A compact diplexer using square open loop with stepped impedance resonators

Abstract

I. IntroductionThe recent advance of RF/microwave applications has stimulated the rapid develop of new communication system. The research work is focused on the third generation (3G) mobile communication systems. The diplexer are generally employed in order to share one antenna for both transmitting and receiving. Therefore, how to design a bandpass filter and diplexer at low cost and with high performance is currently of great interest. Microstrip bandpass filters can be easily mounted on a dielectric substrate and can provide a more flexible design of the circuit layout [1]. The square open loop resonators filters have been known for years. The compact high performance microwave bandpass filters are highly desirable in the wireless communications systems [2]. The design of different filters and diplexer was dicussed, many authors [3]-[8] based diplexer is preferable because of its compact size. For bandpass filter based diplexer designs, the spurious response of a bandpass filter gives poor isolation performance; consequently, a technique to suppress this spurious response is required. Therefore, it is desirable to develop new types of square open loop resonators. In this paper, a compact diplexer using square open loop with stepped impedance resonators is introduced as an alternative for the miniaturization of the filter and diplexer structure. The square open loop 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. II. Bandpass filter design The filter illustrated in Fig. 1(a) shows a conventional square open-loop resonator [2]. The proposed filter is based on square open-loop resonator consists of four identical patches with attached to an inner corner of the square open-loop is shown in Fig. 1(b). The use of a capacitive patches to reduce the size of the microstrip

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