Monopole antenna

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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 (see image antenna).

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 ideal ground system for AM broadcasters 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 guy-wire supported radio antennas are simply called monopoles if they stand alone. If multiple monopole antennas are used in order to control the direction of Radio Frequency (RF) propagation, they are called directional antenna arrays.

The RF feed-point impedance across this antenna base insulator is well defined and is approximately as shown in the chart.

Approximate feed-point impedance

In the United States, the Federal Communications Commission (FCC) requires that the transmitter power input to the antenna be measured and maintained. The power input is calculated as the square of the measured current, $i$ , flowing into the antenna from the transmission line multiplied by the real part of the antenna's feed-point impedance, $r$ .

P = i 2 r

This impedance is periodically measured to verify the stability of the antenna and ground system. Normally, an impedance matching network matches the impedance of the antenna to the impedance of the transmission line feeding it.

Examples of monopole antennas are: