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Monopole antenna used for broadcasting

A monopole antenna is a type of radio antenna formed by replacing one half of a dipole antenna with a ground plane at right-angles to the remaining half. If the ground plane is large enough, the monopole behaves exactly like a dipole, as if its reflection in the ground plane formed the missing half of the dipole.

When used for radio broadcasting, the radio frequency power from the broadcasting transmitter is fed across the base insulator between the tower and a ground system. The ground system normally comprises 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long and a ground-screen in the immediate vicinity of the tower. All the ground system components are bonded together, usually by brazing or using coin silver solder to help reduce corrosion. Monopole antennas that use guy wires for support are called masts in some countries. In the United States, the term “mast” is not generally used to relate to antennas, so both self-supporting and guyed radio antennas are simply called monopoles if they stand alone. If used in numbers to control the direction of radio frequency propagation, they are called directional antenna arrays.

The radio frequency impedance across this antenna base insulator is well defined and is approximately as shown in the chart.

Approximate feed-point impedance

For this reason, in the United States, the Federal Communications Commission (FCC) requires that the transmitter power be measured and maintained by measuring the current flowing into this common point as the current squared, multiplied the real part of this impedance.

P = i²r

This impedance is periodically measured to verify the stability of the antenna and ground system. Normally a tuning network matches the antenna impedance to the impedance of the transmission line feeding this device.

Examples of monopole antennas are:

• the whip antenna
• the radio mast - when isolated from the ground and bottom-fed

• Cellular repeater
• Signal strength
• Directional antenna
• Folded unipole antenna

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