Tunable High-Selectivity Wideband Bandpass Filter Using Triple-Mode Stub-Loaded Resonators Based on Liquid Crystals

This paper presents the design and realization of a tunable wideband triple-mode bandpass filter (BPF) with two transmission zeros (TZs) using liquid crystal (LC). The proposed filter employs a capacitively coupled open-ended stub-loaded resonator (SLR) to realize the extinct resonant characteristics. In addition, a high-impedance line loaded with a radial stub is introduced to preserve the resonant characteristics and to enable the operation of the LC director under bias voltage, despite the use of open-ended SLRs at both ends simultaneously. Through simulations with respect to the stub length and the dielectric constant of the LC, the proposed filter demonstrates advantages in bandwidth design flexibility and stable frequency-agile tunable response without pole merging or pole isolation. Experimental results show that the center frequency can be tuned from 10.61 GHz to 9.48 GHz with a maximum bias voltage of 30 V, while maintaining a triple-mode response with two TZs. Regardless of the applied voltage, the normalized 3-dB fractional bandwidth remains between 29.11 % and 29.55 %, the peak attenuation level exceeds 24 dB, and the shape factor is less than 1.48, which is the lowest reported value among LC-based BPFs. With its wideband and highly selective response, substantial frequency tuning capability, saturation achieved at moderate bias levels (10–12 V), and consistently low passband insertion loss, the proposed filter demonstrates practical advantages and makes a notable contribution to the integration of reconfigurable RF front-end platforms such as cognitive radio and radar systems.