Umbrella antenna


An umbrella antenna is a 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. 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 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 insulated 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 frame of a giant umbrella – without the cloth – hence the name.

Design

The antenna is supported by a central mast, usually a steel tubular or lattice mast. The conductive mast is usually insulated from the ground and connected at its base to the feedline from the transmitter and serves as the radiating element. At the top of the mast are attached a ring of equally spaced diagonal wires extending down to the ground where they are anchored usually with concrete anchors. A strain insulator inserted in each wire at a certain height blocks the antenna current from reaching the ground, so the antenna consists of the mast and the upper portions of the umbrella wires, while the lower portions just serve to anchor the wires to the ground and may be made of nonconductive rope. The wires may also serve structurally as guy lines to support the mast. Under the antenna is a large ground system consisting of wires buried in the Earth extending radially from the base of the mast out to the edge of the topload wires, connected to the other side of the transmitter feedline.

How it works

The vertical mast 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, so it makes a very electrically short antenna, and by itself would have very low radiation resistance and would be a very inefficient radiator. The oscillating current from the transmitter travels up the mast and splits approximately equally between the topload wires. It is reflected from the ends of the wires and travels back down the mast, forming a standing wave consisting of part of a sine wave. Due to the symmetrical placement of the wires, and ground reflections, the radio waves radiated by the umbrella wires largely cancel out, so the wires themselves do not radiate much radio power. Instead the umbrella-wires function as a capacitive top load adding capacitance to the top of the antenna. The capacitance causes a large current from the transmitter to flow in the vertical mast every RF cycle to charge and discharge it, increasing the current in the mast, increasing the radiation resistance and hence the radiated power. The buried ground wires under the antenna function as the bottom plate of the 'capacitor'.
To tune out the large capacitive reactance of the antenna and make it resonant at the operating frequency so it can be fed power efficiently, a large inductor is placed in the feedline in series with the antenna, at its base. The other side of the feedline from the transmitter is connected to the ground system. The antenna and coil form a tuned circuit. 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. In large umbrella antennas used in the very low frequency band, the bandwidth of the antenna can be less than 100 hertz.
An umbrella antenna can alternatively be fed power by applying the transmitter current to one or more of the radial wires instead of the mast. In this case the central mast is grounded. This design is advantageous in high power antennas which operate at high voltages, because it does not require an expensive mast support insulator that can handle the high voltage. It 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.

Radiation pattern

Umbrella antennas radiate vertically polarised radio waves in an omnidirectional radiation pattern, with equal power emitted in all horizontal directions, with maximum signal strength radiated in horizontal directions, falling monotonically with elevation angle to zero at the zenith. Due to the large topload they are usually more efficient than the other common toploaded antenna, the flattop or T antenna, at low frequencies, and are widely used in the VLF band.
Ground waves are vertically polarized waves which travel away from the antenna horizontally just above the ground. Therefore umbrella antennas are good ground wave antennas, and are used as radio broadcasting antennas in the MF and LF bands.
The gain of an umbrella antenna, like other electrically short monopole antennas, is approximately 3.52 dBi if it is significantly shorter than.
Since the diagonal wires are sloped down, the current in them has a vertical component. This current is in a direction opposite to the current in the mast, so far from the mast the radio waves radiated by it are 180° out of phase with the radio waves from the mast, and partially cancel them. Thus the umbrella wires partially shield the mast, reducing the power radiated. With enough umbrella wires all the radio waves emitted by the portion of mast above the bottom of the umbrella is blocked, and the only radiation is from the portion of the mast below the umbrella.

Applications

Due to their large capacitive topload, umbrella antennas are some of the most efficient antenna designs at low frequencies, and are used for transmitters in the LF and VLF bands for navigational aids and military communication. They are in common use for commercial medium-wave and longwave AM broadcasting stations. Umbrella antennas with heights of 15–460 metres are in service. The largest umbrella antennas are the trideco antennas built for VLF naval transmitting stations which communicate with submerged submarines. 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 wavelength high.
With the progressing world-wide adoption of two new amateur radio bands at 630 metres and 2200 metres, amateurs with adequate real estate have resumed use of this design.

Trideco antenna

The trideco antenna is a huge specialized umbrella antenna used in a few high power transmitters at very low frequency. In a conventional umbrella antenna, the use of the sloping guy wires as the capacitive top load has some disadvantages: First, since the umbrella wires must be anchored to the ground, their length is limited. At low frequencies the length of topload wires required is far longer than can be used for guy wires, without additional supporting masts the wires would sag to the ground. Second, since the wires are sloping, the current in them has a vertical component. This vertical current is in the opposite direction to the current in the mast, so the radio waves radiated by it are 180° out of phase with the mast radiation, and partially cancels it.
In the trideco design the top load wires extend horizontally from the top of the central mast, supported by a ring of 12 masts surrounding the central mast, to create a "capacitor plate" parallel with the Earth, driven at the center. The topload wires are in the form of six rhomboidal shaped panels extending symmetrically from the central mast at angles of 60°, giving the antenna the form of a six-pointed star when seen from above. Instead of using the central mast itself as a radiator, each panel is connected to a vertical radiator wire next to the central mast, and the six radiator wires are fed in phase at the base. This 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. Buried in the ground under the antenna is an enormous radial ground system, which forms the bottom 'plate' of the capacitor with the overhead top load. The antenna must be very large at the VLF frequencies used; the supporting masts are high, and the topload is about in diameter.
The trideco antenna was developed for high power naval transmitters, which transmit on frequencies between 15 and 30 kHz at powers up to 2 megawatts, to communicate with submerged submarines worldwide. It is the most efficient antenna design found so far for this frequency range, achieving efficiencies of 70-80% where other VLF antenna designs have efficiency of 15-30% due to the low radiation resistance of the very electrically short monopole. The inspiration for the design was the umbrella antenna of the 1 megawatt Goliath transmitter built by Nazi Germany's navy in 1943 at Kalbe, Germany. The antenna was invented by Boynton Hagaman of Development Engineering Co. and first installed at Cutler, Maine in 1961. Today trideco antennas are located at a few military bases around the world, such as Cutler naval radio station in Maine, USA, Harold E. Holt Naval Communication Station, Exmouth, Australia, and Anthorn Radio Station, Anthorn, UK. A modified 3 panel antenna was located at NSS Annapolis, Annapolis, Maryland, USA but was decommissioned in 1990.

History

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. Low frequencies were used for long distance transcontinental communication, and antennas were electrically short, so capacitively toploaded antennas were used. Umbrellas developed from large multiwire capacitive antennas used by Guglielmo Marconi during his efforts to achieve reliable transatlantic commmunication. One of the first examples was the umbrella antenna built in 1906 at Nauen Transmitter Station, Germany's first long range radio station, consisting of a steel lattice tower radiator with 162 umbrella cables attached to the top, anchored by hemp ropes to the ground 200 m from the tower. 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.
Umbrella antennas 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, before those systems were shut down.