[en] The final report for the project is comprised of the PhD thesis of Richard L. Savage, Jr entitled: 'Frequency Upshifting of Electromagnetic Radiation via an Underdense Relativistic Ionization Front.' An underdense, relativistically propagating ionization front has been utilized to upshift the frequency of an impinging electromagnetic wave from 35 GHz to more than 173 GHz in a continuously tunable fashion. The source radiation interacted with the ionization front inside a metallic waveguide. The front, simply a moving boundary between ionized and neutral gas, was created as a short, intense pulse of ionizing laser radiation propagated through the gas-filled waveguide. In 1991, W.B. Mori showed theoretically that large upshifts are possible using underdense ionization fronts (underdense implies that the plasma density is lower than that required to reflect the source radiation), where the source wave is transmitted through the plasma/neutral boundary. The authors have extrapolated Mori's analysis to interactions within a waveguide. This is a new technique for generating high-power, short-pulse, tunable radiation, and has potential applications in areas such as time-resolved microwave spectroscopy, plasma diagnostics, and remote sensing