In telecommunications, particularly in
radioRadio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
,
signal strength refers to the
magnitudeMagnitude Is A Part Of An EarthquakesMagnitude may refer to:In mathematics:*Magnitude , the relative size of a mathematical object*Magnitude , a term for the size or length of a vector...
of the
electric fieldIn 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...
at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as
received signal level or
field strength. Typically, it is expressed in
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
per
lengthIn geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...
or
signal powerIn radio transmission, transmitter power output is the actual amount of power of radio frequency energy that a transmitter produces at its output....
received by a reference antenna.
High-poweredNominal power is a measurement of a mediumwave radio station's output used in the United States. AM broadcasters are licensed by the Federal Communications Commission to operate at a specific nominal power, which may be different from the transmitter power output.* For non-directional stations,...
transmissions, such as those used in
broadcastingBroadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
, are expressed in
dBThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
-millivolts per metre (dBmV/m). For very low-power systems, such as
mobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s, signal strength is usually expressed in
dBThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
-microvolts per metre (dBµV/m) or in
decibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s 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
Blanketing is interference caused by strong radio signals. Although the spectral mask of a radio station's transmitter suppresses spurious emissions on other frequencies in the band, being extremely close to a station may allow them to still be strong enough to cause significant interference...
interference may occur on some receivers
- 60 dBµ or 1.0 mV/m: frequently considered the edge of a radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
'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 antennaA dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
in
free spaceFree-space optical communication is an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking."Free space" means air, outer space, vacuum, or something similar...
(
). 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
Cellphone 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 ever to be effectively covered since the cost of erecting a cell tower is too high for only a few customers. Even in high reception areas it is often found that basements and the interiors of large buildings 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 rapid 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
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
since these are attenuated more rapidly by intervening obstacles, although they are able to use
reflectionReflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
and
diffractionDiffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
to circumvent obstacles.
Cell phones in the U.S. operate at around 800
MHzThe hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
and
PCS phonesAt the most basic level Personal Communications Service or PCS describes a set of wireless communications capabilities that allows some combination of terminal mobility, personal mobility, and service profile management...
at 1900 MHz, classified as UHF and low energy microwaves respectively. This has led to the rapid growth in the home
cellular repeaterA cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal...
market. The more advanced models now typically include an external
directional antennaA directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
and an
amplifierA linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio...
(usually operating at 55 dB
gainIn electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
), which is generally enough to turn a very weak signal into a clear one over the local area (from around a thousand square feet to over twenty thousand).
See also
- Cell network
- Cell phone
- Dropped call
- Cellular repeater
A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal...
- Dead zone (cell phone)
- S meter
An S meter is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a subjective system of reporting signal strength from S1 to S9 as part of the RST code...
- Cel-Fi
Cel-Fi is a device, similar to a cellular repeater, designed to eliminate in-building dead zones and improve indoor mobile phone reception.Created by Nextivity, Inc., the Cel-Fi systems are designed with smart antenna technology to seek out the best available signal to maximize signal gain to phone...
External links