Patent GB-1356260-A (GB)
Title: Tunable microwave filters
IPC: H01P1/215
Filed: 1970-11-05
Abstract: 1356260 Filters NATIONAL RESEARCH DEVELOPMENT CORP 1 Nov 1971 [5 Nov 1970] 52796/70 Heading H1W A microwave filter comprises a flat body of microwave magnetic material, means for feeding a right circularly-polarized (RCP) wave perpendicularly to the faces of the body and means for applying a magnetic field in the direction of propagation. The body may be, e.g. a disc of YIG either in single-crystal or polycrystalline form. The filter operates by virtue of partial reflection of the wave at the surfaces of the body (where discontinuities, e.g. irises, may be provided) and is, in effect, an interference filter. The frequency of operation and magnetic field are such that electron-spin resonance modes are not excited. Magnetostatic resonance modes are suppressed by choosing sufficiently small dimensions for the magnetic body or by making the outer circumferential parts of the body from a lossy material e.g. very broad linewidth polycrystalline YIG or suspended iron filings. The mode of operation of the filter and the suppression of spurious modes are discussed at length in the specification. In the embodiment of Fig. 7, a YIG disc 125 is arranged between circular or elliptical irises 111, 123, in the adjacent broad walls of rectangular waveguides 107, 119, the transverse position of the irises being that at which the magnetic vector of the TE 01  waveguide mode exhibits circular polarization whereby a RCP electromagnetic wave is propagated through the body. The faces of the body may be co-planar with the inside surfaces of the waveguides and the irises formed by conductive coatings on these faces. A magnetic field perpendicular to the faces is provided by a system including polepieces 129, 135, windings 133, 139 and shells 131, 137. A narrow band of wave energy is transmitted from waveguide 107 to waveguide 119 and the remainder of the wave coupled to a load which terminates waveguide 107. By virtue of a directional coupling effect the transmitted energy continues in the same direction towards an output of the end of waveguide 119. Waves reflected from the output will not propagate in the disc 125 since they are LCP. These waves are absorbed by a load at the opposite end of waveguide 119. Both the waveguides are tapered to sections of restricted minor dimension in the vicinity of the coupling. In the embodiment of Fig. 8, a magnetic disc 1, provided on either face with irises 2, 3, is arranged in a dielectrically-loaded waveguide 18 and subject to an axial magnetic field provided by the electromagnetic system 13, 15, 19, 21. Conversion and re-conversion between planepolarized and CP waves is provided in known manner by quarter-wave plates 27, 29 and attenuating vanes 25, 31 are positioned and orientated to eliminate waves reflected from the filter which are polarized perpendicularly to the transmitted waves. As shown in Fig. 9a, the invention is applied to slotlines 215, 217 formed by providing elongated gaps in a conductive coating upon a substrate of dielectric material. At a point where the lines approach each other a magnetic disc 205 provided with irises 207, 209 is arranged with its faces parallel to the axes of the lines and perpendicular to the conductive coating. A magnetic field is provided perpendicular to these faces. It can be shown that the requisite CP wave is excited by virtue of the presence of the disc 205, whereby a narrow band of frequencies is directionally coupled from one slotline to the other. A modification of this construction (Fig. 10, not shown) uses so-called "planar waveguides". These are formed by selective removal of portions of a conductive coating and each consists of a narrow ribbon providing a live conductor narrowly separated on either side from co-planar ground conductors. The disc is arranged as in Fig. 9 with the plane faces arranged coincident with the centre lines of adjacent gaps in respective planar waveguides at a point where the two guides approach each other. It can be shown that an electrically neutral plane extends through the centres of the gaps in the guides and that a CP wave arises in this plane. The mode of operation is similar to that of the Fig. 8 embodiment. In both cases, the irises may have circular apertures or elliptical apertures with the major axes lying parallel to the conductive coatings from which the lines are formed.