Patent GB-847471-A (GB)
Title: Inductive coupling means for high frequency apparatus
IPC: H01J23/24
Filed: 1956-09-17
Abstract: 847,471. Microwave filters. VARIAN ASSOCIATES. Sept. 17, 1956 [Sept. 26, 1955], No. 28414/56. Class 40(8). Successive resonant sections of a periodically loaded waveguide, for use in an electron-discharge device or as a filter, are coupled by a negative mutual inductance such that the frequency at which the coupled resonators are in phase is reduced. The invention may be applied to a waveguide loaded with discs each provided with a central hole which provides capacitative coupling between sections for the TM mode of operation. From the Brillouin diagram of Fig. 5 it may be seen that such coupling provides positive group and phase velocities (Vg and V p ) between the cut-off frequencies of the fundamental space harmonic. It is desirable to provide inductive coupling between sections in addition to capacitative coupling, but the result of this is to leave unchanged the lower cut-off frequency, at which the coupled resonators are in phase and to lower the upper cut-off frequency at which the sections are in antiphase. The result is a negative group velocity for the fundamental space harmonic. It is stated in the Specification that this disadvantage may be avoided by reversing the sign of the mutual inductance between coupled resonators whereby the lower cut-off frequency of the fundamental space harmonic is decreased and a positive group velocity together with improved band width are obtained. This may be done by so arranging the resonators and the coupling slot that for the so-called "O" mode of propagation (phase propagation constant zero) the currents on opposite sides of the slot run in the same direction perpendicular to the slot but run in opposite senses around the edges of the slot. In the embodiment of Fig. 10, two adjacent rectangular sections 12, 13 are rotated with respect to each other so that their long diagonals coincide. They are coupled by a central aperture 3 and by pairs of radial slots 15,16 so disposed that the magnetic field lines H 1 , H 2  for "O" mode operation have large components parallel to the slot length running in opposite directions on opposite sides of the wall between the sections. Modifications of this scheme are described in which the sections are parallelograms having coincident longer or shorter diagonals (Figs. 11 to 12A, not shown). In a further modification the sections are equilateral triangles relatively rotated through an angle of 60 degrees (Figs. 15, 15A, not shown). In the modification of Fig. 13, the sections 27, 28 are rectangles having off-set end portions and coupling slots 31. In the modification of Fig. 14 the sections 32, 33 are circular and have pairs of diametrically opposed re-entrant portions which are angularly displaced with respect to each other by 90 degrees. Two pairs of inductive coupling slots 35, 36 are provided. In a further embodiment (Figs. 16, 16A) two circular sections 42, 43 are coupled by a central aperture 3 and radial slots 46, between alternate pairs of which are provided inwardly projecting conductive elements 44. The sections are rotated through 45 degrees with respect to each other. It is stated that the elements 44 cause current to be diverted around the boundary of the slots and thus provide the requisite negative mutual inductance. Specifications 847,472 and 847,473 are referred to.