Patent GB-746305-A (GB)
Title: Improvements in or relating to devices for rotating the plane of pol arization of plane-polarized electromagnetic waves
IPC: H01P1/38
Filed: 1954-02-19
Abstract: 746,305. Wave-guides. WESTERN ELECTRIC CO., Inc. Feb. 19, 1954 [Feb. 27, 1953], No.4948/54. Class 40 (8). A device for effecting a non-reciprocal or antireciprocal rotation of the plane of polarization of plane-polarized microwave energy comprises a birefringent ferromagnetic element subjected to a magnetic field directed transversely to the direction of propagation of the energy. The element 15 comprises a rod composed of a ferromagnetic spinel or "ferrite," e.g. a nickel-zinc ferrite, in powder form mixed with a small quantity of a plastic substance such as teflon and moulded to shape. It is located along the inside wall surface of a circular waveguide 11 in the vertical plane of symmetry. The element 15 is subjected to a transverse magnetic field produced by an electromagnet having polepieces N, S and windings 25, 26. In order to understand the effect of the element it is necessary to assume initially that the magnetic field is zero. Owing to the high dielectric constant of the ferrite material a wave (polarized in the vertical direction R) passing through the guide 11 will exhibit a lower phase velocity than a horizontally polarized wave (polarized in the direction F). The length of the ferrite element 15 may be dimensioned to produce a 180 degree phase difference between two such orthogonally polarized waves. If the effect of the element on the orthogonal components of a wave polarized at an angle α to the " fast " axis F is considered it will be seen that the direction of polarization is rotated through an angle 2α towards the " fast axis as seen along the direction of propagation. Such a rotation is " reciprocal " insofar as, if the energy is reflected back through the device, an equal rotation 2α towards the fast axis takes place and the total rotation is zero. The effect of the magnetic field, which may be explained in terms of the gyromagnetic phenomenon in the ferrite element, is to rotate the " fast " and "retarded" axes F, R through an angle 4 in a clockwise direction to positions F1, Rl for energy propagated from left to right. For energy propagated in the opposite direction these axes are rotated clockwise through the same angle to F2, R2, the absolute rotations being thus opposite for energy propagated in opposite directions. As the field increases # approaches 45 degrees. Following the legends inscribed on Figs. 3, 4 and bearing in mind that, as explained above, the rotation is always in the direction of the fast axis wherever this may lie, it will be seen that for waves polarized in a direction at an angle 0 to the axis F1 the plane of polarization is rotated through a total angle of 4# as the waves pass once in either direction through the device. If the polarization of incident energy for one direction of propagation is coincident with the corresponding fast axis no rotation is produced as the energy is propagated in that direction and the entire rotation of 4# takes place as the energy returns in the opposite direction. If #=2# the whole of the rotation takes place as the energy is propagated in the first direction. Both these devices are fully "non-reciprocal." If #= #, equal rotations are associated with each direction and the device has the same antireciprocal properties as the Faraday-effect rotator of Specification 733,486. In Fig. 5 a birefringent element 48 is arranged in a circular wave-guide 42 connected at either end to pairs of conjugate terminals a, c; b, d formed by rectangular wave-guides 41, 44; 43; 46. These guides are arranged so that they will accept only waves polarized in a direction which is consistent with their dominant TE 10  mode. The element 48 and polepieces N, S are disposed at the angle 67.5 degrees indicated in the Figure, the field being adjusted to produce a 22.5 degrees shift in the fast axes F1, F2 from their no-field position. The fast axis F1 is thus in a vertical plane so that verticallypolarized energy launched from terminal a is transmitted unaffected to terminal b. Since, however, for a vertically-polarized wave launched from terminal b the fast axis F2 is inclined at 45 degrees to the direction of polarization, the wave is rotated through 90 degrees and hence is propagated in terminal c. In a similar manner it can be shown that c is coupled to d and d to a. If the poles of the magnet are reversed the direction of coupling becomes a to d to c to b to a.