Field strength meter

Not to be confused withSignal strength meter.

Intelecommunications,afield strength meteris an instrument that measures theelectric field strengthemanating from atransmitter.

The relation between the electric field and the transmitted power

In ideal free space, theelectric field strengthproduced by atransmitterwith anisotropic radiatoris readily calculated.

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

where

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

$P$is thetransmitter power outputin watts

$d$is the distance from the radiator in meters

The factor${\sqrt {30}}$is an approximation of${\sqrt {\frac {Z_{0}}{4\pi }}}$

where$Z_{0}=376.730313668(57)$$\Omega \quad$is theimpedance of free space.$\Omega$is the symbol forohms.

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

Field strength meter

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 (TVROand RRO) frequencies. Most modern FS meters haveAFandVFcircuits and can be used as standard receivers. Some FS meters are also equipped with printers to record received field strength.

Antennas

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 gainwithfrequency.

Minimum field strength criteria

TheCCIRdefines the minimum field strength for satisfactory reception. These are shown in the table below.(Band II is reserved for FM radio broadcasting and the other bands are reserved forTV broadcasting.)

Frequency band	Minimum field strength in dBμV/m	Notes
Band I	48
Band III	55
Band IV	65
Band V	70
Band II	48	Rural area
Band II	60	Urban area
Band II	70	Large towns