GB850054A - Improvements in or relating to circular polarisers for electro-magnetic waves - Google Patents

Abstract

850,054. Waveguides. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. May 28, 1958 [Aug. 20, 1957], No. 26320/57. Class 40(8). Circularly-polarized wave energy is produced by means of a waveguide section loaded by an axially-polarized soft-ferrite element whereby propagation of waves circularly polarized in only one sense is possible. The soft-ferrite element 1 is spaced from the sides of the circular waveguide section 2 by means of a dielectric substance 3 which may be a hard ferrite or may be air. The dimensions of the section 2 in the vicinity of the element 1 are such that the section would be below cut-off at the frequency utilized if it were completely filled with the dielectric 3. By means of a permanent magnet (e.g. the filling 3, where this consists of hard ferrite) or a solenoid 5, an axial magnetic field is set up in the soft-ferrite element 1 which becomes anisotropic in the sense that waves circularly polarized in opposite directions have different phase constants within the element. Fig 4 shows how the phase constant B for waves polarized in a certain sense varies with the ratio of the radius b of the guide section 2 to the radius a of the soft-ferrite element 1. The curves B and D respectively, and A and C respectively, relate to right and left-hand senses of polarization and are plotted for different sets of values of magnetic field and relative permittivities of the loading elements 1, 3. It can be seen from the form of the curves B and D that a set of parameters may be chosen such that within the range of values Q to Q<SP>1</SP> of b/a only waves polarized in the right handed sense will have a positive phase constant. The apparatus includes means 13 for launching a plant-polarized wave within guide section 9. Such a wave may be resolved into two components, circularly polarized in opposite senses, and only the righthanded one is propagated through the loaded guide section 2. Since the "sense" of rotation of the wave is defined relative to the direction of the magnetic polarizing field, it is possible to reverse the direction of rotation of the propagated wave as seen in the direction of propagation by reversing the magnetic field. The power transmitted by the device is a function of the magnetic field strength and, accordingly, it may be used as an attenuator or modulator by suitably controlling the current in the solenoids 5. By reducing the field to zero it is possible to prevent propagation altogether.