GB738877A - Improvements in or relating to electromagnetic wave signal transmission systems - Google Patents

Abstract

738,877. Wave-guide transmission systems. WESTERN ELECTRIC CO., Inc. May 19, 1953 [May 28, 1952], No. 14018/53. Class 40 (8). [Also in Group XL (c)] A microwave transmission system comprises an element capable of being rendered transparent to electromagnetic waves by a steady magnetic field applied to the element in a direction perpendicular to the magnetic vector of the waves. In the apparatus of Fig. 1 the element 12 comprises a plate made of nickel-zinc ferrite powder in a binder of polystyrene. The plate is arranged at the junction of two waveguides 10, 11 and is situated in the air gap of an electromagnet 13 in such a manner that the magnetic flux in the plate is parallel to the electric vector of the E.M. wave propagated in the guides and the direction of propagation is in a plane perpendicular to the flux. In the Specification there is derived a mathematical expression connecting the effective high-frequency permeability Áeff of a ferromagnetic material with the magnetizing field, the frequency of the E.M. waves and the gyromagnetic ratio for electrons. This expression is used to derive the curve of Fig. 2 which shows the variation of Áeff with the magnetic field strength for a particular frequency. When the field strength is increased above the value necessary to produce gyromagnetic resonance (where Áeff = oc) it is possible to obtain high positive values of the effective permeability Áeff and by choosing a value of the field strength such that Áeff becomes equal to the effective highfrequency dielectric constant of the material it is possible to make the intrinsic impedance of the material equal to that of air (unity). By this means the plate 12 may be matched to air (or to any suitable dielectric) or it may be made highly reflecting by altering the magnetic field to a value such that a mismatch is produced. A gas under pressure or a liquid may be contained on either side of the partition 12. In Fig. 4 a cylindrical ferrite member 41 supported by a ring 80 of polystyrene foam is subjected to the field of an electromagnet 44, the field being parallel to the E vector of the impressed wave. With the magnetic field adjusted to a point in the range below the point of zero effective permeability (see Fig. 2) the potentiometer 48 may be varied to vary the (positive) permeability and hence the phase shift of the wave. A periodically varied phase-shift may be obtained by applying a saw-tooth wave to the winding of the electromagnet. Alternatively, if the magnetic field is increased to produce partial absorption in the neighbourhood of gyromagnetic resonance variations in the magnetic field will produce variations in absorption and hence amplitude modulation of the electromagnetic wave. The Specification also gives a mathematical treatment of the field configuration in the ferrite element of an electromagnetic wave propagated in the direction of the applied magnetic field. It is shown that an impressed plane-polarized wave gives rise to a positive and a negative circularly-polarized wave within the element. The graph shown in Fig. 3 gives the effective permeability as a function of the applied magnetic field for both waves. In Fig. 6 a plane-polarized wave is impressed upon the ferrite element 66, the direction of propagation being the same as the direction of a magnetic field produced within the element by a solenoid 67. It can be shown that there is complete reflection of a wave when the value of the effective permeability is zero or negative provided that, at the same time, there is no absorption due to resonance. Hence the value of the magnetic field in the element 66 can be set (see Fig. 3) to give total reflection of the positive circularly-polarized component of the impressed wave and partial transmission of the negative circularly-polarized component. If the impressed wave is circularly polarized in the positive sense the magnetic field may be adjusted to give either reflection or partial transmission. Hence a varying signal applied to the solenoid can be used to amplitudemodulate the wave in a manner similar to that described with respect to Fig. 4. The electromagnets used in the various constructions can be replaced by permanent magnets and the ferrite element can be replaced by a container of gas or liquid which has a suitable gyromagnetic absorption characteristic with a resonance not too far removed from the frequency of the incident energy. Gases trapped in a clathrate molecular structure are suitable for this purpose.