Patent GB-817349-A (GB)
Title: Circularly polarised microwave lenses
IPC: H01Q15/24
Filed: 1957-04-24
Abstract: 817,349. Radio wave-polarizing systems. MARIO, G. R. P. April 24, 1957 [April 24, 1956; March 27, 1957], No. 13012/57. Class 40 (7). The invention relates to a combined lens and polarization changer construction for effecting an interchange between an electromagnetic rectalinearly polarized wave having a given field pattern and a circularly polarized homogeneous plane wave. Particular constructions transforming wave patterns of TE N1  or TM N1  in a circular guide into a free space circularly polarized wave are considered. The construction comprises three parts. Firstly the face of the lens is divided into a number NM sectors; M being so chosen that each sector is of sufficiently small apex angle for the field pattern to be regarded as having a single direction over the whole area and each sector is provided with a phase-shifting plate of a thickness such that a phase lag proportional to the angle between electric vector of the sector surface and a fixed direction of the wave surface. is produced. Secondly, each sector is covered with its own quarter-wave plate arranged so that the angle formed by the axes of the plate is bisected by the direction of the electric field in the sector. Finally a lens is provided on one side of the sector assembly to transform the wave surface in the guide into a plane wave at the assembly. In one embodiment illustrated in Fig. 3, energy of TE o1  mode in a circular guide 1 is changed into a circularly polarized plane wave which is radiated in a given direction at right angles to the axis of the system. The circular guide is coupled to a flared conical horn 2 at the mouth of which is mounted the lens 9 having a plane face 21 and a convex face 7. The lens is shaped to give a plane wave at the face 21 and the face 7 is provided with circular grooves 10 of a quarter wavelength in depth to prevent reflections. The polarization changer immediately above the face 21 is divided into nine sectors of which six can be seen in the Figure. Each section comprises a quater wave plate having rectalinear grooves 20 inclined at 45 degrees to the radius which bisects the sector. The depth p width b and separation a of the grooves are chosen so that the sector functions correctly as a quarter wave plate at the working frequency. Between the quarter wave sector plates and the surface 21 phase-shifting plates 8 are introduced to make the necessary phase corrections. These plates may be of like sector shape and have parallel faces and a uniform thickness of where n is the square root of the dielectric constant of the plate material. No plate of this type is provided in sector 11, one in sector 12, two in sector 13 and so on. The structure is protected by a dielectric cover 22 in which is secured a mirror 23 inclined at 45 degrees to the vertical. This directs the energy radiated outwardly from the axis in a direction determined by the angular positioning of the cover. The Specification includes a full theoretical discussion of the arrangement including an explanation of why the phase-shifting plates are needed. It also describes briefly with reference to Figs. 4 to 7 (none of which are shown), the constructions necessary where the rectalinearly polarized wave is of the TE H  mode or the TE 31  mode.