Introduction of a Transmission Zero in Directional Filters by Using Unequal Length Coupled Sections for Inter-Resonator Coupling

A novel directional filter topology is proposed where a transmission zero is introduced at no cost of increased total electrical length compared to classical topology. Two coupled line sections per wave-long resonator featuring unequal transmission lines are used for inter-resonator coupling. One of the sections is realized to be close to half-wavelength of electrical length at a center frequency so that its deep null is located at the edge of the filter’s stopband. The approach is theoretically analyzed and validated using a transmission line model of a $2^{\text {nd }}$ order directional filter demonstrator operating at 1 GHz with 4% bandwidth. Simulation results confirmed the filter to exhibit frequency response with extra transmission zero certifying the usability of the proposed approach.