High isolation tunable diplexer based on mixed electromagnetic coupling

In this article, a tunable diplexer based on quarter‐wavelength resonators with mixed electromagnetic coupling is designed. Different coupling types are utilized to generate a transmission zero in the stopband of filter, aiming to improve the isolation of diplexer. The meander line tunable filter with a magnetic dominated coupling forms channel 1 of the diplexer, a transmission zero in upper stopband is realized. The tunable filter dominated by electrical coupling constitutes channel 2 of the diplexer, a transmission zero is produced in lower stopband of the filter. Varactor diodes are loaded at quarter‐wavelength resonators for tunability of the center frequency. The proposed diplexer is both demonstrated by simulation and measurement. Experimental results show that the designed tunable diplexer has a frequency range of 0.85‐1.2 GHz and an isolation of greater than 42 dB.