The Bel is named after Alexander Graham Bell, and is the base 10 logarithm of the ratio of two power levels. Originally, the Bel was used to compare two audio power levels in terms of the response to them by the human ear because the human ear responds logarithmically to audio power levels.

In the wireless world DeciBels (or dB) are used as a convenient common unit to convert wireless measurements to. Some measurements that you will see represented as dB are:

• dBm - decibel milliwatts
• dBi - decibel isotropic radiator (referring an Isotropic Antenna)
• dBw - decibel watts

A handy reference chart of dBm -> mW equivillents is available on dBmChart.

• Confusion here, can someone clarify? If dB is used to compare, compare with what? E.g., dBm is decibel milliwatts (as we see in some of the specs of the Prism2Cards) is relative to what?

  • To followup, evidently it is relative to 1 mW. Which squares with what I've seen: (expt 10 1.5) -> 31.62, or 15 dBm is equivalent to 31.62 mW. That makes sense now. Thanks for the clarification. -- RussellSenior

  Secondly, I thought I heard DrEngelson say that in dBi the "i" stood for electrical current, but the sound decibels were awfully low, so I could easily have that wrong. Maybe someone could check. -- RussellSenior

DrEngelson did say the 'i' in dBi meant current. I believe he was confusing 'I' which normally used as an abbreviation for current in Ohm's Law, V=IR, with the real meaning of 'i' in dBi. It means 'isotropic radiator', which is some sort of theoretical antenna that is useful for comparing against other antennas. A good summary is at http://www.ydi.com/decibels-radio-systems.php -- DonPark

http://www.ydi.com/deployinfo/wp-decibels.php <link corrected [CategoryGlossary]