Umbrella antenna
Capacitively top-loaded wire monopole for low-frequency transmission.
An umbrella antenna is a type of wire monopole that uses capacitive top-loading. It typically consists of a mast fed at its base, with several radial wires attached at the top that slope downward. One side of the transmitter feedline connects to the mast, while the other connects to a ground system of radial wires buried beneath the antenna. These antennas are used for transmitting below 1 MHz, in the MF, LF, and especially VLF bands, where building a full-size quarter-wave monopole is impractical or impossible.
The antenna is supported by a central steel tubular or lattice mast. At the top, a ring of equally spaced radial wires extends diagonally toward the ground, where each is attached via a strain insulator to a non-radiating wire or rope anchored to the ground. These umbrella wires can also serve as guy lines to support the mast. Several feeding methods exist. In base feed, the mast sits on a thick ceramic insulator, keeping it isolated from the ground, and the feedline connects to the mast’s base; the steel mast itself acts as the radiator. For high-power antennas, the mast is grounded, the umbrella wires are insulated where they meet the mast, and they connect to vertical radiator wires hanging parallel to the mast, which are fed at the bottom. This avoids the difficulty of insulating the mast base against very high voltages. Beneath the antenna, a large ground system of buried radial wires extends from a terminal at the mast base to the edge of the umbrella wires.
Alternatively, in radial feed, the antenna is powered by applying transmitter current to the ends of one or more radial wires instead of the mast, with the central mast grounded. This eliminates the need for a mast support insulator and avoids an isolator in the mast’s aircraft warning light power cables. This method was used in three large umbrella antennas for the Omega navigation system (operating at 10–14 kHz) to avoid insulating the mast base against the 200 kV antenna potential.
Because the antenna is shorter than a quarter wavelength, it has capacitance. To cancel this capacitive reactance and make it resonant for efficient power feeding, a loading coil is connected in series with the feedline at the antenna’s base.
In operation, the vertical mast (or vertical radiator wires) functions as a resonant monopole.
- Frequency bands
- MF, LF, VLF (below 1 MHz)
- Typical height range
- 15–460 metres
- Gain over perfect ground
- approximately 3.52 dBi
- Bandwidth example
- less than 100 hertz in large VLF antennas
- Maximum signal improvement
- up to 6 dB from no top loading
Lore & Background
The umbrella 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 anchored to the ground. The umbrella wires may also serve structurally as guy lines to support the mast. Several methods of feeding power exist: in base feed, the mast is supported on a thick ceramic insulator and the feedline is attached to the base; in high power antennas, the mast is grounded and the umbrella wires are insulated from the mast and attached to vertical radiator wires fed at the bottom. Alternatively, in radial feed, the antenna can be fed by applying transmitter current to the ends of one or more radial wires instead of the mast, avoiding the need for a mast support insulator.
Since the antenna is shorter than the resonant length of one-quarter wavelength, it has capacitance. To cancel the capacitive reactance and make it resonant, an impedance matching inductor called a loading coil is connected in series with the feedline at the base. The vertical mast or vertical radiator wires function as a resonant monopole antenna. The umbrella wires function as a capacitive top load, increasing the current in the vertical mast and potentially doubling the total current and quadrupling the radiated power. The diagonal wires have a vertical component of current opposite to that in the mast, so the radio waves from the umbrella wires partially cancel those from the mast, reducing radiated power. With enough umbrella wires, all radiation from the portion of mast above the bottom of the umbrella is blocked.
Reader's Guide
Umbrella antennas are notable for their use at low frequencies where a full-size quarter-wave monopole is impractical. They are among 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, as well as for commercial medium-wave and longwave AM broadcasting stations. The largest umbrella antennas are the trideco antennas built for VLF naval transmitting stations. Eight umbrella antennas 350 metres high are in use at the German VLF transmitter DHO38, operating at about 20 kHz. With the adoption of two new amateur radio bands at 630 metres and 2200 metres, amateurs have resumed use of this design. The trideco antenna is a huge specialized umbrella antenna used in a few high power military transmitters at VLF, addressing limitations of conventional umbrella antennas: the sloping guy wires used as capacitive top load have limited length and their vertical current component partially cancels mast radiation.
Frequently Asked Questions
What is an umbrella antenna?
An umbrella antenna is a capacitively top-loaded wire monopole designed for low-frequency radio transmission. It features a central mast fed at the base with several radial wires fanning downward from the top, resembling the ribs of an open umbrella.
Which frequency bands does an umbrella antenna cover?
These antennas are built for operation below 1 MHz, spanning the VLF, LF, and MF bands. In very large VLF installations, the usable bandwidth can be narrower than 100 hertz.
How tall are umbrella antennas in practice?
Typical structures range from about 15 metres up to roughly 460 metres in height, depending on the target frequency. They are supported by a central steel tubular or lattice mast.
How does an umbrella antenna create its radiating field?
One conductor of the feedline connects to the mast while the other ties into a buried radial ground system beneath the structure. The top-loaded radial wires add capacitive loading, improving signal by up to 6 dB compared to an untopped monopole and yielding roughly 3.52 dBi gain over perfect ground.
Why build an umbrella antenna instead of a full quarter-wave monopole?
At VLF, LF, and MF frequencies a true quarter-wave vertical would be impractically tall or simply impossible to erect. The umbrella design achieves useful radiation at a fraction of that height, making it the go-to choice for stations that must transmit below 1 MHz.
More in Antenna Types, Part 3 1-17
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced