It’s the outside shell of an antenna. The big plastic gumdrop you see in the picture is a satellite dish for marine use. It’s got a lot of moving parts to help it aim itself automatically and keep a good aim while on the high seas. It’s covered in a plastic dome to keep the elements away from parts that could short out or corrode if they get wet.
Most of the time the term “radome” is used for dish-type antennas. It was originally used for radar installations, and in fact the word is a combination of the words “radar” and “dome.” Military radomes do more than just protect the dish inside; they keep prying eyes from telling what the dish is pointing at.
Of course civilian radomes are far less insidious and generally radomes are only used when there’s a dish with moving parts that needs to be protected. In most cases the components of a dish are waterproof enough that there isn’t a problem with the elements, but when you throw salt water into the mix it gets to be a little dicey.
Radomes are often white in color but they don’t have to be. The only thing that’s important is that they are totally transparent to the frequencies they pick up, and sometimes that means they can be any color, even black, if the coloring agents respond differently to RF than they do to visible light.
Sometimes, antennas are “radomized” as well. Our Televes and Antennas Direct antennas are both covered in sleek-looking plastic. It’s not just for show, at least not in the Televes model – there’s a powerful amplifier and delicate components in there that could be easily damaged by a storm. So the whole business is covered in a weather-tight enclosure that has no effect on the antenna’s ability to perform.