Design of a Compact Wideband Microstrip Bandpass Filter Using Multiple-mode Resonator

In this letter, a compact wideband microstrip filter using a multiple-mode resonator (MMR) is present. The basic filter is composed of only one resonator with two symmetrical connecting lines. The resonator operates as not only a resonant element but also an open stub. By properly adjusting the lengths of the stub, the first three resonant modes of this MMR can be evenly allocated within the 4.8-to-10.2 GHz wideband while the 4th resonant frequency is raised above 13 GHz. It results in the formulation of a novel WMBF with compact-size and widened upper-stopband. Simulated and measured results are found in good agreement with each other, showing improved WB bandpass behaviors with the insertion loss lower than 0.5 dB, return loss higher than 12 dB, and maximum group delay variation less than 0.5 ns.