Experimental Verification of the Low Transmission Loss of a Flat-Roofed Sine Waveguide Slow-Wave Structure

The sine waveguide (SWG) is presented as a potential slow-wave structure (SWS) for the millimeter-wave (mm-wave) and terahertz (THz) traveling-wave tube (TWT) because of the advantages of wide bandwidth, natural electron beam tunnel, and easy fabrication. In particular, this theoretical study indicates that the transmission loss of the SWG is far less than that of the folded waveguide SWS, which is widely employed in the mm-wave and THz TWT. Here, the flat-roofed SWG slow-wave circuit working in the ${W}$ -band has been machined using nano-computer numerical control technology, which guarantees micron tolerances and surface roughness of tens of nanometers using carbide tooling with 100- ${\mu }\text{m}$ diameter. The cold test results show that the transmission coefficient ${S}_{{{21}}}$ of the entire high-frequency slow-wave circuit is more than −4.1 dB with a total length of 123.84 mm in the frequency range from 90 to 100 GHz, and the transmission loss is less than 0.36 dB/cm.