Pete, I think that you are may be confusing the dBm with the definition of
the decibel and how meters are calibrated and referenced..
It is always true, by definition, that 0 dBm represents a power level of
one milliwatt. That is independent of any resistance value involved. What
is does depend upon, however, is the amount of current or voltage associated
with that resistance value that determines where the 0 dB level will be.
Consider the definition of the dB and see what how many dB are represented
by a power level P of one milliwatt::
dB = 10 log P2/P1 = 10 log (10^-3)/(10^-3) = 0 dB
So a power level of one milliwatt is 0 dBm. This is independent of any
resistance values since only power is involved in the equation.
When a voltage or current meter, for example has a dB scale on it, it may be
calibrated such that 0 dB on that scale corresponds to one milliwatt in 50
ohms or equally commonly one milliwatt in 600 ohms. Or perhaps some other
resistive reference.
Some of my meters are intended for r-f work and so their 0 dB reference is
to one milliwatt in 50 ohms for convenience. An example is the HP 411B RF
Millivoltmeter.
Others are for audio work and are calibrated to a reference of one milliwatt
in 600 ohms for a reading of 0 dB. An example is the Ballantine 303-07 AC
Voltmeter.
But 0 dBm is always a power level of one milliwatt regardless of the
resistance associated with it. The voltage or current associated with that
power level will,however, depend upon the resistance involved.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>
----- Original Message -----
From: <AC5E@aol.com>
To: <tentec@contesting.com>
Sent: Friday, July 25, 2003 5:22 PM
Subject: [TenTec] RE: dBm
>
>
> Unless they have changed the definitition since they changed cycles per
> second to Hertz; a dBm is "decibels referenced to one milliwatt at a
specified
> impedance." Back in those days, unless some other impedance was specified
a dBm
> was taken to be 1 milliwatt into 600 ohms for professional audio work.
>
> + 67 dB is 2,000,000 times ( as close as I can read the scale on this
> slipstick, or so close not to matter) the 0 dB level, and - 67 dB would
be one 2
> millionth of the 0 dB level of 1 milliwatt at the specified impedance. If
you
> don't have a calibrated milliwatt meter, that's about 5 millivolts on an
AC
> voltmeter or 5 mv p-p on a calibrated o-scope.
>
> 73 Pete Allen AC5E
>
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