Reduced-size multilayer X-band filters with stacked resonators on a flexible organic substrate

This work presents the design of two four-pole, quasi-elliptic microwave bandpass filters with stacked open-loop resonators on an emerging low-loss organic dielectric material. Both filters are based on a multilayer microstrip structure that uses low-cost and flexible liquid crystal polymer substrates to produce a compact size. The first filter, which has a bandwidth of 17% at 9.3 GHz, was designed to control the high coupling strength between overlapped resonators without sacrificing its compact size. It has a measured 1.7 dB of insertion loss. The second filter uses the same topology, but its measured parameters include a bandwidth of 9% at 10.5 GHz and an insertion loss of 4.5 dB. To achieve the 9% bandwidth, two metallic patches were added between layers to limit the higher coupling between the overlapped resonators. Both bandpass filters have a footprint area reduction of at least 25% when compared to an equivalent single-layer, open-loop resonator filter.