Characteristic impedance extraction of spoof surface plasmon polariton waveguides

Frequency-dependent characteristic impedance extraction and compact excitation of spoof surface plasmon polariton (SSPP) waveguides are achieved in this work. The behavior of SSPP waveguides cannot be directly described using the transmission-line model due to the non-transverse electromagnetic modes. By investigating the near-field distribution of a typical SSPP waveguide, we find a special position where the field component in the longitudinal direction is nearly equal to zero, and the reflection is completely caused by impedance mismatch rather than mode mismatch. Then, the characteristic impedance curve of the SSPP waveguide is extracted. Based on the obtained characteristic impedance and the special excitation position, transition between the microstrip and SSPP waveguide in the desired frequency bands can be achieved without using an additional gradient-corrugation transition structure. Compared to the conventional gradient-corrugation strategy, the proposed transition has a much more compact size and better transition performance. Meanwhile, the accuracy of the proposed method is not impacted by the internal discontinuities and local couplings of typical or atypical SSPP waveguides, which is a complicated problem in other methods. The simulated and measured results demonstrate the validity of the presented method. Hence, the characteristic impedance extracted by the presented method is accurate and makes compact transformation and systematic integration of SSPP waveguides more practicable.