A compact size and wide stopband microstrip quadruplexer with a common crossed resonator is proposed in this paper. The resonator mentioned is theoretically analyzed and proved to be able to resonance at three different frequencies, which can be easily modified by changing the length of the corresponding stub of the resonator. This tri-mode resonator is proved to have the capacity of being shared by three different bandpass filters in a quadruplexer in this paper. Then an additional channel is designed to be coupled to the other side of the feed line of the common input port. Compared to conventional ones, the proposed quadruplexer has a more compact structure, cause no extra matching network is needed, and the number of resonators is reduced effectively. Moreover, a wide stopband is obtained by making the resonators work at the same fundamental frequencies but different higher order frequencies. Besides, open circuit stubs are also used to suppress the harmonic frequencies. To demonstrate the design procedure, a quadruplexer with a third order Chebyshev response in each channel is fabricated and measured. The measured result is in good agreement with the simulated one, showing an attenuation of 20 dB up to 10.16 times of the first channel frequency.
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