Intelecommunications, afield strength meter is an instrument that measures theelectric field strength emanating from atransmitter.

In ideal free space, theelectric field strength produced by atransmitter with anisotropic radiator is readily calculated.^[1]

${\displaystyle \text{E}\approx \frac{\sqrt{30\cdot P}}{d}}$

where

${\displaystyle \text{E}}$ is the electric field strength in volts per meter

${\displaystyle P}$ is thetransmitter power output in watts

${\displaystyle d}$ is the distance from the radiator in meters

The factor${\displaystyle \sqrt{30}}$ is an approximation of${\displaystyle \sqrt{\frac{Z_0}{4\pi }}}$

where${\displaystyle Z_0=376.730313668(57)}$ ${\displaystyle \mathrm{\Omega }}$is theimpedance of free space.^[2]${\displaystyle \mathrm{\Omega }}$ is the symbol forohms.

It is clear thatelectric field strength is inversely proportional to the distance between the transmitter and the receiver. However, this relation is impractical for calculating thefield strength produced by terrestrial transmitters, where reflections andattenuation caused by objects around the transmitter or receiver may affect the electrical field strength considerably.

A field strength meter is actually a simple receiver. The RF signal is detected and fed to amicroammeter, which is scaled indBμ. The frequency range of the tuner is usually within the terrestrial broadcasting bands. Some FS meters can also receive satellite (TVRO and RRO) frequencies. Most modern FS meters haveAF andVF circuits and can be used as standard receivers. Some FS meters are also equipped with printers to record received field strength.

When measuring with a field strength meter it is important to use a calibrated antenna such as the standard antenna supplied with the meter. For precision measurements the antenna must be at a standard height. A value of standard height frequently employed for VHF and UHF measurements is 10 metres (33 ft). Gain correction tables may be provided with the meter, that take into account the change ofantenna gain withfrequency.

TheCCIR defines the minimum field strength for satisfactory reception. These are shown in the table below.^[3] (Band II is reserved for FM radio broadcasting and the other bands are reserved forTV broadcasting.)

• Electromagnetic radiation meters
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