Novel Compact UWB Bandpass Filters Design With Cross-Coupling Between $\lambda / 4$ Short-Circuited Stubs

Two novel, compact, and highly frequency selective ultra-wideband (UWB) bandpass filters (BPFs) based on an improved model are presented in this letter. The improved model contains four short-circuited stubs connected by transmission lines. A cross-coupling between the first and last short-circuited stubs is introduced to obtain new pairs of transmission zeros on both sides of the passband. The relationship between the coupling factor and the position of the transmission zeros was analyzed and good agreement was obtained between the analysis and model simulation results. Based on the model, two UWB BPFs are designed with 51% (3.1 to 5.2) GHz and 108% (3 to 10) GHz relative bandwidth and small size of 15 mm × 15 mm and 19 mm × 14 mm, respectively. The insertion loss of the (3.1 to 5.2) GHz filter is less than 0.4 dB in passband and the other one is less than 3 dB. The simulation and measurement results agree well.