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Two typical topology shapes of the microstrip meander line (MML) including round U-shaped and V-shaped slow-wave structure (SWS) for ${D}$ -band traveling wave tubes (TWTs) are comparatively investigated, fabricated, and tested in this article. As a critical factor applied in millimeter-wave frequency, the high-frequency losses of the MML SWS are emphatically discussed. The two types of golden SWSs are both fabricated with the thin-film circuit technology on quartz substrates. After the surface roughness test and the cavity fabrication, the SWS samples are assembled and tested. According to the experimental test results, the ${S}_{11}$ parameters of the U-shaped and V-shaped transmission models are below −13.5 dB versus −15 dB, and the “cold” ${S}_{21}$ parameters are over −6.4 dB versus −4.9 dB, which are approached to the simulation results with the consideration of the silver conductive paste (SCP). At 140 GHz, the tested transmission losses for the U-shaped SWS and V-shaped SWS are about 0.35 and 0.29 dB/mm, respectively. Although particle-in-cell (PIC) simulations for the two SWSs both predict more than 100-W output power, the V-shaped MML TWT shows better bandwidth and loss performances than the U-shaped MML TWT. [ABSTRACT FROM AUTHOR]