A radome (radar dome) is a weatherproof enclosure used to protect an antenna. It is used mainly to prevent ice (especially freezing rain) from accumulating directly onto the metal surface of the antenna. In the case of a spinning radar dish antenna, the radome also protects from debris and rotational irregularities due to wind.
For stationary antennas, excessive amounts of ice can de-tune the antenna to the point where its impedance at the input frequency rises drastically, causing VSWR to rise as well. This reflected power goes back to the transmitter, where it can cause overheating. A foldback circuit activates to prevent this, however it causes the station's output power to drop dramatically, reducing its range.
A radome prevents this by covering the antenna's exposed parts with a sturdy, weatherproof material, typically fibreglass, which keeps the ice far enough away from the antenna to prevent any serious issues. A radome does however add to the wind load and the ice load , in addition to its own weight, and so must be planned for when considering overall structural load.
For this reason, and the fact that radomes may be unsightly if near the ground, heaters are often used instead. Usually running on DC, the heaters do not interfere physically or electrically with the AC of the radio transmission.
For radar dishes, the single, large, ball-shaped dome (usually geodesic) also protects the rotational mechanism, and the sensitive electronics, and is heated in colder climates to prevent icing.