Mode Composite Substrate Integrated Coaxial Line Based on Substrate Integrated Coaxial Line and Periodic L-Shaped SSPP Structure

In this article, a mode composite substrate integrated coaxial line (MCSICL) is proposed for transmitting fundamental mode and first high-order mode in dual frequency bands. The MCSICL consists of substrate integrated coaxial line (SICL) and periodic L-shaped spoof surface plasmon polariton (SSPP) structure etched on the inner conductor of the SICL. By introducing the L-shaped SSPP structure, the bandwidth of the fundamental mode of the SICL can be suppressed without changing the cutoff frequency of the first high-order mode. Therefore, the proposed MCSICL can transmit signals at the low-frequency band with the fundamental mode and at the high-frequency band with the first high-order mode. The equivalent circuit model is built to analyze the propagation properties of the MCSICL. In addition, to excite the two modes independently, a grounded coplanar waveguide (GCPW)-to-MCSICL transition and a microstrip line (ML)-to-MCSICL transition are designed. The MCSICL with the transitions is fabricated under the printed circuit boards (PCBs) process. Measurement results show that the MCSICL can propagate the fundamental mode from dc to 30 GHz and the first high-order mode from 42 to 60 GHz, and the isolation between the two modes is larger than 30 dB.