A compact diplexer using square open loop with stepped impedance resonators

In this paper, a compact diplexer using a square open loop with stepped impedance resonators is proposed to develop the microstrip square open loop resonators bandpass filter. The filter is based upon a square open loop resonator consisting of two identical patches which is attached to the inner corner of the square open loop for size reduction. The compact miniaturized two poles square open loop resonators are used to design filters and a diplexer for IMT-2000 bands application. 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 applications in the IMT-2000 mobile communication systems.