Umbrella antenna

An umbrella antenna is a capacitively top-loaded wire monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards. They are used as transmitting antennas below 1 MHz, in the MF, LF and particularly the VLF bands, at frequencies sufficiently low that it is impractical or infeasible to build a full size quarter-wave monopole antenna. The outer end of each radial wire, sloping down from the top of the antenna, is connected by an insulator to a supporting rope or (usually) insulated cable anchored to the ground; the radial wires can also support the mast as guy wires. The radial wires make the antenna look like the wire frame of a giant umbrella (without the cloth) hence the name. The antenna is supported by a central steel tubular or lattice mast. The top of the mast is attached to a ring of equally spaced radial wires extending diagonally to near the ground, where each is attached with a strain insulator to a length of non-radiating wire or rope which is anchored to the ground. The umbrella wires may also serve structurally as guy lines to support the mast. There are several different methods of feeding power from the transmitter to the antenna: In base feed, the mast is supported on a thick ceramic insulator which keeps it insulated from the ground, and the feedline from the transmitter is attached to the base of the mast. The mast serves as the monopole radiator. Alternately, in high power antennas, the mast is grounded, the umbrella wires are insulated where they connect to the central mast, and are attached to vertical radiator wires that hang down parallel to the mast which are fed at the bottom. This construction is used in high power antennas in which the very high voltage on the antenna would make it difficult to insulate the mast from the ground. Under the antenna is a large ground (Earthing) system connected to the opposite side of the feedline, consisting of wires buried in the Earth extending radially from the base of the mast out to the edge of the umbrella wires.

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Umbrella antenna