We have modified the frequency of a propagating R.F. signal using a pulsed plasma discharge. In our experimental work, we have utilized two frequency shifting mechanisms. The first is to use the plasma as an nonphysical plunger to upshift the frequency of the signal.
Mark Rader, Mark Rader,
"Frequency modulation of free space RF signals", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 157611 (1 October 1991); doi: 10.1117/12.2297750; https://doi.org/10.1117/12.2297750