A Novel Millimeter-Wave Ultra-Wideband Bandpass Filter Using Microstrip Dual-Mode Ring Resonators Loaded with Open Tuning Stubs of Different Lengths

In this paper, a millimeter-wave ultra-wideband (UWB) bandpass filter is developed by cascading two microstrip dual-mode ring resonators loaded with λ₀/4 and 3λ₀/4 open tuning stubs, respectively. The ring resonators are analyzed by using the even-and odd-mode method, and closed-form formulas are derived to facilitate the design of transmission zeros of these resonators. The filter has a midband frequency of 25.5GHz and a 3-dB bandwidth of 5GHz (20%). Due to the multiple transmission zeros produced in both the lower and upper stopbands, the filter exhibits sharp attenuations near the passband and very wide stopbands. Over 13-35GHz, the Federal Communications Commission's indoor limit is satisfied quite well.