Compact tri-band bandpass filter using three stub-loaded open-loop resonator with wide stopband and improved bandwidth response

A compact three stub-loaded open-loop resonator-based tri-band bandpass filter with better out-of-band suppression is presented. The position and length of the three stubs loaded to an open-loop resonator determine the position of the three passbands. Mathematical analysis has also been performed to calculate the resonance condition of the proposed resonator. To demonstrate the concept, two second-order filters are fabricated having three passbands centred at f_o = 1.93 GHz, f₁ = 3.6 GHz, f₂ = 4.89 GHz and f_o = 1.59 GHz, f₁ = 3.12 GHz, f₂ = 4.02 GHz, respectively. The fabricated filters show better 3 dB bandwidth as well. The simulation and measured results are in good agreement.