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(P_{2}/P_{1})

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

'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '