Design and RF Characterization of the Co-Planar Slow Wave Structure for Millimeter-Wave BWO Applications

This article investigates the operation principle and dispersion properties of the co-planar slow wave structure (SWS). The SWS exhibits an additional phase shift of 2 pi seen by the electrons, owing to its physical configuration. The backward wave mode is the fundamental mode, naturally conducive to the backward wave oscillation. To analyze the characteristic properties of the SWS, an equivalent circuit model is proposed, consider-ing models of the transmission lines and discontinuities. The theoretical calculation results are in good agreement with the simulations. Moreover, a design of the SWS at V band is proposed with a -15-dB S-11 frequency range from 0 to 70 GHz. Particle-in-cell (PIC) simulation for the backward wave oscillator (BWO) based on the proposed SWS is studied. The results show that the proposed SWS has a tunable frequency range of 17 GHz and a maximum output power of 60 W at 69.5 GHz. In addition, the proposed SWS is fabricated using microfabrication techniques and the S-parameters are measured with the probe station. The developed SWS has the characteristics of wide bandwidth and high coupling impedance, which is suitable for the application of millimeter-wave BWO.