dBm, dBi and dBd
. . . .
dBm
dBm (dB) - Unit of comparison (Absolute)
- Decibles milliwatt (dBm) provides a comparison of a signal to 1 milliwatt of power.
- 0 dBm is equal to 1 milliwatt. 1 milliwatt is the reference point.
- dBm is a measurement that is compared to a known value, 1 milliwatt, it is actually a measure of absolute power.
- Decibels (relative) are referenced to 1 milliwatt (absolute), think of a dBm as an absolute assessment that measures change of power referenced to 1 milliwatt.
- Any absolute power measurement of –dBm indicates amplitude less than 1 mW.
- Most 802.11 radios usually ranges from 1 mW to 100 mW.
- A transmission amplitude of 100 mW is equal to + 20 dBm.
- Because of FSPL, received signals will always measure below 1 mW. A very strong received signal is –40 dBm, which is the equivalent of 0.0001 mW (1/ 10,000th of 1 milliwatt).
dBi
dBi (dB) - Unit of comparison (Relative)
dBi (dB) - Unit of comparison (Relative)
- Decibels isotropic (dBi) is the comparison of gain or increase in power of antenna when compared to an isotropic antenna.
- dBi measures the antenna gain. The dBi value is measured at the strongest point, or the focus point, of the antenna signal.
dBd
dBd (dB) - Unit of comparison (Relative)
dBd (dB) - Unit of comparison (Relative)
- Decibels dipole (dBd) is the comparison of gain or increase in power of antenna when compared to an dipole antenna.
- A common antenna used on access points is the half-wave dipole antenna.
- The half-wave dipole antenna is a small, typically rubber-encased, general-purpose omnidirectional antenna. A 2.4 GHz half-wave dipole antenna has a dBi value of 2.14.
Comparing dBi and dBd Antennas
- On the rare occasion that you do run into an antenna measured with dBd, just add 2.14 to the dBd value and you will know the antenna’s dBi value.
- A standard dipole antenna has a dBi value of 2.14.
dBd -> dBi - If an antenna has a value of 3 dBd, this means that it is 3 dB greater than a dipole antenna.
- To calculate the value of a dBb antenna to dBi antenna, all you need to do is add 3 to 2.14.
- A 3 dBd antenna is equal to a 5.14 dBi antenna.
dBi -> dBd - If an antenna has a value of 6 dBi, this means that it is 6 dB greater than a isotropic antenna.
- To calculate the value of a dBi antenna to dBd antenna, all you need to do is subtract 2.14 from 6.
- A 6 dBi antenna is equal to a 3.86 dBd antenna.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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