Square Fan-Shaped SSPPs and Its Application in Microwave Filters Design

In this article, a new type of square fan-shaped spoof surface plasmon polaritons (SSPPs) is proposed and applied to design compact microwave filters. Compared with rectangular-shaped SSPPs unit, the proposed square fan-shaped SSPPs unit has the lower asymptotic frequency of the fundamental mode (Mode0) in same periodic, enabling the design of compact SSPPs-based filter. Moreover, the frequency range of the high-order modes of the proposed SSPPs unit can be largely narrowed and the frequency band of the first-order mode (Mode1) approached nearly to Mode0, making it the possible not only in design of lowpass filter with ultrawide stopband by using Mode0 but also the dual-band filter by adopting the Mode0 and Mode1 simultaneously. First, a wide stopband lowpass filter is designed based on the Mode0 of the proposed double-side symmetric square fan-shaped SSPPs. The wide stopband is realized by the mutual stagger of the high-order modes of the transmission SSPPs and the transition SSPPs. By adjusting the size of the lower part of the proposed SSPPs unit, the double-side asymmetric square fan-shaped SSPPs unit is then obtained and the frequency range of the Mode1 can be regulated freely. Based on the double-side asymmetric SSPPs unit, a dual-band filter is designed. For verification, a well-designed lowpass filter and dual-band filter are fabricated and measured. The measured results agree well with the simulations, validating the proposed structure and design method.