John Smith I wrote:

I have seen a large neon bulb in a microwave and the patterns formed in
the gas ... and it only left me dreaming of the "neon room" and full
sized antennas in such ... (yes, vhf-uhf-shf if you can't afford a BIG
room.)

We do this sort of testing at JPL. We have a large bell jar (6 ft across
or so) that we put antennas and high power RF components in and then
pump down to a vacuum or fill with simulated Mars atmosphere.

Mars gas is mostly CO2, Argon, and CO.. so not far off your idea of a
neon room.