Three element compact broadband parallel-coupled microstrip bandpass filter of simple configuration

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 centre resonator, without ground plane aperture at PCML structure and capacitive open-ended stub at the centre resonator, is proposed. The proposed design is further optimized by adjusting the length and width of the centre resonator. Three broadband bandpass filters with PCML structure of various couplings have been designed. It was found that the simulated and measured insertion and return losses responses showed good agreement with operating bandwidth of over 80%, return loss of better than -16 dB and 250% wide upper stopband.