I am building a wooden kayak. To soften partially cured epoxy and break its grip, you can heat it with a hair dryer. I don't know if this will work when the epoxy has cured for months or years. Beware: if the epoxy does soften, it will most likely be a sticky mess. Do not get it on your skin.
You might want to visit some epoxy manufacturers' websites for more information. Try West Systems, MAS and System 3. They may have tips for cleaning the site after you remove the antenna.
Hey Ian, look what I found on System Three's site:
What solvent can I use to clean up cured epoxy resin products?
Cured epoxy systems are very chemical resistant, and need to be removed with an epoxy-type paint
stripper containing methylene chloride, or by a combination of heat for softening followed by scraping. Uncured epoxy resins and hardeners can be cleaned up with ketones, alcohols, or lacquer thinner. White vinegar will clean up unmixed resin components.