Umbrella antenna
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.
[2][3][4] 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 a terminal at the base of the mast out to the edge of the umbrella wires.
In order to cancel the capacitive reactance and make it resonant so it can be fed power efficiently, an impedance matching inductor called a loading coil is connected in series with the feedline at the base of the antenna.
The oscillating current from the transmitter travels up the mast and divides approximately equally among the topload wires.
The ground wires buried or laid on the earth under the antenna function as the corresponding bottom plate of the giant 'capacitor'.
In the best case, this can double the total current, and quadruple the radiated power, increasing the signal up to 6 dB from the level it would be with no top loading.
Their large reactance and low resistance usually give the antenna a high Q factor, so it has a narrow bandwidth over which it can work.
Below are several grounded mast umbrella antenna variations developed by the US military in the 1970s for use on the low frequency band.
This eliminates the difficult problem of insulating the mast from the ground at the extremely high voltages used.
It also allows the possibility of shutting down power to one of the panels, and lowering it to the ground for maintenance while the rest of the antenna is operating.
[8] The antenna was invented by Boynton Hagaman of Development Engineering Co. (DECO) and first installed at Cutler, Maine in 1961.
Umbrella antennas 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.
One of the first antennas that used this design was the tubular 420 foot (130 m) mast erected in 1905 by Reginald Fessenden for his experimental spark gap transmitter at Brant Rock, Massachusetts with which he made the first two-way transatlantic transmission, communicating with an identical antenna in Machrihanish, Scotland.
[10] The wires attached to the top (either 4 or 8, depending on source) were electrically connected to the mast and stretched diagonally down to the surface, where they were insulated from the ground.
Another early example is the umbrella antenna built in 1906 by Adolf Slaby at Nauen Transmitter Station, Germany's first long range radio station, consisting of a 100 metres (330 ft) steel lattice tower radiator with 162 umbrella cables attached to the top, anchored by hemp ropes to the ground 200 m from the tower.
Umbrella antennas were used at most OMEGA Navigation System transmitters, operating around 10 kHz, at Decca Navigator stations and at LORAN-C stations, operating at 100 kHz with central masts approximately 200 metres tall, before those systems were shut down.
