What does the dBm mean on wireless adapters. Is it better if it is under 0 or above 0?

Answer 1

dB is decibels and is a "relative" term because it measures the ratio of two different power levels. dBm is decibels referenced to 1 milliwatt so it is an "absolute" term. Remember the rule of 3's and 10's. It's a logarithmic scale so a difference of plus or minus 3dB is twice or half the power, and plus or minus 10dB is 10 times or 1/10th the power.

So 0 dBm is 1 mW and 3dBm=2mW and 6dBm=4mW and 10dBm=10mW. Then 20dBm=100mW and 30dBm=1000mW or 1 Watt.

Note that your antenna also has a gain measured in dBi, here the little "i" is for isotropic, which describes a perfect omnidirectional antenna which is only possible in theory. Most standard rubber ducky antennas on home access points are 2.2dBi for example.

There are limitations placed on the maximum output power and maximum antenna gain by the FCC. They call the radio itself the "intentional radiator" and they call the power radiated by the antenna the EIRP, or equivalent isotropically radiated power which also takes the antenna gain into account.

In the 2.4GHz band from 2.4 to 2.4835 GHz the intentional radiator is limited to 30dBm (1 Watt) and the EIRP to 36dBm (4 Watts). So if you have a radio output power of 1 watt, the highest antenna gain you can use is 6dBi for example. If one goes up by 3dB (double the power) then the other must drop by 3dB (half the power) so you cannot exceed 36dBm or 4 watts.

dBm becomes important to wireless network designers because the FCC has these limits on the maximum radiated power, such as 4 watts which is 36dBm. So if your transmitter outputs 1 watt which is 30dBm and you hook up a high gain 12dBi antenna, that's a total of 42dBm and you're breaking the rules by 6dBm.

Answer 2

dBm is decibel relative to 1 milliWatt. Above zero means higher signal level than under 0. 1 milliwatt is 0dBm. The equation is

dBm = 10*log10(mW).