Don't be fooled by the negative sign, it doesn't result in a negative number, but rather a very small number. Here's how it works:
-40 decibel milliwatts
= -4 bel milliwatts
= (10 to the power of -4) milliwatts
= .0001 milliwatts
= 100 nanowatts
You can never get decibels to result in a negative number, never ever.
Going the other way, if you wanted to convert the power of the Alfa to dBm, it'd be:
= 2.69897 bel milliwatts (I got this figure by taking the Log10 of 500)
= 26.9897 decibel milliwatts
So it's about 27 dBm
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