Patent GB-987593-A (GB)
Title: Waveguide filter
Inventor: CRAVEN GEORGE FREDERICK
IPC: H01P1/213
Filed: 1962-04-30
Abstract: 987,593. Waveguide hybrid couplings. STANDARD TELEPHONES & CABLES Ltd. April 30, 1962, No. 16380/62. Heading H1W. [Also in Division H3] Waveguide junctions of the type shown in Figs. 1, 2, 4 are provided with means for varying the resonant frequency of the coupling slots 2, 3 and means for adjusting the impedance of the slots as seen from the main waveguide 1. The junctions each comprise longitudinal and transverse resonant coupling slots 2, 3 arranged at the same axial position in the same or in opposite broad walls of a rectangular waveguide 1. The slots communicate with branches which may comprise either a single circular waveguide 8, Fig. 1, transmitting orthogonal modes or separate rectangular waveguides 4, 5, Fig. 2; 6, 7, Fig. 4. A modification is described (Fig. 3, not shown) in which the waveguides 6, 7 are arranged end-on to the broad walls of the main waveguide 1. It can be shown that, at the resonant frequency of the slots, the whole of the input from one end of the waveguide 1 is coupled into the branches. The two ends of the waveguide 1 may therefore be considered as conjugate arms of a hybrid junction, the branch waveguides forming the other two conjugate arms. A further main waveguide with associated branches, similar to and forming a mirror image of the arrangements shown, may be used to complete a system having the properties of a directional coupler. Thus, in the arrangement shown in Fig. 17, the whole of the input energy from arm A at the resonant frequency of the slots 2, 3, 200, 300 appears at the output arm F. At other frequencies, the output is derived at the arm B. The circuit may thus be used as a channel-dropping filter. A similar arrangement may comprise two junctions of the type shown in Fig. 1 using a resonant cavity or cavities of the kind shown in Fig. 10. In an alternative arrangement, the slots 200, 300 are tuned to a local oscillator frequency and the oscillator is connected to the arm E. An intermediate signal frequency is derived from the crystals 35. The slots may be tuned by means of movable capacitative plates 11, Fig. 13, or eccentrically rotatable discs (Fig. 8, not shown). Alternatively, tuning screws 16, 19, Figs. 9, 10, may be associated in the manner shown with resonant cavities 17 in the branch guide or guides. In the embodiments of Figs. 1, 2, 4 the slots are tuned by screws 9, 10 arranged one half or one quarter-wavelength from their associated slots. A detailed discussion of the impedance matching conditions between the various branches to obtain desired distributions of power is given in the Specification. In general, the impedance transformation effected by the slots may be adjusted by varying the distance of the slots from the axis of the waveguide. Thus, the longitudinal slot 20, Fig. 13, is formed in a plate 21 which may be moved towards and away from the axis of the circular guide 8. The transverse slot 2 may have a similar adjustment. Alternatively and as shown, the distance of the transverse slot 2 from the centre of guide 8 is adjusted by an eccentric rotation of the guide which is provided for this purpose with arcuate mounting slots 23. A further means of adjusting the effective transformation ratio of the slots consists in providing one of the diaphragms 18, Fig. 10, bounding the cavity 17 with an elliptical aperture and clamping the diaphragm between two sections of waveguide after angular adjustment. Fig. 18 shows a directional coupler essentially consisting of two junctions of the kind shown in Fig. 4. Corresponding references are used on the two Figures for analogous parts. In addition to screw tuning members 9, 10 the embodiment of Fig. 18 makes use of pistons 37 in the ends of the coupling guides 6<SP>1</SP>, 71.