Broadband and compact multi-pole microstrip bandpass filters using ground plane aperture technique

A ground plane aperture technique is developed for effective enhancement of the capacitive coupling factor in a parallel-coupled microstrip line (PCML). By applying a so-called ´short-open calibration´ (SOC) scheme in the fullwave method of moments (MoM) algorithm, this PCML with two external lines is characterised by an equivalent J-inverter network with its susceptance and two electrical line lengths. Extracted parameters indicate that the coupling factor appears to be frequency-dependent and its maximum value rises rapidly as the aperture is widened. With the introduction of a single microstrip line section between two identical PCMLs, a broadband and compact multi-pole microstrip bandpass filter is proposed for the first time, and its electrical behaviour is studied and optimised on the basis of its equivalent circuit network. The network-based optimised results are confirmed by an EM simulation of the entire filter layout, featuring ultra-broadband and four-pole bandpass behaviour. Further, a single capacitively loaded line section is utilised to formulate a multi-pole bandpass filter, and its electrical effects are also discussed for filter design. The predicted and measured results confirm attractive properties of the proposed multi-pole filter with BW=60%. |S₁₁|<-16 dB and 220% wide upper stop-band