GB925396A - Improvements in or relating to electric wave filters - Google Patents

Abstract

925,396. Band-elimination filters. STANDARD TELEPHONES & CABLES Ltd. May 29, 1959, No. 18358/59. Addition to 845,957. Class 40 (8). An electric wave band-elimination filter for transmitting a wave having a frequency lying in a first frequency range and for substantially suppressing a wave having a frequency lying in a second frequency range lying above the first range, comprises a three-sectioned ridge waveguide 1, 2, 3 having ordinary rectangular waveguide terminations 7, 8 coupled to its ends 1, 3, the ridge waveguide transmitting waves of frequencies in both ranges, a principal tuning screw 13, Fig. 5, and first and second resistive elements 14, 15, Fig. 4, for absorbing energy reflected by the screw 13, coupled to the waveguide 2 at points on opposite sides of the screw, which are each spaced from the screw by an integral number of half wavelengths in the guide 2 at the mean frequency of the second frequency range. Each waveguide termination 7, 8 is provided with one or more auxiliary tuning screws 20, 21 and is coupled to the ridge waveguide by an adjustable metal probe 11, 12, Fig. 2, secured to the ridge 6 of the waveguide and passing through holes in the ridge waveguide and the waveguide termination. The waveguide section 2 is coupled to each of the resistive elements 14, 15, consisting of a rectangular waveguide stub terminated by a resistive element 27, by a rectangular slot 16, 17 with its axis normal to the axis of the waveguide 2 and resonant at the mean frequency of the second range. The waveguide stubs 14, 15 are of such dimensions as to have a cut-off frequency lying above the highest frequency of the first range. Each stub 14, 15 is provided with one or more auxiliary tuning screws 18, 19. The resistance of the resistive element 27 is equal to the characteristic impedance of the waveguide stub. The distance d 2 between the centre of the slot 16 and the inside surface of the wall 25 of the stub 14 is equal to half the wavelength in the stub. The distance between the probe 11, 12 and the adjacent closed end of the ridge guide 1, 3 is half the ridge guide wavelength at the mid-frequency of the first range. The position of the holes 22 in the terminations 7, 8 is such that the impedance presented at the open ends of the flanges 9, 10 should be equal to the characteristic impedance of the terminations. The tuning screw 13 can be replaced by three equally spaced screws and an additional slot and stub line as in the parent Specification.