Patent GB-643738-A (GB)
Title: Improvements in directional aerials
IPC: H01Q19/12
Filed: 1947-08-14
Abstract: 643,738. Aerials. CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. Aug. 14, 1947, No. 22667. Convention date, Feb. 29, 1940. [Class 40 (vii)] A directive aerial system for transmitting or receiving circularly-polarized radio waves comprises a concave reflecting surface, generated by rotating about its axis a half-parabola, the parameter of which increases by a half-wavelength as a complete rotation is made while the focus remains fixed, and a dipole aerial placed at the focus with its longitudinal axis arranged along the axis of the reflector. The parameter of the generating parabola may increase uniformly with the rotation so that a section of the reflector perpendicular to the axis is as shown in Fig. 3, or it may increase discontinuously, remaining constant over an arc subtending an angle of 2#/n and then increasing by #/2n in the adjacent arc, n being any integer and Fig. 4 illustrating a section of the reflector obtained when n is 3. Fig. 1 shows a section of the system including the axis of the reflector. The dipole d is situated at the focus o. It can be seen from Fig. 1, considering the transmission case, that the radiation reflected from the lower half-parabolic section in any axial plane lags on radiation reflected from the upper half-parabolic section by a half-wavelength and that the directions of the electric vectors in the reflected radiation are as shown by the arrows in the Figure. Moreover, radiation reflected in a plane inclined to the one illustrated will differ in phase by an amount equal to the angle between the planes and the resultant emergent radiation will be circularly polarized. Since the dipole d does not radiate along its axis, nearly all the radiation therefrom is intercepted by the reflector. Moreover, the portion of the reflector about the apex reflects very little energy and it may be cut away as illustrated in Figs. 3 and 4. The Specification as open to inspection under Sect. 91 describes a system for radiating spherical waves wherein the parabolic profile of the reflector is replaced by a hyperbolic or elliptical profile. This Specification also describes a system for radiating or receiving circularly polarized radiation comprising a plurality of angularly spaced aerial elements 20, Fig. 1 (Cancelled), mounted at the focus of a parabolic reflector 22 and fed with suitably phased currents. As shown, a plurality of generators 24 is used, their relative phasings differing by 2#/3; however, a single generator may be used and the phase-differences introduced by suitable networks. In a modification of this system a rectangular wave-guide is energized by a plane-polarized wave inclined at 45 degrees to the guide walls and the dimensions of the guide are such that the two components of the wave transmitted down the guide differ in phase by 90 degrees at the end thereof. These are then caused to excite a pair of crossed dipoles or are radiated through apertures such as shown in Fig. 5 (Cancelled), thus resulting in circularly-polarized radiation. In Fig. 4 (Cancelled, not shown), is illustrated a system having two feed-points for the waveguide, one feed-point being used for radiating and receiving radiation polarized in one direction of rotation, while the other feed-point is used for oppositely polarized radiation. This subject-matter does not appear in the Specification as accepted.