You might try looking at atmospheric absorption data. This covers microwave to terahertz and beyond. Other than notches here and there you have to go through kilometers of the stuff to get much absorption.

Thanks,
What I'd hope to find is a very lossy gas. i.e. 5 dB per inch loss at 1 Ghz type numbers.
Water might be a good idea, but I need it ultra light.
I want to fill a balloon with a gas and test it in a waveguide for loss.

Maybe another forum could provide some info, lets see what happens here first though.

I've searched the internet for a few hours and written to gas suppliers with no luck so far.

I'd prefer  if the gas is somewhat friendly if it escapes. Although that probably means it's not very lossy.

kch

From the atmospheric data we could deduce that atmospheric gases are no use. This must include trace gases becuase although sparce we can still drive through km of the stuff.

Nitrogen N2 78.08%
Oxygen O2 20.95%
*Water H2O 0 to 4%
Argon Ar 0.93%
*Carbon Dioxide CO2 0.0360%
Neon Ne 0.0018%
Helium He 0.0005%
*Methane CH4 0.00017%
Hydrogen H2 0.00005%
*Nitrous Oxide N2O 0.00003%
*Ozone

+Freon?

>5 dB per inch loss at 1 Ghz type numbers
My instinct on this is "not a hope in hell". It would either have to be conductive or lossy as a dielectric. It would have to have such a high density that it would be liquid at best. Remember that waveguide impedance is around 300 ohms.

You stand more chance at mmwave because the guide would be longer in terms of half-wavelengths.

You're probably correct logbook.
A mist/spray of water vapor has been used with some success, but it requires substantial distances, 10-30 feet.

Thanks,
kch