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. One side of the feedline supplying power from the transmitter is connected to the mast, and the other side to a ground (earthing) system of radial wires buried in the earth under the antenna. 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 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 conductive steel 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 a terminal at the base of the mast out to the edge of the umbrella wires.

Alternatively, in radial feed, the antenna can be fed power by applying the transmitter current to the ends of one or more of the radial wires instead of the mast. In this case the central mast is grounded. As with wire feeders, this avoids the need for a mast support insulator, and also does not require an isolator in the power cables for the mast's aircraft warning lights. This construction was used in three large umbrella antennas for the obsolete Omega navigation system which operated at 10–14 kHz, to eliminate the very difficult problem of insulating the mast base against the 200 kV antenna potential.

Since the antenna is shorter than the resonant length of one-quarter wavelength, it has capacitance. 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 vertical mast, isolated from the ground, or the vertical radiator wires, functions as a resonant monopole antenna. At the low frequencies used, the height of the mast is much less than its resonant length, one quarter wavelength (${\displaystyle \ {\tfrac {1}{4}}\lambda \ }$), so it makes a very electrically short antenna; it has very low radiation resistance and without the topload wires would be a very inefficient radiator. The oscillating current from the transmitter travels up the mast and divides approximately equally among the topload wires. It is reflected from the ends of the wires and travels back down the mast. The outgoing and reflected current superpose, forming a standing wave consisting of the tail part of a sine wave.

Due to ground reflections and the symmetrical placement of the topload wires, measured far from the antenna the radio waves radiated from the umbrella-like spoke wires largely cancel each other out, so the spoke wires themselves radiate almost no radio power. Instead the umbrella-wires function as a capacitive top load replacing some or all of the capacitance that would be provided by the top of a full-length quarter-wave mast. The ground wires buried or laid on the earth under the antenna function as the corresponding bottom plate of the giant 'capacitor'. The added capacitance increases the current in the vertical mast due to the extra charge required to charge and discharge the top load each half of the RF cycle. 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.