This letter presents a high isolation substrate integrated waveguide (SIW) diplexer using dual-mode resonators. Each dual-mode SIW resonator can generate one transmission pole (TP) and one transmission zero (TZ) for the channel filter. The frequencies of the TZs can be easily controlled by the perturbation vias. One channel filter's two TZs' frequencies are designed to cover the other channel filter's passband. Thus, high isolation performance is achieved. The proposed diplexer is designed and fabricated with lower and upper channels centered at 8.04 and 9.07 GHz, respectively. Simulated and measured results are in good agreement for the two channels with better than 49/53 dB channel isolation.
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