Plasmas can interact with a R.F. signal. We have theoretically predicted and experimentally proven that it is possible to use this interaction to shift the frequency of a fixed oscillator in free space.
Mark Rader, Mark Rader,
"Microwave frequency modification using plasmas", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 15146L (1 December 1990); doi: 10.1117/12.2301647; https://doi.org/10.1117/12.2301647