A Novel Coplanar Slow-Wave Structure for Millimeter-Wave BWO Applications

A novel coplanar slow-wave structure (SWS) for application in millimeter-wave backward-wave oscillator (BWO) is proposed in this article. Unlike the traditional microstrip meander-line SWS with metal layers on both top and bottom surfaces of the substrate, the proposed SWS has a metal layer only on the top surface of the substrate. This significantly reduces the dielectric loading effect. The SWS is quite suitable for fabrication using PCB or microfabrication techniques and can be directly fed with coplanar waveguide (CPW), which has a lower loss than microstrip line. A design of the SWS presented here covers a -15-dB S
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frequency range from 13.7 to 120 GHz. Simulation results for a BWO based on the proposed SWS show a tunable frequency range of 32.7 GHz and a maximum output power of 22.7 W at 114 GHz. To verify the proposed concept, a scaled version of the SWS is fabricated and measured S-parameters are presented. With a good potential for fabrication using microfabrication techniques, this structure can be a good candidate for millimeter-wave BWO applications.