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Abstract-A novel RF interference rejection technique using four-port network is presented in this paper by using two diplexers combined together. This technique offers the signal isolation of 68.46 dB between transmitter and receiver module, which is the best figure ever reported. The four-port network exploits both high and low-Q factor filters for the cost reduction. The design tolerance with phase deviation between 180 o and 183 o of four-port network was investigated and the novel concept still has signal isolation (S32) of better than 65.47 dB, which is still superior compared to the existing diplexer. Finally, RF interference rejection technique can be used in wireless communication systems whereas small size, low losses and low complexity are required.

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A dual-mode dual-band bandpass filter with high cutoff rejection using an asymmetrical transmission zeros technique is presented here. Two dual-mode filters are combined to form a dualband filter by sharing the input and output coupled-feed line, which is more flexibly designed and maintains a small circuit size. Controllable asymmetrical transmission zeros (TZs) at lower-and uppersideband locations of dual-band filters are designed to achieve the high-selectivity dual-mode dual-band bandpass filter. Unwanted signals are suppressed by the location of the TZs between the first and second passbands, which gives a much-improved signal selectivity for the dual-band bandpass filter. The two passbands are centered at 1.8 and 2.4 GHz, respectively. The first and second passbands' insertion losses are only 0.9 dB and 1.1 dB, respectively, and the measured return losses are better than 20 dB. Three transmission zeros are located between the two passbands, which achieve rejection levels about 40 dB attenuations from 1.9 to 2.3 GHz.

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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