and directed therealong. In another arrangement, two aerials are installed, one on each side of the runway, and operated in different time intervals or on different frequencies. The system would also include an elevation aerial as previously described, and operating similarly to the azimuth aerial or aerials.