Design of Microstrip Lossy Filter Using an Extended Doublet Topology

A lossy three-pole microstrip bandpass filter using an extended doublet network is proposed in this letter. Nonuniform Q distribution is considered by reducing the $Q_{u}$ of one resonator, resulting in a flatter passband with an enhanced selectivity. Also, the novel feeding network adopted brings about two extra transmission zeros and a wide stopband that further improve the filter performance. Good agreement between the simulated and measured results can be observed to validate the design.