In telecommunications, field strength meter is a measuring device which measures the electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 caused by a transmitter

Transmitter

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

.

The relation between the electric field and the transmitted power

In ideal free space, the electric field strength

Field strength in free space

Field strength in free space is a term in telecommunications. It is the field strength caused by a half wave dipole under ideal conditions. The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength....

 produced by a transmitter

Transmitter

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

 with isotropic radiator

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 a sphere centred on the source...

 is readily calculated


where

E is the electric field strength in volts per meter

P is the transmitter power output

Transmitter power output

In radio transmission, transmitter power output is the actual amount of power of radio frequency energy that a transmitter produces at its output....

 in watts

d is the distance from the radiator in meters

It is clear that field strength is inversely proportional to the distance between the transmitter and the receiver. However, this relation is impractical for calculating the field strength produced by terrestrial transmitters, where reflections and attenuation 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. After a tuner circuit, the signal is detected and fed to a microammeter, which is scaled in dBų. 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 have AF

Audio frequency

An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...

 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 10 metres (32.8 ft). Gain correction tables may be provided with the meter, that take into account the antenna's change of gain with frequency.

Minimum field strength criteria

The CCIR defines 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 for TV broadcasting.)

Frequency band	Minimum field strength in dBu	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