High-Selectivity Single- and Dual-Band BPF Using a Cross Shaped Coupled-Line Resonator

A novel cross-shaped resonator composed of parallel coupled-lines for the design of high-selectivity single- and dual-band bandpass filters (BPF) is presented in this paper. The proposed resonator consists of four pairs of quarter-wavelength parallel coupled-lines two of which having one leg short-circuited. Even and odd mode analysis is performed for the exact derivation of the transmission poles. By adding a quarter wavelength open-circuited stub at the input and output ports two passbands can be created on each side of the operation frequency. To validate the theoretical analysis, the two filters are designed and simulated demonstrating the simple design procedure. Both single- and dual-band BPFs demonstrate an exceptional insertion loss of 0.23 and 0.77 dB respectively. The single –band BPF has a center frequency of 2.94 GHZ and a -3dB fractional bandwidth (FBW) of 50% with good filtering performance, sharp roll- off factor and return loss better than 25dB for the entire passband. The dual- band BPF is centered at 2.2/3.6 GHz having -3dB FBW of 10%/6.7% respectively.