telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

, a field strength meter is an instrument that measures the

electric field strength An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field f ...

emanating from a

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

The relation between the electric field and the transmitted power

In ideal free space, the

produced by a

with an

isotropic radiator An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over ...

is readily calculated. :

\mbox \approx \frac

where :

\mbox

is the electric field strength in volts per meter :

P

is the

transmitter power output In radio transmission, transmitter power output (TPO) is the actual amount of power (in watts) of radio frequency (RF) energy that a transmitter produces at its output. This is not the amount of power that a radio station reports as its power, as ...

in watts :

d

is the distance from the radiator in meters The factor

\sqrt

is an approximation of

\sqrt

where

Z_0=376.730 313 668(57)

\Omega\quad

is the

impedance of free space The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic fie ...

\Omega

is the symbol for

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (b ...

s. It is clear that

is inversely proportional to the distance between the transmitter and the receiver. However, this relation is impractical for calculating the

field strength In physics, field strength means the ''magnitude'' of a vector-valued field (e.g., in volts per meter, V/m, for an electric field ''E''). For example, an electromagnetic field results in both electric field strength and magnetic field strength ...

produced by terrestrial transmitters, where reflections and

attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...

caused 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 a

microammeter An ammeter (abbreviation of ''Ampere meter'') is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit ...

, which is scaled in dBμ. The frequency range of the tuner is usually within the terrestrial broadcasting bands. Some FS meters can also receive satellite (

TVRO Television receive-only (TVRO) is a term used chiefly in North America, South America to refer to the reception of satellite television from FSS-type satellites, generally on C-band analog; free-to-air and unconnected to a commercial DBS prov ...

and RRO) frequencies. Most modern FS meters have AF and VF circuits 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 . Gain correction tables may be provided with the meter, that take into account the change of antenna gain with

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

Minimum field strength criteria

The CCIR defines the minimum field strength for satisfactory reception. These are shown in the table below.K.H.Kaltbeitzer: ''Technical Monograph 3104'', EBU Technical Centre, 1965, p.24 (Band II is reserved for FM radio broadcasting and the other bands are reserved for

TV broadcasting Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began wi ...

References

{{Analogue TV transmitter topics Electromagnetic radiation meters Broadcast engineering