Patent GB-878016-A (GB)
Title: Improvements in or relating to frequency selective microwave branching filters
IPC: H01P1/213
Filed: 1958-09-17
Abstract: 878,016. Waveguide impedance networks. MARIE, G. R. P. Sept. 17, 1958 [Nov. 12, 1957], No. 29774/58. Class 40(8) A frequency-selective microwave branching filter comprises two rectangular waveguides having one of their broader walls common and coupled by two spaced directional couplers, each including two juxtaposed lengths of rectangular waveguide which are coupled through at least one aperture in a wall common to both of the waveguide lengths, and phase-shifting means in at least one of the guides between the directional couplers, wherein the or each phase-shifting means includes a dielectric medium at least a major portion of the length of which extends across the whole cross-sectional area of the guide. The branching filter shown in Fig. 2 comprises two rectangular guides 1 and 2 juxtaposed along their broad common side 3-or a single guide divided into two similar guides by a wall parallel to its broader sides can be used. Wall 3 is provided with two groups of coupling slots 4, 5 and 4 1 , 5 1 , the slots being spaced by a quarter of the wavelength in the guide for the middle frequency of the entering wave. The opening 11 is used as input, the openings 21 1  and 22 1  as outputs, and the opening 12 is closed by a reflectionless termination. The guide sections 6 and 7 between the couplers are phase shifter sections, the section 6 containing a piece of a first dielectric material and the section 7 containing a piece of a second dielectric material. One of the dielectric materials can be air and the lengths of the dielectric pieces can be different or equal. The ends 8 and 9 of the dielectric pieces have reduced cross-sections to reduce reflections across the separation surfaces between air and solid dielectric. In modifications more than two directional couplers, each pair being separated by two dielectric pieces can be used. (And connections can be made in parallel or cascade. The attenuation, as a function of the frequency, of the wave issuing through one of the outputs with respect to the input wave can be made to conform approximately to a predetermined law by connecting several branching filters in cascade. An assembly of branching filters of the type shown in Fig. 2 for separating four channels is shown in Fig. 5 and comprises two rectangular guides 14 and 15, two directional couplers 16 and 17 and the phase-shifter 18, 19. Guide 15 is closed by a reflectionless termination 20 at its input corresponding to the energy input opening 40. A guide 23 is coupled to the output of the guide 14 by directional couplers 24 and 25 and a phase-shifter 26, 27. A guide 33 is coupled to the output of guide 15 by directional couplers 34 and 35 and a phase shifter 36, 37. The guides 23 and 33 are closed at their input ends. by reflectionless terminations 50 and 51. In the construction shown in Fig. 6 the phase shifters are formed by two resonant cavities, the first located in guide 41 between the irises 45 and the second in guide 42 between the irises 46, the guides being coupled by directional couplers 43 and 44. Energy entering at 47 which is outside the pass-bands of the resonant cavities issues from left-hand end of guide 42 to an output coupling circuit. Alternatively a matched termination can be provided at this load. The dielectric phase shifters 18-19, 26-27 and 36-37 of Fig. 5 can be replaced by resonant cavities as in Fig. 6 acting as band-pass filters. If the waves reflected by these filters are in phase for each pair of filters they are reflected towards the terminations 20, 50 and 51 which can be replaced by circuits. The mathematical theory of the device is developed.