Umbrella antenna
Encyclopedia
An umbrella antenna is a top-loaded electrically lengthened
monopole antenna, consisting in most cases of a mast fed against ground, 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.
Umbrella antennas radiate vertically polarised ground waves, which are in common use on frequencies below 1.6 MHz for AM broadcasting and communications. Typical mast heights are less than 1/40 wavelength to 1/10 wavelength depending on the requirement. The added umbrella-wires improve the current distribution on the vertical mast, thus increasing the radiation resistance, and they also increase the antenna capacitance and hence the bandwidth. Umbrella antennas with heights of 15 to 350 metres are in service. (Umbrella antennae up to 428 metres in height were used for the OMEGA Navigation System
, but have since been demolished after the system was shut down in 1997.)
The umbrella name was given because the antenna looks like an umbrella without the cloth. The function of the umbrella wires can be performed in part or in full by guy wires for the central mast, but in most cases, for antennas which are short in comparison to the radiated wavelength
, additional umbrella wires are required.
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.
Umbrella antennas are used as single mast antennas and are in common use for commercial medium-wave AM transmissions as well as for LF
and VLF transmissions. 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.
Electrical lengthening
Electrical lengthening is the modification of an aerial which is shorter than a whole-number multiple of a quarter of the radiated wavelength, by means of a suitable electronic device, without changing the physical length of the aerial, in such a way that it corresponds electrically to the next...
monopole antenna, consisting in most cases of a mast fed against ground, 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.
Umbrella antennas radiate vertically polarised ground waves, which are in common use on frequencies below 1.6 MHz for AM broadcasting and communications. Typical mast heights are less than 1/40 wavelength to 1/10 wavelength depending on the requirement. The added umbrella-wires improve the current distribution on the vertical mast, thus increasing the radiation resistance, and they also increase the antenna capacitance and hence the bandwidth. Umbrella antennas with heights of 15 to 350 metres are in service. (Umbrella antennae up to 428 metres in height were used for the OMEGA Navigation System
OMEGA Navigation System
OMEGA was the first truly global radio navigation system for aircraft, operated by the United States in cooperation with six partner nations.-History:OMEGA was originally developed by the United States Navy for military aviation users...
, but have since been demolished after the system was shut down in 1997.)
The umbrella name was given because the antenna looks like an umbrella without the cloth. The function of the umbrella wires can be performed in part or in full by guy wires for the central mast, but in most cases, for antennas which are short in comparison to the radiated wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, additional umbrella wires are required.
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.
Umbrella antennas are used as single mast antennas and are in common use for commercial medium-wave AM transmissions as well as for LF
Lf
Lf or LF may stand for:* FlyNordic IATA airline designator* Nippon Broadcasting System, a radio station in Tokyo, Japan - JOLF* Lactoferrin, a protein* Laissez-faire* LeapFrog, an educational toy company...
and VLF transmissions. 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.
