In
telecommunications
Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
, a field strength meter is an instrument that measures the
electric field strength emanating from a
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
.
The relation between the electric field and the transmitted power
In ideal free space, the
electric field strength produced by a
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
with an
isotropic radiator is readily calculated.
:
where
:
is the electric field strength in volts per meter
:
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.
TPO is a concept related to effective radiated power (ERP), but refers t ...
in watts
:
is the distance from the radiator in meters
The factor
is an approximation of
where
is the
impedance of free space.
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 (1 ...
s.
It is clear that
electric field strength 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 refers to a value in a vector-valued field (e.g., in volts per meter, V/m, for an electric field ''E'').
For example, an electromagnetic field has both electric field strength and magnetic field strength.
Field str ...
produced by terrestrial transmitters, where reflections and
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
caused by objects around the transmitter or receiver may affect the electrical field strength considerably.
Field strength meter
A field strength meter is actually a simple receiver. The RF signal is detected and fed to a
microammeter, 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 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. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
.
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.)
References
{{Analogue TV transmitter topics
Electromagnetic radiation meters
Broadcast engineering