An alternative coupling matrix arrangement for capacitively loaded multi-mode microstrip diplexers having close channel bands

In this paper, a design procedure for multi-mode diplexers having closely located bands and an alternative coupling matrix arrangement suitable for all types are reported. The proposed topology includes the interdigital capacitive fingers as a loading element. In addition, the theoretical approach is based on the combination of a polynomial synthesis procedure and the coupling matrix synthesis method. The validation is shown by the design and fabrication of new microstrip diplexers having two different dual/quad modes bandpass filters to explain how the required selectivity and the isolation between the close channels are obtained. To obtain the best performance without an extra matching circuit, the capacitively loaded loop resonators used as a channel filter are fed by a T-junction. To obtain a sufficient coupling effect a serial capacity is used in the input/output. The proposed resonator is investigated by using the even–odd mode analysis to evaluate the resonance frequencies, transmission zeros, and return loss level. An interdigital loading element not only achieves dual-mode characteristics without the requirement of any extra surface area but also moves the transmission zeros to the desired side of the passband. The proposed diplexers have advantages such as compact size, good selectivity, interconnection capability, and high isolation.