Compact broadband PCML bandpass filter with broad upper stop band

A simple broadband parallel coupled microstrip line (PCML) bandpass filter with compact design is proposed. A PCML structure with two feeding network of various widths is characterized by an equivalent J-inverter network. The extracted parameters indicate that the normalized J susceptance and equivalent electrical length are frequency dependent. A pair of PCML structure with middle resonator, without ground plane aperture at PCML structure and capacitive open-ended stub at the middle resonator, is proposed. The proposed design is further optimized by adjusting the length and width of the middle resonator. Three broadband bandpass filter with PCML structure of various coupling strengths have been designed. The simulated and measured insertion and return loss responses show good agreement with bandwidth of over 80%, return loss of better than -16 dB and 250% broad upper stopband.