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

'One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike - and yet it is the most precious thing we have.'