Dual-/triple-wideband microstrip bandpass filter using independent triple-mode stub-loaded resonator

A dual-wideband and a triple-wideband microstrip bandpass filters (BPFs) using independent triple-mode stub-loaded resonators are presented in this letter. The resonator consists of a half-wavelength transmission line, which resonates at the first resonant mode, and two identical quarter-wavelength loaded stubs generating the other two resonant modes. The dimensional adjustments of a particular resonator only affect itself without influencing the other two resonators, which results in good independent property. The measurement results verify that the fabricated dual-wideband BPF possesses the central frequencies of 2.39 GHz (return loss: 18.2 dB) and 5.13 GHz (return loss: 30.1 dB), and the fractional bandwidth (FBW) of 16.2% (insertion loss: 0.25 dB) and 16.6% (insertion loss: 0.31 dB) were obtained. For the fabricated triple-wideband BPF, the FBW was 16.7% (insertion loss: 0.24 dB) at 2.50 GHz (return loss: 22.8 dB), 14.4% (insertion loss: 0.33 dB) at 3.68 GHz (return loss: 19.7 dB), and 13.2% (insertion loss: 0.40 dB) at 5.04 GHz (return loss: 21.5 dB).