In
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 tha ...
, particularly in
radio frequency engineering
Radio-frequency (RF) engineering is a subset of electronic engineering involving the application of transmission line, waveguide, antenna and electromagnetic field principles to the design and application of devices that produce or use signals w ...
, signal strength refers to the
transmitter power output as received by a reference antenna at a distance from the transmitting antenna.
High-powered transmissions, such as those used in
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 ...
, are expressed in
dB-
millivolts per metre (dBmV/m). For very low-power systems, such as
mobile phone
A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
s, signal strength is usually expressed in
dB-
microvolts per metre (dBμV/m) or in
decibels above a reference level of one
milliwatt (
dBm). In broadcasting terminology, 1 mV/m is 1000 μV/m or 60
dBμ (often written dBu).
;Examples:
*100 dBμ or 100 mV/m:
blanketing {{Unreferenced, date=December 2009
Blanketing is a term used predominantly in the US to refer to receiver blocking, which is interference caused when a strong unwanted off-channel radio signal prevents the reception of another (wanted) transmission ...
interference may occur on some receivers
*60 dBμ or 1.0 mV/m: frequently considered the edge of a
radio station
Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radi ...
's protected area in North America
*40 dBμ or 0.1 mV/m: the minimum strength at which a station can be received with acceptable quality on most receivers
Relationship to average radiated power
The electric
field strength at a specific point can be determined from the power delivered to the transmitting antenna, its geometry and radiation resistance. Consider the case of a center-fed half-wave
dipole antenna
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole w ...
in
free space
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
, where the total length L is equal to one half wavelength (λ/2). If constructed from thin conductors, the current distribution is essentially sinusoidal and the radiating electric field is given by
:
where
is the angle between the antenna axis and the vector to the observation point,
is the peak current at the feed-point,
is the permittivity of free-space,
is the speed of light in a vacuum, and
is the distance to the antenna in meters. When the antenna is viewed broadside (
) the electric field is maximum and given by
:
Solving this formula for the peak current yields
:
The average power to the antenna is
:
where
is the center-fed half-wave antenna's radiation resistance. Substituting the formula for
into the one for
and solving for the maximum electric field yields
:
Therefore, if the average power to a half-wave dipole antenna is 1 mW, then the maximum electric field at 313 m (1027 ft) is 1 mV/m (60 dBμ).
For a short dipole (
) the current distribution is nearly triangular. In this case, the electric field and radiation resistance are
:
Using a procedure similar to that above, the maximum electric field for a center-fed short dipole is
:
RF signals
Although there are cell phone base station tower networks across many nations globally, there are still many areas within those nations that do not have good reception. Some rural areas are unlikely to ever be covered effectively since the cost of erecting a cell tower is too high for only a few customers. Even in areas with high signal strength, basements and the interiors of large buildings often have poor reception.
Weak signal strength can also be caused by
destructive interference of the signals from local towers in urban areas, or by the construction materials used in some buildings causing significant attenuation of signal strength. Large buildings such as warehouses, hospitals and factories often have no usable signal further than a few metres from the outside walls.
This is particularly true for the networks which operate at higher
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 ...
since these are attenuated more by intervening obstacles, although they are able to use
reflection and
diffraction to circumvent obstacles.
Estimated received signal strength
The estimated received signal strength in an active
RFID tag can be estimated as follows:
:
In general, you can take the
path loss exponent into account:
:
The effective
path loss depends on
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 ...
,
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary s ...
, and environmental conditions.
Actually, one could use any known ''signal power'' dBm
0 at any distance r
0 as a reference:
:
Number of decades
:
would give an estimate of the number of decades, which coincides with an average path loss of 40 dB/decade.
Estimate the cell radius
When we measure cell distance ''r'' and received power pairs,
we can estimate the mean cell radius as follows:
:
line-of-sight propagation
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions travelin ...
, unless reflection occurs.
See also
*
Cel-Fi
*
Cell network
*
Cell phone
*
Cellular repeater
A cellular repeater (also known as cell phone signal booster or cell phone signal amplifier) is a type of bi-directional amplifier used to improve cell phone reception. A cellular repeater system commonly consists of a donor antenna that receives ...
*
Dropped call
*
Dead zone (cell phone)
*
Field strength in free space
*
Field strength meter
*
Received signal strength indication
In telecommunications, received signal strength indicator (RSSI) is a measurement of the power present in a received radio signal.
RSSI is usually invisible to a user of a receiving device. However, because signal strength can vary greatly and a ...
*
S meter
*
Signal (electrical engineering)
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
*
Mobile phone signal
*
Mobile coverage
References
External links
Global map of cell phone signal by network. Based on crowdsourced data.Crowd sourced map of cell and wifi signals. Data release under the Open Database License.{{Refend
Radio electronics
Mobile technology
es:Repetidor celular
id:Penguat sinyal selular