A micromachining process has been developed to create high aspect ratio coplanar waveguides (HARCs). The process creates tall Si mesas using deep reactive-ion etching and converts them into solid SiO2 mesas using thermal oxidation. Tall Au conductors are deposited using electroplating and planarized using lapping and chemical-mechanical planarization. The solid SiO2 mesas form the dielectric gap between the tall Au conductors, resulting in HARCs with a planar surface. The tall conductor sidewalls created from the high aspect ratio process reduce the transmission line resistance, which allows the lines to have lower loss at low impedances compared to conventional transmission lines. Transmission lines with characteristic impedances of 16-21 ?? have been fabricated on high-resistivity Si. Transmission line characteristics were measured from 1 to 50 GHz and showed an attenuation of 1.0-1.4 dB/cm at 10 GHz. Measurements were compared to HFSS simulations and showed reasonable agreement over the frequency range.