Radiated Power Measurement

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Power Ratio dBm or dBmW
The power compared to a 1 milliwatt (1mW) source, expressed in decibels (dB)

Formula:

$$ x = 10 \log_{10}(P/1 \ \mathrm{mW}) \, $$

Where P is the power in watts (W)

This roughly equates to:
 * Double the power = 3dB increase in dBm
 * Halve the power = 3dB decrease in dBm

more information can be found here