Compact filtering structure with four transmission zeros for extended stopband performance

In this work we present a new compact filtering structure with four transmission zeros in the stopband. The proposed filtering topology consists of a coupled line directional coupler connected in parallel to a transmission line. This structure can be used to design filters with lowpass (LPF) or bandstop (BSF) responses. This can be achieved by controlling the coupling factor of the coupled line coupler and the characteristic admittances of both the coupler and the interference transmission line. Design equations and graphs are provided for the proposed topology. The validity of the design method is demonstrated by the design and implementation of two microstrip filters: a BSF centered at 2 GHz and a two section LPF with a cutoff frequency of 1 GHz, both with sharp rejection and expanded stopband performance.