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Alan wrote:
Assume a convenient spool of coaxial cable. Take your standard Bird
wattmeter and measure power going into the cable using a slug that has
had the wattage range rubbed off. Adjust the signal source as needed
to get a suitable reading.

Move to the other end of the cable, and measure the power coming out
(and presumably into a load), using the same slug. Note that the meter
indicates 1/2 the power level.

You can pretty safely say that there was about 3 dB loss in the cable.

You cannot say whether 1 watt was applied at the source end, or 10 watts.

You just made a dB reading without any absolute reference.

One might suggest the absolute reference is the power reading on the
input. It might say "1" without you having any way of knowing 1 *what*
(1 watt, 1 milliwatt, 1 kilowatt?) but you do know it was 1 of whatever
the scale is with that slug. Since the formula for calculating dB
factors out the units, the scale doesn't matter as long as it's the same
for both the source and load measurements.

Assume the same experiment, except that there is no connector at the
source end of the cable so you *cannot* measure the power at that point.

How do you determine the dB loss in the cable when you can't determine
the reference power at the source?
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

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Doug Smith W9WI writes:

Alan wrote:
Assume a convenient spool of coaxial cable. Take your standard Bird
wattmeter and measure power going into the cable using a slug that has
had the wattage range rubbed off. Adjust the signal source as needed
to get a suitable reading.
Move to the other end of the cable, and measure the power coming
out
(and presumably into a load), using the same slug. Note that the meter
indicates 1/2 the power level.
You can pretty safely say that there was about 3 dB loss in the
cable.
You cannot say whether 1 watt was applied at the source end, or 10
watts.
You just made a dB reading without any absolute reference.

One might suggest the absolute reference is the power reading on the
input. It might say "1" without you having any way of knowing 1
*what* (1 watt, 1 milliwatt, 1 kilowatt?) but you do know it was 1 of
whatever the scale is with that slug. Since the formula for
calculating dB factors out the units, the scale doesn't matter as long
as it's the same for both the source and load measurements.

Assume the same experiment, except that there is no connector at the
source end of the cable so you *cannot* measure the power at that
point.

How do you determine the dB loss in the cable when you can't determine
the reference power at the source?

You can't, and that was my point. Although Alan did come up with
a good correction to my statement.
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% % 'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

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(Alan) writes:

In article Randy Yates writes:
(Alan) writes:

Or, you just do the readings with an instrument that does not have
absolute calibration. i.e., something that can only do relative readings.

Whoa. There is no such thing, if the readings are in "dB" (i.e., not
"dBm" or "dBW' or whatever).

If you're actually measuring some real, physical thing to come up with
a dB value, e.g., the loss in a cable, then you must have an absolute
reference level somewhere.

Nonsense.

In the cable loss case, the reference level
is the power input to your cable in the measurement and the referred
level is the power level measured at the output of the cable. In other
words, you can't just measure the power coming out of the cable and
pull a dB value of your hat (or any other place).

Again, nonsense.

Assume a convenient spool of coaxial cable. Take your standard Bird
wattmeter and measure power going into the cable using a slug that has
had the wattage range rubbed off. Adjust the signal source as needed
to get a suitable reading.

Move to the other end of the cable, and measure the power coming out
(and presumably into a load), using the same slug. Note that the meter
indicates 1/2 the power level.

You can pretty safely say that there was about 3 dB loss in the cable.

You cannot say whether 1 watt was applied at the source end, or 10 watts.

You just made a dB reading without any absolute reference.

Although I don't think "nonsense" is the appropriate response, you've
made a good point, Alan. I stand corrected.

You do not need an absolute reference even when COMPUTING dB. And you
sure as hell don't need one when INTERPRETING dB.

In both cases, this is assuming the system is reasonably linear.
--
% Randy Yates % "Though you ride on the wheels of tomorrow,
%% Fuquay-Varina, NC % you still wander the fields of your
%%% 919-577-9882 % sorrow."
%%%% % '21st Century Man', *Time*, ELO
http://home.earthlink.net/~yatescr

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Doug Smith W9WI writes:

Alan wrote:
Assume a convenient spool of coaxial cable. Take your standard Bird
wattmeter and measure power going into the cable using a slug that has
had the wattage range rubbed off. Adjust the signal source as needed
to get a suitable reading.
Move to the other end of the cable, and measure the power coming
out
(and presumably into a load), using the same slug. Note that the meter
indicates 1/2 the power level.
You can pretty safely say that there was about 3 dB loss in the
cable.
You cannot say whether 1 watt was applied at the source end, or 10
watts.
You just made a dB reading without any absolute reference.

One might suggest the absolute reference is the power reading on the
input. It might say "1" without you having any way of knowing 1
*what* (1 watt, 1 milliwatt, 1 kilowatt?) but you do know it was 1 of
whatever the scale is with that slug.

Then that is NOT an absolute reference! It is precisely the same
situation as being given 0 dB! You know it's one of the reference
power, but you don't know what that power is!
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% % 'Waterfall', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

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On Sun, 1 Oct 2006 07:26:04 +0000 (UTC), (Alan) wrote:

Again, nonsense.

Assume a convenient spool of coaxial cable. Take your standard Bird
wattmeter and measure power going into the cable using a slug that has
had the wattage range rubbed off. Adjust the signal source as needed
to get a suitable reading.

Move to the other end of the cable, and measure the power coming out
(and presumably into a load), using the same slug. Note that the meter
indicates 1/2 the power level.

You can pretty safely say that there was about 3 dB loss in the cable.

You cannot say whether 1 watt was applied at the source end, or 10 watts.

You just made a dB reading without any absolute reference.

In an earlier post, I said

"Sure - you (or your gear) must know (absolute levels) *during* the measurement".

In this case, the wattmeter knows...

Steve Maki