A Broadband Crossover Based on Spoof Surface Plasmon Polaritons Transmission Lines With a Wide Optimizable Coupling Coefficient

The crossover structure is an essential component for two-channel signal routing. In this article, a broadband crossover based on spoof surface plasmon polariton (SSPP) transmission lines (TLs) has been proposed for the first time, which can overcome the inherent tradeoff between optimizable coupling coefficients and the return loss degradation in conventional microstrip. By exploiting the unique phase difference between adjacent grooves in the SSPP structure, two frequency-dependent optimization methods are revealed. Close to the plasmon frequency (18 GHz), the coupling can be controlled through managed phase difference by displacing the teeth centers around the overlap. Wide optimizable coupling coefficients of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{31} =5.6$</tex-math> </inline-formula> dB are achieved, maintaining Bloch impedance and limiting return loss degradation to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{11} =1$</tex-math> </inline-formula> dB and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{33} =3.3$</tex-math> </inline-formula> dB. In the lower frequency band, a nonuniform SSPP TL is proposed to optimize the conventional capacitive coupling by linewidth modulation. By integrating these two methods, an expanded optimizable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{31}$</tex-math> </inline-formula> range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{31} =8.6$</tex-math> </inline-formula> dB across the 0.1–18-GHz band is obtained. Meanwhile, the degradation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11}$</tex-math> </inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{33}$</tex-math> </inline-formula> is limited to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{11} =1.8$</tex-math> </inline-formula> dB and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta S_{33} =6.5$</tex-math> </inline-formula> dB, and the maximum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11}$</tex-math> </inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{33}$</tex-math> </inline-formula> remain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le -10$</tex-math> </inline-formula> dB in the 0.1–11-GHz band. These results highlight the potential of this broadband crossover for SSPP-based broadband microwave systems and provide a foundation for multichannel signal routing and related fields.