Richard Clark wrote:
"---where dBm is frequently a short hand for dB (re1 mW into 600
Ohms)---."

I agree, but I`ve worked with communicatiobs systems in which the
builder specified signal (test-tone) levels throughout in dBm regardless
of the impedance at the particular point. Incinvenient, maybe, but often
the conversion factor is specified for reading with a dB meter
calibrated for 600 Ohms.

dB is a power ratio. dBm is a power level for which the 0 dBm reference
is 1 milliwatt. The impedance is not specified. 2 milliwatts is
approximately +3 dBm and 1/2 milliwatt is approximately -3 dBm.

The a-c voltmeter which reads in dB is likely calibrated at 600 Ohms
impedance. Volts vary as the square root of the power. +6 dB is 4X the
power but only double the voltage.

dBm is frequently specified as a test-tone level at various points
throughout a communications system. The dB meter will only be correct
when the impedance at the measurement point imatches the impedance for
which the meter is calibrated. When the impedance is different, The
system builder will often give a correction factor to be used with a
600-Ohm dB meter.

Knowing the power level in milliwatts, the a-c volts are easily
calculated from the square toot of PR.

Best regards, Richard Harrison, KB5WZI