Although an excellent electrical insulator, epoxy, very slowly, can adsorb water up to 6% by mass. A better choice would be a vinyl ester resin which is almost unaffected by moisture.

Cyanate resins are noteworthy for their low dielectric constant, low loss, and low moisture absorption. You can get prepreg from YLA.

Why don't you coat it with parylene. Parylene is typically 0.3 mils thick and often used for circuit boards and is vapor deposited, so it gets everywhere. It's tough, so tough that trying to remove it requires a sharp knife.
It's more expensive than standard conformal coat. If you make a complete antenna, cover the connector, then parylene coat it, it'll keep the moisture out pretty well.

Google results hints Parylene C is the best. Here's a quick Google result.

"After testing many coatings, Parylene, a vapor deposited conformal coating, was found to be the most suitable for the MVD. It is moisture resistant and because it can penetrate materials, also adds mechanical robustness [9]. Vapor deposition assures a uniform and homogeneous coating over the entire structure [9]. Parylene C was chosen for maximum moisture protection and to increase the mechanical robustness of the foam. The thickness of the Parylene layer was optimized to maximize the robustness and moisture resistance while minimizing the mass. "

kch (antennas)