Design of microstrip dual-band filters using a double- diplexing configuration

Two microstrip filters are diplexed at the input and output ports to realize a dual-band characteristic. The center frequencies, bandwidths and orders can be quite arbitrary since the two individual filters are realized independently. T-junctions are used to connect filters in parallel and extended microstrip arms are added to each filter for minimizing the mutual couplings at design frequencies. Two-port Y-parameter analysis is performed to determine the length of the arms. Three experimental circuits are fabricated and measured to validate the design idea. All the measured results agree well with the simulation.