| Q: | I'm confused about dB ratings listed in the specifications for wireless LAN equipment. What is dB and what is the difference between dBi and dBm? | | A: | 1. deciBels - dB, or one tenth of a Bel, is a unit of mesurment that looks at the ratio of one value to another. Gain or loss can be measured in dB. The dB scale is an exponential scale using the formula log(ratio)*10. This means that 3 dB is about twice the power, 10 dB is 10 times the power, 13 dB is about 20 times the power and 20 dB is 100 times the power.
2. dBm - dBm is deciBels referenced to a value of 1 miliWatt of power. Power over or under 1mW would be plus or minus dBm respectively. If you have a transmitter that produces 1 watt of power that would be 1000 times more than 1 mW so that converts to 30 dBm.
3.dBW - dBW is deciBels referenced to a value of 1 Watt of power. Power over or under 1 Watt would be plus or minus dBW respectively.
4.Effective Isotropic Radiated Power - EIRP - Effective Isotropic Radiated Power defines the gain of an antenna over an "isotropic antenna" that would radiate equally in all directions.
If you have an antenna that radiates better in one direction than another, it would have gain in this direction. The amount of gain would be shown as "dBi" or dB gain (or loss) over an "isotropic antenna".
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