A compact diplexer using square open loop with stepped impedance resonators

A compact diplexer using square open loop with stepped impedance resonators is proposed here. The resonator consists of two identical patches with attached to inner corner of the square open loop. The bandpass filter and diplexer are used an alternative technique for the implementation of transmission zeros using an asymmetric feed structures. A prototype diplexer is designed at 1950 MHz and 2140 MHz. The diplexer channels are 3.07% and 2.8% fractional bandwidth. The measured insertion losses are about 3.0 dB for the two channels with better than 45 dB isolation. It is very useful for the mobile communication systems.