Compact Dual-wideband Bandpass Filter Using Stub Loaded Zero-degree Feed Coupling Structure

A novel compact dual-wideband bandpass filter is proposed in this paper, by employing stub loaded ring-shape Uniform Impedance Resonator (UIR) and embed-inside T-shape resonator which contributes to reducing circuit size. Odd-even mode method can be used to analyze the resonance characteristic, since the structure is symmetrical. The stub-load UIR can produce two resonant frequencies of f odd1 and f even1 , and the T-shape resonator can also introduce two resonant frequencies of f odd2 and f even2 . By adjusting the position of odd-even mode resonant frequencies and the coupling degree between UIR and T-shape resonator, two wide passbands can be realized. In design, a pair of zero-degree feed coupling structure is utilized to generate two transmission zeroes at the lower and upper of the first passband, improving the frequency selectivity of the first. The 3 dB frequency bands range of the first passband and the second are 2.5 GHz–3.3 GHz and 5.1 GHz–6.2 GHz, and the fractional bandwidths are 29% and 19%, corresponding to center frequencies at 2.8 GHz, 5.8 GHz, respectively. Comparing with the previous works, the proposed dual-wideband bandpass filter has a compactness of 12 × 10.45 mm2 and has lower insertion loss of 0.1 dB and 0.12 dB, corresponding to the first and the second pass-band.