Signal strength
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In telecommunications, and particularly in radio, signal strength transmitted signal is being received, measured, or predicted, 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, this is measured as signal electric field strength of voltage per length or signal power received by a reference antenna. Higher powered transmissions such as broadcasting use units of dB-millivolts per metre (dBmV/m). Very low-power uses such as mobile phones are most often expressed in dB-microvolts per metre (dBµV/m) or in decibels above a reference level of one milliwatt (eg − 80 dBm).
In broadcasting terminology 1 mV/m is 1000 µV/m, or 60 dBµ (often written dBu).
- Some examples
- 100 dBµ or 100 mV/m: blanketing interference may occur on some receivers
- 60 dBµ or 1.0 mV/m: Is 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
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[edit] Formula
The electric field strength can be calculated from the effective radiated power, ERP,[1] of the antenna and its distance, d (here, based on a resistance of 50 Ω):
where E is in volts per metre, and d is in metres.
[edit] 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.
Cell phones in the U.S. operate at around 800 MHz and PCS phones at 1900 MHz, classified as UHF and low energy microwaves respectively. This has led to the rapid growth in the home cellular repeater market. The more advanced models now typically include an external directional antenna and an amplifier (usually operating at 55 dB gain) – 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).
