I would think a thin clear polythene bag would so the trick. The dielectric constant will be high so it is preferable if there is lots of air in the bag ie the bag doesn't rest on the reflectors/directors at too many points.

Expanded polystyrene is lossless but too porous.

Cotton is not waterproof!

Don't use a black ESD polythene bag or an lined ESD poly bag either.

Thanks. I'll look into it. Last question: what if
i need a material that has to make a close fit and
actually touch the elements like a vaccum sealed bag?

Any suggestions?

Thx again,
Frank

You can put material around any antenna without changing it much if the material is electrically thin (1/100 wavelengths thickness).
Most plastic bags are 5 mils, that's 1/2000 th of a wavelength at 1 ghz, you'll never even notice it (unless you're really high power, then the bag would heat).

At worst case for thicker material, the frequency of the antenna changes and is a bit lower, shortening the antenna will get you back to the starting frequency.

kch

One reason radomes are placed away from the conductor is to reduce the noise produced by electrostatic discharge of the rain droplets as they hit.  After the material is wetted it is much less of a problem.  If you can live the very slight increase in noise, then it seems to be OK from the previous post.  Have you considered those dry coat materials?  It seems metals and bike shops have those in many communities for reasonable prices.