A filter with good impedance matching for both in-band and a wide range of out-of-band is reported in this paper. Thus, the proposed filter offers low reflection for a wide range of frequencies, and it can be called as quasi-reflectionless filter. Also, the proposed filter improves the passband flatness significantly. The quasi-reflectionless filter consists of n-pole conventional U-shaped bandpass filters with absorptive stubs (ABSs) placed at the input and output ports. Each part in the whole filter is individually investigated. The U-shaped resonator is studied first, and then the ABSs are analyzed mathematically and simulated to optimize the attenuation rejection. Several parameters that have an influence on the overall performance are investigated. Different n-pole filters are simulated to simply enhance the out-of-band rejection without affecting the passband response. The filter response is furthermore improved by introducing two transmission zeros using the cross-coupling between the two ABSs. To validate the proposed idea, the 3-pole U-shaped quasi-reflectionless BPF is fabricated on an FR4 substrate at the operating frequency of 3.5 GHz. The filter has measured responses very close to the simulated ones.
Bahaa Hamzah Alkhuwaildi,
"Third Order U-Shaped Quasi-Reflectionless Bandpass Filter," Progress In Electromagnetics Research C,
Vol. 123, 27-43, 2022. doi:10.2528/PIERC22061203
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