Patent GB-818665-A (GB)
Title: Improvements in or relating to ultra-high frequency directional coupler filters
IPC: H01P1/213
Filed: 1957-11-19
Abstract: 818,665. Filters;directional couplers and non-reciprocal waveguide devices. MARIE, G. R. P. Nov. 19, 1957 [Nov. 19, 1956], No. 36037/57. Class 40 (8). A directional-coupler filter comprises two rectangular waveguides coupled by one or more cavities resonating at the same frequency in two modes TE oPq  and TE opq , where p and q are respectively even and odd. As shown in Figs. 2 and 3, rectangular guides 10 and 11 are coupled by a square-section cavity 12, the guides and the cavity having a common median plane extending parallel to the broad sides of the guides. The narrow dimension of the cavity is equal to or less than the narrow dimension of the guides. A TE 01  wave propagated in the guides 10, 11 at the resonant frequency of the cavity is coupled to the cavity by apertures 5, 6, 7, 8 symmetrically arranged about a median plane of the cavity normal to the waveguide axes. The electric field pattern of the TE 023  and TE 032  modes excited in the cavity is shown in Figs. 2 and 3 where the dotted lines represent planes of zero electric intensity. The resonant frequency of the cavity for each mode may be varied independently by movable pairs of dielectric rods 13, 14 and 15, 16, each pair being disposed to lie in a plane of zero electric intensity for the mode it is desired not to affect. It is necessary that the loaded Q's of the cavity for each mode should be equal and these may be adjusted by adjusting the sizes of the coupling apertures. If the pairs of apertures 7, 8 are spaced apart a distance equal to one third of the side of the cavity, they do not affect the loaded Q of the cavity for the symmetrical mode of Fig. 2. The central apertures 5, 6 do not affect the asymmetrical mode. It may be shown that energy at the resonant frequency of the cavity is completely transferred between outputs 1 and 3 or 2 and 4 whilst energy at other frequencies propagates directly between outputs 1 and 2 or 3 and 4. If the cavity is loaded along its axis of symmetry by a ferrite rod biased by an axial magnetic field of sufficient intensity to cause gyromagnetic resonance near the resonance frequency of the cavity, the filter will have non-reciprocal properties, i.e. the cavity will have one resonant frequency for energy transferred from 1 to 3 or 4 to 2 and a different resonant frequency for energy transferred from 3 to 1 or 2 to 4. Broad-band operation may be obtained by a coupling using a number of cavities in parallel spaced apart in the axial direction of the waveguides by an odd number of quarter-wavelengths at the central frequency of the pass-band and tuned to different frequencies within the pass-band. In the modification of Fig. 6, the waveguides 31, 32 are designed to transmit a TE o2  wave and the cavity 30 (in all respects similar to the cavity 12, Figs. 2, 3) is coupled to the broad sides of the waveguides along their centre lines. In the band-separating system of Fig. 7 a TE o1  wave in circular waveguide 40 is transformed to a TE o2  in rectangular waveguide 58. Connected to the rectangular guide are three filters 48, 49, 50 each comprising two parallel-connected cavities and associated respectively with a receiver 53, a second receiver 54 and a transmitter 55. Transition sections 45, 46, 47 interposed between the filters and the devices 53, 54, 55 transform between the TE 02  mode and the TE 01  mode utilized in the devices. If the filters 48, 49 are made non-reciprocal, it is possible to use for the transmitter 55 a frequency lying within the bandwidths of the receivers 53, 54 since the filters will offer a high impedance to waves propagating towards the circular guide 40. The biasing magnets for the ferrite rods are shown at 51, 52. The mode-transforming sections 41, 45, 46, 47 are described in Specification 805,510. Specification 810,057 also is referred to.