Units of Power and Comparison
| Units of power (absolute) | Units of comparison (relative) |
|---|---|
| (W) watt | (dB) decibel |
| (mW) milliwatt | (dBi) decibels relative to an isotropic radiator |
| decibels relative to 1 milliwatt (mW) | (dBd) decibels relative to a half-wave dipole antenna |
Watt (W)
A watt is the standard unit of power in the International System of Units (SI). One watt is equal to 1 ampere (amp) of current flowing at 1 volt. (W=volt*amp) In RF, it is used to measure the total power of a signal.
Milliwatt (mW)
Milliwatt is a subunit of the watt and is equal to one-thousandth (1/1000) of a watt. It is commonly used to express small RF power levels. Most 802.11 devices transmit at power levels between 1 and 100 mW.
Decibel (dB)
A dB is a relative expression and a measurement of change in power (difference in two units of power), not a unit of power. In wireless networking, decibels are used either to compare the power of two transmitters, or more often, to compare the difference or loss between the EIRP output of a transmitter's antenna and the amount of power received by the receiver's antenna.
Decibel came from the term bel, which in the telephony industry was used to define power loss at a ratio of 10 to 1 between the power of two sound. ( a bunch of logarithms dealing with the powers of 10)
Decibels relative to an isotropic radiator (dBi)
The gain of power from an antenna compared to an isotropic radiator is called decibels isotropic (dBi), or simply antenna gain. It's a relative measurement, not absolute power, indicating the increase in power relative to an isotropic radiator. Antennas focus their energy in one direction, resulting in positive dBi values indicating gain, not loss. An antenna's dBi value is measured at its strongest point, typically the focus point of its signal. Antennas with a dBi value of 0 are known as "no-gain" or "unity-gain" antennas.
Decibels relative to a half-wave dipole antenna (dBd)
Another scale to describe antenna gain is decibels dipole (dBd), representing gain relative to a dipole antenna. It indicates the increase in gain compared to a dipole antenna's signal. Since dipole antennas are omnidirectional, dBd measures omnidirectional antenna gain, not unidirectional. Like dBi, dBd is a relative measurement of gain, not power.
The half-wave dipole antenna is a small, typically rubber or plastic encased, general-purpose omnidirectional antenna. A 2.4 GHz half-wave dipole antenna has a dBi value of 2.14. what happens when you want to compare two antennas and one is represented with dBi and the other with dBd? This is actually quite simple. A standard dipole antenna has a dBi value of 2.14. If an antenna has a value of 3 dBd, this means that it is 3 dB greater than a dipole antenna. Because the value of a dipole antenna is 2.14 dBi, all you need to do is add 3 to 2.14. So a 3 dBd antenna is equal to a 5.14 dBi antenna.