A wide passband filter consisting of 3-stage ssymmetrical coupler can be realized, where the admittance inverter parameters in equivalent circuit of asymmetrical coupled microstrip lines are given as a function of a fundamental design parameter of a bandpass filter. An experimental filter was designed over 22 percend bandwidth centered at 9GHz and optimized using Super-Compact. Measured forward transmission and forward reflection closely matched design data but measured insertion loss was slightly higher (0.7dB) than the designed value.