GB1455155A - Microwave transmission line and devices using multiple coplanar conductors - Google Patents

Abstract

1455155 Microwave strip-line circuits RCA CORPORATION 7 Dec 1973 [14 Dec 1972] 56791/73 Heading H1W [Also in Division H3] Devices are constructed which utilize the ability of a so-called "co-planar" transmission line to propagate wave energy in both even and odd modes. Such a line consists of a dielectric substrate carrying on one surface a ribbon conductor spaced on either side from two similar, parallel conductors usually of greater width than the central conductor. The even mode is associated with symmetrical electric fields extending from the central conductor to the two outer conductors, and the odd mode with an electric field extending between the outer conductors and asymmetric about the centre of the line. These modes will propagate within a chosen frequency band provided the relative widths of the central conductor and its spacing from the outer conductor lie within a certain range (fully discussed in the Specification. In particular, if the characteristic impedances associated with the various modes are to be independent of the thickness t of the substrate, then t must be greater than 4b where b is the distance between the centre-line of the central strip and the nearer edges of the outer strips. In the directional coupler of Fig. 5, the coplanar line 51, 55, 57 has a length at the centre of the band of operation. Input/ output terminals are provided by co-axial sockets 66, 67, 68, 69 having outer conductors connected to the outer line conductors 55, 57 and inner conductors connected by wires 70, 71, 72, 73 to the central line conductor 51. The coupler is designed in accordance with known procedures in terms of the odd and even mode impedances of the line. The balun shown in Fig. 6 comprises a co-planar line 81, 88, 89 of length #/4. Central conductor 81 and outer conductors 88, 89 in common provide input terminals for an unbalanced transmission line. One of the outer conductors (89) and a common connection 79 to the other outer conductor 88 and the central conductor 81 provide output terminals to a balanced transmission line. At the unbalanced end, a wire 76 connects the outer conductors. The necessary impedance conditions for effective operation over a band of frequencies are discussed in the Specification.