GB1189974A - Improvements in Electrical Waveguide Arrangements - Google Patents

Abstract

1,189,974. Filters and switches. STANDARD TELEPHONES & CABLES Ltd. 7 March, 1968, No. 12220/69. Heading H1W. A band-stop filter comprises a rectangular waveguide having an H-plane bifurcation dividing the guide into two branches one of which is effectively dimensioned beyond cut-off. As shown, the rectangular waveguide 1 is divided, over a certain length 21, by a central H-plane septum 2 into two waveguides 3, 4 of reduced height. The waveguide 4 has a width equal to that of waveguide 1 and propagates in the normal manner. The width of waveguide 3 is reduced, e.g. by thickening the narrow walls, to such an extent that its cut-off frequency lies above the operative frequency and only an evanescent wave is propagated in it. It can be shown that a length l of such a waveguide presents an inductive reactance to the wave whereby if the length is terminated by the conjugate capacitative reactance, it behaves as a tuned section and there is full energy transfer within a certain pass-band. Each length l is provided with a capacitative tuning screw 7 whereby the waveguide 3 behaves as a two-section band-pass filter. Each section is associated with a phase-shift of #/2 so that, if the septum 2 has a length equal to one guided wavelength, there exists a differential phaseshift of # between the waveguides 3 and 4. This results in zero output from the guide 1 at the pass frequency of waveguide 3. At other frequencies, the waveguide 3 ceases to propagate, but since the waveguide 4 propagates at all operative frequencies, an output is derived from the waveguide 1. The device therefore behaves as a band-stop filter. If the narrow walls of the waveguide 3 are loaded with ferrite or garnet, the cut-off frequency may be varied by means of a variable transverse magnetic field provided by a permanent magnet or an electromagnet. This variation enables the device to be operated as a switch. The screws 6 are for matching purposes. The necessary differential phase-shift may be obtained for various lengths of septum 2 and various numbers of sections 1.