A miniaturized UWB bandpass filter with tunable cut-off frequencies employing rectangular open-loop defected ground structure

This article presents a compact rectangular impedance resonator (RIR)-based ultra-wideband bandpass filter with an improved reflection coefficient within the passband and controllable cut-off frequencies. The proposed filter consists of anti-parallel coupled RIR coupled with rectangular open-loop defected ground structure (ROL-DGS) etched on the ground plane. The ROL-DGS ensures better in-band and out-off-band characteristics with reduced size due to its high Q-factor. The short- and open-circuit stubs control the lower (f(CL)) and upper (f(CU)) 3-dB cut-off frequencies, respectively, by originating transmission zeros independently with the help of variations in design parameters. As a result, the filter having a compact size of 0.216 lambda(CL) x 0.176 lambda(CL) is achieved, where lambda(CL) is the corresponding guided wavelength at the lower cut-off frequency f(CL) of 2.95 GHz. Further, an equivalent circuit is also modeled to explain the working of the proposed filter, and equivalent lumped parameters were extracted. Simulated results reveal that the reported filter passes the frequencies effectively within 2.95-11.44 GHz with fractional bandwidth of 117.9%. The simulated insertion loss values within the passband are less than 1.17 dB, and the reflection coefficient is better than -19.6 dB. Further, the proposed filter is fabricated, and it was found that the measured and simulation results are in well agreement.