Wide‐band dual‐mode bandpass filter using a modified square‐loop resonator

A novel and compact microstrip dual‐mode bandpass filter with multi‐transmission zeros is designed, fabricated, and measured in this letter. The presented filter consists of a modified square‐loop resonator (MSLR) with dual‐mode resonant characteristic and an improved input/output defected microstrip structure with a dual‐bandgap feature, which is overlapped with the MSLR at the input/output tapping location. Finally, a planar wideband filter with low insertion loss and high selectivity is simulated by the full‐wave electromagnetism solver, and verified by our circuit experiment. The measured results of proposed filter have agreed well with the simulated ones, and have shown an excellent electric characteristic with −3 dB fractional bandwidth 33.9% at the mid‐band frequency, minimum insertion loss 0.5 dB, maximum return loss better than −20 dB, and four transmission zeros nearby the lower and upper pass‐band edges. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:618–621, 2017