Warped-mode multiway combline directional couplers

A new general matrix theory of mode warping is advanced for directional couplers with nearestneighbour interaction and an arbitrary number of lines. The design procedure involves defining the form of the normal-mode vector, the type of `taper parameter¿¿ and then computing changes in the elements of the `wave-propagation¿¿ matrix due to differential changes of normal-mode vector. The wave-propagation matrices may thus be obtained at any point of the warped-mode coupler. These computed wave-propagation matrices at certain positions of the coupler are then used for obtaining the theoretical power against frequency plots at the output ports as well as for computing the physical dimensions of the coupler. A specific design using five coupled lines with the centre frequency at C band has been fabricated to demonstrate the power of the technique. Here using coupled comb/herringbone lines on microstrip, a normal mode of a single-line input is warped into a five-line mode with equal power splitting. Computed and measured results are presented for an experimental five-line coupler fabricated in open microstrip line form for working within 1.5 to 8.5 GHz. Applications and implications of the theory are discussed.