Patent GB-1298344-A (GB)
Title: Non-reciprocal microwave transmission device
IPC: H01P1/387
Filed: 1969-11-21
Abstract: 1298344 Circulators and isolators WESTERN MICROWAVE LABORATORIES Inc 21 Nov 1969 [21 Nov 1968 7 Nov 1969] 57020/69 Heading H1W The rate of circulation of waves propagated in a circulator is such that the wave is concentrated at the edge of the circuit member. The "circulation" is defined in the Specification as the rate of change of direction of the wave as it propagates, e.g. between ports 1 and 2 of the 3-port planar circuit member of Fig. 5. This rate of change (d#/dy) is such that for a given distance between the ports (arc length C) the wave would, if boundary conditions permitted, circulate through an angle #<SP>1</SP>. Now the rate of change of the arc C is such that the angular displacement between ports 1 and 2 is #, and if the circulator is designed such that #<#<SP>1</SP>, the wave will tend to concentrate on the edge defining the arc C. The Specification discusses the theoretical conditions necessary to produce the desired result. The curves defining the edges of the circuit members may be of any geometrical form, including straight lines, provided they contain no discontinuities, and the invention may be applied to circulators having any number of ports. Typical configurations of circuit members are shown in Figs. 3, 5, 12, 14a, 14b, (the circulator incorporating configuration 14a differs from a normal circulator in that the nth port is not coupled to the first port). The circulators are of normal construction with symmetrical ground planes on either side of the planar circuit member and polarized ferrite discs interposed between the circuit member and the ground planes. They may be modified to incorporate the features described in Specification 1,298,345 according to which undesired TE modes of propagation are suppressed by means of a layer or layers of lossy material. These layers may be applied to the outer surfaces of the ground planes which are slotted in the direction of propagation of the desired TEM modes whereby transverse currents associated with the TE modes are absorbed (see Figs. 9, 10, not shown). Alternatively, areas of lossy material may be disposed on the surface the ferrite discs near the edges of, and in the same plane as, the circuit member (see Fig. 11, not shown). In the application of the invention to an isolator, one of the ports of a circulator using any of the configurations described is underlaid by a layer of lossy material whereby the port is internally terminated. It is stated that the lossy material in these modifications of the invention is also effective to prevent moding. An isolator incorporating an unbalanced Y-circulator is shown in Figs. 25, 26 where the circuit member 104 has three ports 106, 108, 110 joined by two curved edges and a straight edge. The port 110 is dissipatively terminated and, to this end, a layer 112 of lossy material having good heat-conductive properties (e.g. silicon carbide) is provided between port 110 and the ground plane 114. The portion of the circuit member 104 which provides the forward transmission path between ports 106, 108 overlies a ferrite element 101 formed from a material having optimum bandwidth and insertion loss characteristics whilst that portion of the circuit member which propagates the reverse wave overlies a ferrite element 102 having optimum isolation characteristics. The magnetic field is provided by a permanent alloy magnet 116 and a layer 121 of highly permeable magnetic material forming part of the ground plane is inserted between the magnet and the ferrite elements. The rest of the ground plane 114 is made of a non-magnetic material. In a modification of this isolator (Fig. 30, not shown), the ports 106, 108 are provided with locally widened portions to improve matching at the connections to external lines.