Wideband bandpass filter design with three-line microstrip structures

A systematic procedure is described for designing wideband bandpass filters based on parallel coupled three-line microstrip structures. It starts with modal analysis of a three-line coupled microstrip circuit. A database of modal eigenvoltage coefficients and modal characteristic impedances for a specified value of substrate ε_r is established. The relation between the circuit parameters of a three-line coupling section and an admittance inverter circuit is derived, so that the filters can be synthesised by a standard procedure. As compared with traditional parallel coupled lines, the proposed three-line design has the following two important features for the design of wideband filters: the tight line spacings of end stages can be greatly relaxed; and the stopband rejections are improved. Four filters with fractional bandwidths from 40% to 70% are fabricated on substrates with low and high dielectric constants. Prediction and measurement results are in good agreement.