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cage aerial
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 and mediumwave 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.This antenna type is known in America as "Folded unipole" and extensively studied by John H. Mullaney
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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