Compact Wideband Bandpass Filter Based on the Perturbed Quintuple-Mode Resonator

This letter presents a systematic mode-control technique for quintuple-mode bandpass filters using multimode resonators. A central grounding via is introduced to lower the frequency of the TM 01 mode by altering the E-field distribution. Additionally, radial slot perturbations and selective radius variations are incorporated, which independently tune the frequencies of the four higher-order modes and separate closely spaced resonances. A precise phase de-embedding method is utilized to extract external quality factors and optimize tapped-line coupling structures. To validate this approach, a quintuple-mode bandpass filter based on a perturbed multimode resonator is implemented, achieving a center frequency of 4.50 GHz and a fractional bandwidth of 54.4%.