GB700868A - Improvements in or relating to high frequency radio aerials - Google Patents

Abstract

700,868. Aerials. ELLIOTT BROS. (LONDON), Ltd. Feb. 15, 1952 [Feb. 16, 1951], No. 3884/51. Class 40 (7) A directional aerial comprises a primary element which radiates or receives radio waves polarized in a predetermined direction, a main deflector, and in the energy path between this reflector and the primary element an auxiliary reflector which reflects wave energy of the aforementioned predetermined polarization but transmits energy polarized perpendicularly thereto, there being further associated with the main reflector a device for changing the polarization of radiation incident thereon through 90 degrees upon reflection. Consequently, considering a transmitting aerial, energy from the primary element is deflected by the auxiliary reflecter on to the main reflector, energy reflected therefrom having its polarization direction rotated 90 degrees so that it radiates into space without obstruction from the auxiliary reflector. In Fig. 1, energy is radiated from a waveguide 2 which passes through the plane main reflector 1 and is directed upon the auxiliary reflector 5. This comprises a plurality of parallel metal plates 6 arranged in a suitable dielectric medium and spaced apart by less than one-half the operating wavelength. The radio energy, which is polarized parallel to these plates, is reflected therefrom on to a device 9 which again comprises a plurality of metal plates 10 spaced apart by suitable dielectric material by less than a half wavelength. The plates 10 are disposed at 45 degrees with respect to the plates 6 so that the radiation incident thereon is split into two components, one polarized parallel to the plates 10 being reflected and the other polarized perpendicularly being transmitted by the device 9 and reflected from the main reflector 1. The excess path length of this component over the former directly reflected component is arranged to be a half wavelength so that the resultant radiation derived from the two reflected components is polarized in a direction perpendicular to that radiated from the waveguide 2 and consequently passes into space without obstruction from the auxiliary reflector 5. An alternative embodiment is shown in Fig. 9 where vertically polarized radiation from the waveguide aperture 13 is reflected by the auxiliary reflector 14 on to the main reflector 12. The auxiliary reflector comprises a plurality of metal plates spaced apart by less than a half wavelength and has a concave inner surface. Associated with the main reflector 12 are metal plates 16 which are inclined at 45 degrees with respect to those of reflector 14 and are spaced apart by more than a halfwavelength. The plates are attached to the reflector 12 and their depth is such that the components of polarization perpendicular and parallel thereto differ in phase by 180 degrees upon emergence from the plates after reflection at the reflector 12. In consequence of this the vertically polarized radiation incident upon the reflector 12 is reflected as horizontally polarized radiation and passes freely through the reflector 14. Scanning of the radiated beam may be achieved by suitable motion of the auxiliary mirror 14. In Fig. 9 the main reflector is concave and metal plates 16 are shaped to conform to the curved surface of the reflector. Figs. 11 to 14 (not shown) illustrate modifications wherein one at least of the reflectors has a plane reflecting surface, additional focusing being provided in Figs. 12 to 14 (not shown) by a phase-advance lens. In Fig. 15 (not shown), both reflectors have concave surfaces, a phase-advance lens being employed in addition.