Design of wideband codirectional couplers and their realisation at microwave frequencies using coupled comblines

The combline directional coupler (c.d.c.) is a recently described planar device suitable for m.i.c. applications which couples power in a forward direction through controlled phase matching of the coupled waves. As a result high coupling levels (up to almost 0 dB) may be achieved at high frequencies (up to 20 GHz) without the need for close conductor spacings. The bandwidth of a basic c.d.c. is approximately 30%. We describe here two methods of broadbanding the c.d.c. which retain the advantages just mentioned and which enable operation over almost a decade to be achieved. Results for a 1.9¿¿19 GHz, 3 dB power dividing coupler are presented. Both methods involve a spatial variation of the coupler parameters, and one of them, using the theory of warped modes, makes possible directional couplers which have a combination of frequency range and coupling level flexibility not at present obtainable by any other means. Particular attention is paid to practical details; design tables and methods are given which enable experimental couplers to be produced with any desired coupling level. We present experimental examples illustrating the various types of coupler design discussed and compare their performances. An indication is also given of the way in which the broadband design methods used for the c.d.c. may be applied to other types of planar forward directional coupler.