If you use “iwconfig” on Linux to get statistics on your wireless LAN you’ll note that the “Signal Level” is expressed in negative decibels. On the surface that would appear confusing – how can decibels be negative?!
Fortunately an anonymous poster explains at Toms Hardware:
Absolute power of a signal is measured in wattage. The bel or decibel system can only describe relative power- a gain of 3 dB means your signal is 2 times as strong as it was before, but the dB scale doesn’t define where you’re starting from or what your ‘zero’ is. So, we specify dBm, indicating that our scale is relative to 1 milliWatt of power. 0 dBm = 1 mW.
The reason you see negative values is that you’re representing small but positive numbers, on a logarithmic scale. In logarithms, the value indicated represents an exponent… for example, under a log 10 scale, a value of -2 represents 10 to the -2 power, which equals 0.01. Likewise, a negative dBm means that you’re applying a negative exponent in your power calculations; 0 dBm equals 1 mW of power, so -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth. It’s a lot easier, and more useful in some calculations, to describe a weak signal as -100 dBm as opposed to 0.0000000001 mW.
Pretty simple when it’s explained that way!