A miniaturized ultra‐wideband filter with ultra‐wide stopband and high selectivity using a capacitive loading stub in a multimode structure

Abstract

AbstractIn this paper, a miniaturized ultra‐wideband filter with ultra‐wide stopband and high selectivity using a capacitive loading stub in a multimode structure is proposed. The second, third, and fourth harmonics of the proposed filter are all suppressed on account of using a capacitive loading stub, which makes it have the characteristic of an ultra‐wide stopband. In addition, there are two transmission zeros located at the upper stopband close to the passband, which improves the selectivity of the proposed filter. The proposed filter is fabricated and measured, and the result shows the filter with the center frequency f0 1.185 GHz has 151% 3‐dB fractional bandwidth. Its upper stopband is extended to 9.18 GHz (7.75 f0) and the 20‐dB roll off rate is 212.5 dB/GHz. What's more, the size of the proposed filter is 0.175 λg × 0.18 λg, where λg is the guided wavelength at the center frequency f0.