A Cage aerial is a radio antenna, which consists of the top portion of a tower or mast and of several parallel wires, which are radially arranged around the lower part of the mast. one advantage of the cage aerial is that the supporting tower can be grounded, allowing it to be used for other radio services, such as a support for VHF or UHF antennas. A grounded tower also simplifies the installation of aircraft warning lamps. Cage aerials have been built in different variants for broadcasting stations in the longwave

Longwave

In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...

 and mediumwave

Mediumwave

Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...

band.

The cage is electrically one quarter of the operating wavelength long. It is connected to the mast at its upper end. This way it isolates the lower part of the mast (λ/4 stub) and makes the upper part of the mast the radiator. Very often the typical height of such an antenna is no problem as the height of the mast is selected for the TV or FM antennas on top.

Example: At 1000 kHz the wavelength is 300 m. Therefore the minimum length of the cage antenna is a bit more than 150 m; 75 m for the radiator, 75 m for the cage and some metres to make the lower end of the cage inaccessible from the ground as the lower end of the cage carries a very high RF voltage.