Signal strength in telecommunications

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

Relationship to Radiated Power

The electric field strength at a specific point can be determined from the average power delivered to the antenna, its geometry and radiation resistance. Consider the case of a half-wave dipole antenna. If constructed from thin wires, the current distribution is essentially sinusoidal and the resulting radiating (far-field) electric field is given by

$E_{\theta }(r)={-jI_{\circ } \over 2\pi \varepsilon _{\circ }c\,r}{\cos \left(\scriptstyle {\pi \over 2}\cos \theta \right) \over \sin \theta }e^{j\left(\omega t-kr\right)}$

where $\scriptstyle {\theta }$ is the angle between the antenna axis and the vector to an observation point, $\scriptstyle {I_{\circ }}$ is the peak current, $\scriptstyle {\varepsilon _{\circ }}$ is the permittivity, $\scriptstyle {c}$ is the speed of light, and $\scriptstyle {r}$ is the distance to the antenna. When the antenna is viewed broadside ($\scriptstyle {\theta =\pi /2}$), the electric field is maximized and is given by

$\vert E_{\pi /2}(r)\vert ={I_{\circ } \over 2\pi \varepsilon _{\circ }c\,r}.$

Solving this formula for the peak current yields

$I_{\circ }=2\pi \varepsilon _{\circ }cr\vert E_{\pi /2}(r)\vert .$

The average power to the antenna is

${P_{avg}={1 \over 2}R_{a}\,I_{\circ }^{2}}$

where $\scriptstyle {R_{a}}$ is the antenna’s radiation resistance. Substituting the formula for $\scriptstyle {I_{\circ }}$ into the one for $\scriptstyle {P_{avg}}$ and solving for the electric field yields

$\vert E_{\pi /2}(r)\vert ={1 \over \pi \varepsilon _{\circ }c\,r}{\sqrt {P_{avg} \over 2R_{a}}}\,.$

Using $\scriptstyle {R_{a}=73.13\,\Omega }$ (the radiation resistance for a half-wave dipole), $\scriptstyle {\varepsilon _{\circ }=8.85\times 10^{-12}\,F/m}$ and $\scriptstyle {c=3\times 10^{8}\,m/S}$, the maximum electric field strength from a half-wave dipole antenna radiating an average power of $\scriptstyle {P_{avg}}$ is

$\vert E_{\pi /2}(r)\vert ={9.91{\sqrt {P_{avg}}} \over r}\,.$

Therefore, if the average power to a dipole antenna is 1W, then the maximum electric field at 3m (10 ft) is 3.3V/m. If the power is 10mW, it is 0.33V/m at 3m.

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).

See also

• Cell network
• Cell phone
• Dropped call
• Cellular repeater
• Dead zone (cell phone)

External links

Cell Phone Signal Strength Map