|This article does not cite any references or sources. (December 2008)|
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 six or more wires are connected at the top, sloping downwards. The outer end of each wire is terminated by an insulator, usually placed at about one third the total height to isolate the lower support-wire or rope which is 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 at frequencies below 1.6 MHz, particularly in the LF and VLF bands, at frequencies sufficiently low that it is impractical to build a full size quarter-wave monopole antenna. Typical mast heights are less than 1/40 wavelength to 1/10 wavelength depending on the requirement. The function of the added umbrella-wires is to add capacitance to the top of the antenna, improving the current distribution on the vertical mast to increase the radiation resistance, and making the antenna resonant at lengths of less than a quarter wavelength. Umbrella antennas with heights of 15 to 350 metres are in service. (Umbrella antennas up to 428 metres in height were used for the OMEGA Navigation System, but many have since been demolished after the system was shut down in 1997.)
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.
Umbrella antennas are used as single mast antennas and are in common use for commercial medium-wave AM broadcasting as well as for LF and VLF transmitters for broadcasting and navigational aids. They were used at most OMEGA Navigation System transmitters, operating around 10 kHz, and at LORAN-C stations, operating at 100 kHz with central masts approximately 200 metres tall. Eight umbrella antennas 350 metres high are in use in an array at the German VLF communications facility, operating at about 20 kHz with high radiation efficiency even though they are less than 1/40 wavelength high.
They were invented during the wireless telegraphy era, about 1900 to 1920, and used with spark-gap transmitters on longwave bands to transmit information by Morse code. Small umbrella antennas were widely used with portable transmitters by military signal corps during World War I, since there was no possibility of setting up full-sized quarter-wave antennas.
It is also possible to build an umbrella antenna with a grounded tower. Such an antenna requires a separate feeder to the umbrella wires, which are also insulated against the mast and the ground.