Microstrip diplexer with low channel-frequency ratio

The paper presents a microstrip diplexer constructed by joining two stub-perturbed ring resonators at a T-junction. Each ring resonator acts as a bandpass filter with a transmission zero lying very close to the passband of the other filter, thereby increasing the isolation between the two channels. A short stub is additionally added at the T-junction to improve the matching at the common port. The diplexer is able to achieve a channel separation of 50 MHz, with the two channels centered at 2.49 GHz and 2.37 GHz respectively. This translates to a center frequency ratio of 1.05 between the two channels, which is the smallest amongst the published works. The insertion loss in both the channels is below 2.4 dB. The return loss at the three ports is around 20 dB, with the in-band channel isolation being greater than 20 dB.