Uniplanar tunable bandpass filter using centrally‐loaded slot‐line resonator

A novel uniplanar tunable bandpass filter (BPF) is designed by using the centrally loaded slot‐line resonator. From the voltage‐wave distribution along the resonator, the proper location for the loading element is determined to be the center of the slot‐line resonator, where the voltages of the fundamental signal and second harmonic are maximum and zero, respectively. As a result, the fundamental frequency can be tuned, whereas the second harmonic remains almost unchanged. For the first time, the properties of the centrally loaded slot‐line resonator are analyzed by using the even‐ and odd‐mode method, and their respective resonant frequencies are derived. The demonstrated tunable BPF can give >30% frequency tuning range with acceptable insertion loss when a varactor is used as the loading element.