# Re: measuring cable loss

On Sat, 11 Aug 2007 07:58:36 -0400, "Jimmie D"
<GFENDER@xxxxxxxxxxxxxxx> wrote:

They did it the normal way

Hi Jimmie,

Given the long and winding road to this point, it would give me pause
that suddenly something became "normal." The remainder of your post
On Thu, 9 Aug 2007 08:13:45 -0400, "Jimmie D" <GFENDER@xxxxxxxxxxxxxxx> wrote:
The normal procedure for doing this is to inject a signal at one end and
measure the power out at the other.
For the sake of clarity (normality aside), what you originally
described Thursday is called "insertion loss."

On Fri, 10 Aug 2007 01:17:31 -0400, "Jimmie D" <GFENDER@xxxxxxxxxxxxxxx> wrote:
Power delivered to the antenna but be maintained with in +- 1 db in this
case that power is 100 watts. Power is normally
checked at the TX and recorded after allowing for line loss as "power at
the antenna".
This again defines "insertion loss."

and by measuring the return loss and they decided the "return loss
method" worked better.

A description of the classic self-fulfilling prophecy.

I presume you mean this to be "the normal way," but it doesn't really
describe a method or procedure (a "way"); instead, it describes an
outcome. There are many "ways" to measure a characteristic called
"return loss." Some "ways" are more accurate than others.

Having introduced this term, "insertion loss," there remains one more
term to consider: "reflection loss." This and "return loss" can be
found scaled on the common form of the Smith Chart.

The distinction to these terms are that "return loss" and "reflection
loss" are a single port characteristic (that port being the "load"
which, of course, is NOT the antenna, but rather the line and the
antenna). "Insertion loss" is a two port characteristic that properly

ALL such losses are defined by the system within which they reside.
This means you have to also characterize the impedances of BOTH the
load and the source. This last requirement is often dismissed in this
forum where the determination of the source's Z is frequently rejected
as being an impossibility (even when it is specified by the equipment
designer).

When Zsource = Zline = Zload, then many complexities are removed. I
have seen others ask you the characteristic Z of the load with no
response by you; and I am certain you have no comfortable assurance
would be immaterial if Zline = Zload.

Not sure what better means at this point. accurate
enough and easier and faster would constitute better.

This, too, simplifies what is an exceedingly difficult determination
(of "return loss," "reflection loss," or "insertion loss") for the
accuracy you originally suggested. Accurate, easy, and fast are not
normally words used in conjunction except in advertising promotions.

The accuracy of any power determination is related to the known Z of
2. The source;
3. The detector.

At 1 GHz, these determinations are not so easily dismissed as trivial,
nor confirmed by dragging a \$20,000 analyzer into the shop. The
analyzer answers the problem of knowing its own source Z, but it does
not answer what that source Z is of the transmitter (again, only a
necessity in the face of returned power).

Now, given no one has actually correlated accuracy to any metric here,
and given that accuracy is determined in large part by the three Zs
above; then a little more discussion is in order. Using only two (the
detector and the load could be interchanged for the simpler analysis):
Zsource = 100 Ohms

view in fixed font:

Error = ------------------------------

Error = +0.42dB to -0.78 dB

These errors are independant of other errors such as instrumentation
error (meter linearity, conversion problems, ...) or operator errors
(reading the meter - a mirrored scale is required to keep this below
5%). Modern instrumentation (if you have the \$\$\$\$) solves some of
this, others dismiss it as a trivial concern and rely on name brand
(Bird is frequently uttered to achieve perfection).

Now, as to the variability in the error wholly associated with just
the Zs (providing you can accurately determine them - yes, a game of
infinite regress). The allowable error of 1dB is nearly wiped out
with some very possible characteristics and you haven't even begun
balancing the error budget. With luck (a fictional village where
every armchair technician resides) the error induced by mismatches
could be 0. That luck demands you know the length of the line (again,
with some accuracy - I enjoy the irony here too). The variation built
into the Error computation is from not knowing that length (as is
common, few know this with enough precision in wavelengths). At 1
Ghz, the characteristic of
aproximately 200ft of coax
is apocryphal.

73's
Richard Clark, KB7QHC
.

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