Compact and high-isolation microstrip diplexer using distributed coupling feeding line

A compact and high-isolation microstrip diplexer using the distributed coupling feeding line is presented. The lower frequency channel filter is based on a short stub-load resonator which has a transmission zero at the center frequency of the upper channel. A pair of hairpin line resonators with the pseudo-interdigital structure are applied to design the upper frequency channel filter which creates a transmission zero at the center frequency of the lower channel. The center frequencies and bandwidths of both channel filters can be controlled independently. The obtained transmission zeros lead to high isolation. A distributed coupling feeding line is utilized to combine the lower and upper channel filters. Taking advantages of the distributed coupling feeding line, no extra matching network is required and each channel filter can be designed independently. Finally, a diplexer working at 3 and 5GHz with over 40dB isolation is fabricated and tested. The simulated and measured results are in good agreement.