A diplexer with direct coupled resonant junction

Conventionally, diplexers are designed by connecting two filters of different frequencies using a transmission T‐line junction which acts as a distribution network serving the individual filters. Here, a novel diplexer has been proposed and designed to utilize a directly coupled resonator as the distribution network. The directly coupled resonator is designed using two square open‐loop resonators of the same resonant frequency of 1.95 GHz. Coupling of the two resonators is achieved using a λ/4 transmission line to directly couple them. This process made the resonator pair to possess a dual‐mode resonator function, resonating at 1.85 and 2.05 GHz. Two bandpass filters are then designed in the corresponding modes of the resonator pair with 5% fractional bandwidth and coupled accordingly. This technique allowed the resonator pair to contribute to the resonant poles of the diplexer while functioning as the distribution network.