Umbrella antenna

An umbrella antenna is a top-loaded electrically lengthened 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. The outer end of each wire is connected by an insulator to a supporting rope or cable anchored to the ground. The radial wires can also function as guy wires to support the mast. The radial wires make the antenna look like a giant umbrella without the cloth, accounting for the name.

They are used as transmitting antennas below 1 MHz, in the LF and particularly the VLF bands, at frequencies sufficiently low that it is impractical to build a full size quarter-wave monopole antenna. Either the central mast itself, insulated from the ground, is connected to the transmitter and serves as the radiating element, or vertical wires parallel to the mast. At the low frequencies used the height of the mast is typically 1/10 to 1/40 of a wavelength, so it makes a very electrically short antenna, and by itself would have very low radiation resistance and be a very inefficient radiator. The function of the umbrella-wires is to add capacitance to the top of the antenna, improving the current distribution on the vertical mast radiator to increase the radiation resistance and radiated power. The umbrella wires serve as the plate of a capacitor, with the ground serving as the other plate, which is charged and discharged by the radio frequency current from the tramsmitter. Umbrella antennas with heights of 15 to 460 metres are in service. The tallest umbrella antennas are actually used by Lualualei VLF transmitter, INS Kattabomman and the CHAYKA-transmitters at Inta and Dudinka.

Umbrella antennas radiate vertically polarised ground waves in an omnidirectional radiation pattern. Because they are short compared to a wavelength of the radio waves, they have low radiation resistance and are usually inefficient, radiating only a fraction of the power supplied by the transmitter.

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Wikipedia