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Signal strength is in dBm so the lower the better it is.
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.