Patent GB-804518-A (GB)
Title: Waveguide assembly including a member of ferromagnetic material
IPC: H01J17/04
Filed: 1956-11-02
Abstract: 804,518. Waveguides. SPERRY RAND CORPORATION. Nov. 2, 1956 [Nov. 4, 1955], No. 33477/56. Class 40 (8). [Also in Group XL (c)] A microwave duplexer or circulator comprises a waveguide and means for launching in the waveguide a circularly polarized wave having a clockwise or counter-clockwise sense of rotation, the waveguide being provided with a unidirectional transmission element capable of reflecting a wave having one sense of rotation and transmitting a wave of the opposite sense. The unidirectional element comprises a disc 12 of ferrite situated in the field B of a solenoid 13. The magnetic field is of such a strength that the ferrite disc offers a permeability of zero to a circularly polarized wave of positive rotation and a finite permeability to a circularly polarized wave of negative rotation. The waveguide 11 is of circular cross-section and is coupled at one end to rectangular waveguides 17, 23 whose axes and whose broad walls lie in orthogonal planes. A vertically polarized wave from transmitter 16 is fed through the rectangular guide 17 into the circular guide 11 where it is converted by a polarizer 14 to a circularly polarized wave. The polarizer 14 is of the kind designed to produce a 90 degree phase shift between equal orthogonally-resolved components of a plane polarized wave. The circularly polarized wave rotates in a clockwise sense i.e. the sense of rotation is positive when viewed in the direction of the field. Hence the wave encounters an effective permeability of zero in the element 12 and is reflected back to the polarizer 14. On passing through the polarizer it is reconverted to a plane polarized wave having a direction of polarization at right angles to that of the original wave from the transmitter 16. Since such a wave is compatible with the guide 23 but not with the guide 17, it propagates along the guide 23 to the antenna 22. A wave received by the antenna 22 is coupled as a horizontally polarized wave into the circular guide 11 and converted by the polarizer 14 into a circularly polarized wave having a counter-clockwise sense of rotation, e.g. a negative rotation with respect to the direction of the field B. The wave encounters a finite effective permeability in the ferrite disc 12 and is propagated through the disc to a second polarizer 15 which converts it to a vertically polarized wave. The wave then propagates along a rectangular guide 19 to a receiver 18. The polarizer 15 also serves to convert into a horizontally polarized wave any part of the positively-rotating transmitted wave which leaks past the disc 12. This unwanted wave is incompatible with the guide 19 and is absorbed by lossy material 26 disposed in slots near the top and bottom portions of the guide 11. Mismatch of the antenna 22 may cause reflection of a portion of the transmitted energy. This reflected wave has the same properties as a received-signal wave and, consequently, will propagate in the guide 19 towards the receiver 18. To prevent damage to the receiver a T-R switch 27 is arranged to fire upon the incidence of the reflected wave. The switch does not respond to the weaker echo signals. In a modification (Fig. 4, not shown) the polarizer 15 is omitted and the right-hand end of the guide 11 terminated by a non-reflecting absorptive load. The apparatus now functions merely to isolate the transmitter 16 from reflected energy which is diverted towards the lossy termination. A liquid coolant such as water may be used to cool both the termination and the ferrite element. In the modification of Fig. 6 (a " circulator ") the rectangular guides 17, 23 are duplicated to form a symmetrical system. In this system the directions of coupling are: 17 to 23, 23 to 19, 19 to 30 and 30 to 17.