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A slow-wave structure (SWS) consisting of a planar helix with straight-edge connections and incorporating a coplanar waveguide feed has been designed for operation at W-band and has been fabricated using microfabrication technique. On-wafer cold measurements have been carried out on a number of fabricated SWSs, and the results are reported here for the first time. The parameters measured are return loss, attenuation, and phase velocity, and the results cover a frequency range of 70-100 GHz. Cold-test parameters of the SWS have been also obtained using simulations, and the effects of fabrication, such as surface roughness, have been accounted for by estimating effective conductivity of different parts of the microfabricated structures. The measured and simulated results match well. The effects of silicon wafer resistivity have been also discussed. Planar helical SWSs fabricated in this manner have application in traveling-wave tubes operating at millimeter wave and higher frequencies.