DESIGN METHOD FOR COUPLED TRANSMISSION LINE FILTERS HAVING AN ATTENUATION POLE AT ORIGIN.

Although a number of design theories have been reported on halfwave coupled, interdigital and hair-pin type circuits, most of them are approximate. This paper considers their equivalent circuits consisting of physically realizable elements with positive values using the equivalent transformation in distributed coupled lines. The equivalent circuits of the above coupled lines which have a simple attenuation pole at the origin become identical to, or the dual of, simple short-circuited stubs as far as their attenuation characteristics are concerned. Application of the simple short-circuited stubs to the transformer was done by S. Ikeno. In this paper the Ikeno theory is simplified when the input and output resistances are identical and the transfer characteristics are of Wagner type. Simple nomographs are given for nonequal input and output resistances and a coupled transmission line satisfying the requirement is easily obtained by the use of transfer functions. Using this procedure a short-circuited interdigial bandpass filter is designed.