There are three terms, idb, pdb, and vdb. They are not the same. idb and pdb
have the same value: 10*log10 (i/i0), while vdb is 20*log10(v/v0).

Dan


Roy Lewallen wrote:
Robert11 wrote:

Hello:

When a table gives the attenuation (at some freq.) for a length of a
particular coax in db,
are they referring to db in voltage or db in power ?


Both. When voltage and power ratios are measured across the same single
impedance, the ratio of voltages in dB is the same as the ratio of power
in dB. That is, dB in voltage equals dB in power if only one impedance
is involved. That's why there are different formulas for dB voltage and
power ratios, to make this happen. And it's the case if a coax cable is
terminated in its characteristic impedance so the input and output
impedances are the same. It's under this condition that cable loss is
specified, so the dB loss represents the loss of both voltage and current.

But when you compare the power at the output of a coax cable with the
power at the input and the cable isn't terminated in its characteristic
impedance, output and input impedances can be very different. The power
loss will be somewhat higher than the specification (probably not much,
when the matched loss is only 2 dB), but you can have much less or more
voltage at the output than the input. So the power and voltage loss in
dB can be very different, and you can actually have voltage gain --
although it can be argued that defining dB for a ratio of voltages
across two different impedances is a bit shaky and perhaps not too
meaningful.


Is is correct to ask a question as simply as the following:

If the attenuation is given as, e.g., 2 db, what Percentage therefore
of a received signal is "lost"
going thru the coax length ?


100 * (1 - 10^(-dB/10)) ~ 37% is the fraction of power lost.

100 * (1 - 10^(-dB/20)) ~ 21% is the fraction of voltage lost.

These assume that the coax is terminated with its characteristic impedance.

And you don't need to put "lost" in quotation marks. It is truly lost as
a signal, having been turned into heat.

Roy Lewallen, W7EL