What exactly is a dBm and how would you measure it?

Asked by: Steve Drake

Answer

It is often convenient to express power or voltage ratios, (or any ratio for that matter), using a logarithmic scale. For example the definition of a power ratio in terms of decibels is:

Power ratio in dB = 10*Log(P₂/P₁)

for example if the output power of a device is found to be half of the input power then we could express this as -3dB (plug in some numbers to see this).

Now to answer your question concerning dBm. We use dBm to express absolute values of power relative to a milliwatt. The definition is

Measure of power in dBm = 10*Log(power in milliwatts / 1 milliwatt)

So if you wanted to express your power in terms of dBm instead of milliwatts you would use the above equation.

For example if your power level were 1 milliwatt, you could also call this power level 0 dBm. Or if your power level were 100 milliwatts you could instead say 20 dBm.
Answered by: Bryan Benson, B.S., Physics Grad Student, CSUF