Compact Bandpass Transmission Line With Mode Matching Based on Fractal SSPPs Structure

A novel fractal spoof surface plasmon polaritons (SSPPs) unit is proposed to construct a miniaturized transmission line (TL) in this article. The proposed fractal SSPPs unit has a lower cutoff frequency than the conventional rectangular SSPPs unit, even with the same periodic unit size. Due to the compact fractal SSPPs structure, the limitation of the lower cutoff frequency of the SSPPs-based TL combining with microstrip-to-slotline transition (MST) structure is able to be eliminated, making the frequency range of the designed TL more flexible. Also, by optimizing the coverage of the MST structure and SSPPs structure, the commonly used transition SSPPs portions for momentum matching can be removed; thus, the circuit size is further reduced greatly. Finally, by cooperating the high-pass response of the adopted MST structure and the natural low-pass property of the proposed fractal SSPPs, a bandpass TL with the controllable upper and lower cutoff frequencies is realized. Moreover, by rationally designing the MST structure, the high-order modes of SSPPs are suppressed, leading to a broad upper stopband. The well-designed bandpass TL is fabricated and measured. The experimental and simulation results are in excellent agreement, effectively validating the proposed structure and design method. The size of the fabricated TL is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.43\lambda _{\mathrm {g}} \times 0.32\lambda _{\mathrm {g}}$</tex-math> </inline-formula>, indicating the advantage of miniaturization and can be well used in communication systems.