Analysis of small-sized winding type directional coupler

Directional couplers are utilized in many communication systems and measuring instruments. The coupler is usually made of two-conductor coupled transmission-lines in the VHF and UHF ranges. Since the coupler becomes large in these frequency ranges, the bifilar line winding can be used to reduce its size. Then, the phase velocity of u- and b-modes become unequal v/sub u//spl ne/v/sub b/, which leads to deterioration of the isolation characteristics of the coupler. In this paper, we analyse the three or more conductor winding type directional couplers, whose isolation characteristics are high. The theoretical and experimental characteristics agree well.