In telecommunications, particularly in radio, signal strength refers to the
magnitude of the electric field at a reference point that is at 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 voltage
per length or signal power received by a reference antenna. High-powered
transmissions, such as those used in broadcasting, are expressed in
dB-millivolts per metre. For very low-power systems, such as mobile
phones, signal strength is usually expressed in dB-microvolts per metre or
in decibels above a reference level of one milliwatt. In broadcasting
terminology, 1 mV/m is 1000 µV/m or 60 dBµ.
Examples 100 dBµ or 100 mV/m: blanketing
interference may occur on some receivers 60 dBµ or 1.0 mV/m: frequently
considered the edge of a radio station'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 bruh 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 is 1
mV/m. 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 frequency since these are attenuated
more rapidly 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 a mobile device can be estimated as
follows: More general you can take the path loss
exponent into account: If the mobile device is at cell radius
distance from the cell tower the received power is estimated as -113 dBm.
The effective path loss is depending on the frequency, the topography, and the
environmental conditions. Actually one could use any known signal
power dBm0 at any distance r0 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 dBmm pairs, then we can estimate the mean cell radius as
follows: Specialized calculation models exist to
plan the location of a new cell tower, taking into account local conditions and
radio equipment parameters. Take also into consideration that mobile radio
signals have line-of-sight propagation, unless reflexion would occur.
See also Cel-Fi
Cell network Cell phone
Cellular repeater Dropped call
Dead zone Field strength in free space
Received signal strength indication S meter
Signal Mobile phone signal
Mobile coverage References
External links