A Novel Compact Microstrip Diplexer with Closely Spaced Channels

A novel microstrip diplexer with closely spaced channels is introduced by using stub loaded dual-mode square loop resonators (DMSLRs). The proposed resonator is constructed by locating open-circuited stubs with narrow slits to the midpoints of a conventional DMSLR to achieve size reduction. By changing the number of narrow slits, resonant frequencies of the proposed resonator can be controlled. In order to design a diplexer with low channel frequency ratio, two stub loaded DMSLRs having different number of narrow slits are coupled to input and output ports by thin film capacitors. The designed diplexer was also fabricated and measured with a channel frequency ratio of 1.10. The channels were measured at 1.78 GHz and 1.96 GHz with the fractional bandwidths (FBW) of 5.4% and 5.3%, respectively. The measured insertion losses of the channels are 1.99 dB and 1.78 dB. Isolation between the output ports was obtained as better than 28 dB.