Miniaturized Diplexer With High Selectivity Based on Transition-Free Effective Surface Plasmon Polaritons

This letter presents a miniaturized high-selectivity diplexer utilizing transition-free effective surface plasmon polaritons (ESPPs). The design leverages the steep asymptotic characteristics of ESPPs to enhance frequency selectivity while eliminating mode-conversion structures between rectangular and ESPP waveguides, achieving a compact footprint. Experimental results show both channels exhibit in-band insertion loss (IL) below 0.45 dB, return loss (RL) exceeding 16 dB, and interchannel isolation greater than 40 dB. Compared to conventional diplexers and those reported in the literature, the ESPP diplexer offers a more compact size ( $2.402\lambda \times 2.429\lambda \times 0.552\lambda $ ) and superior selectivity, with shape factors of 1.45 and 1.207, respectively. These results highlight the ESPP diplexer’s potential for space-constrained communication systems, offering a high-performance and miniaturized solution.