GB799163A - Improvements in or relating to band frequency selective wave guide filters for use on very high frequencies - Google Patents

Abstract

799,163. Band-selective waveguide filters. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 9, 1956 [June 24, 1955], No. 18317/55. Class 40 (8). A band-selective waveguide filter comprises a length of waveguide containing two or more spaced resonant elements whose axes are arranged approximately transverse to the waveguide and out of parallelism with each other, the distance between the elements being less than one quarter-wavelength at mid-band frequency. In the simplest embodiment there are two elements consisting of spaced diaphragms having dumb-bell-shaped slots whose axes are arranged respectively at 60 degrees and 120 degrees to the broad face of the guide. Since the elements are at an angle to each other the filter depends for its operation upon evanescent modes and the elements are spaced by a distance (determined by experiment) much smaller than the usual quarter-wavelength to obtain critical coupling. The filter may be used as a coupling between a pair of colinear waveguides 6, 7, Fig. 3, which are twisted at an angle of 90 degrees with respect to each other. Arranged between the guides 6, 7 is a square section coupling cavity 11 terminated at either end by resonant elements 10, 12 in the form of dumb-bell-shaped slots. Each slot may be arranged at an angle of 60 degrees to the broad faces of the adjacent guide. The filter may also be used as a coupling between two guides 13, 14, Fig. 4, whose axes are at right-angles to one another and which are, in addition, twisted at 90 degrees with respect to each other. Portions of the end walls of the guides form the boundaries of a coupling cavity 15 and are provided with dumb-bell slots 20, 22. Each slot may be arranged at angle of 60 degrees to the broad faces of the adjacent guide. In the constructions of Figs. 3 and 4 coupling may be adjusted by varying the dimensions of the coupling cavity or the angle between the slots. The slotted diaphragms need not be exactly parallel or perpendicular to each other. The invention may be applied to provide coupling across a rotating joint. Band-stop filters may be constructed by replacing the slots by dipoles mounted on transverse partitions made of insulating material in mesh form. Ridge waveguides and circular waveguides may be used. By providing a large number of mutually inclined resonant elements the guide will act as a highly dispersive loaded line.