I disagree that it's not of any practical use. The MFJ antenna analyzer
uses essentially this method, and on every occasion where I've used it
(mostly on RG-58 cable, occasionally on RG-213), it agrees within a
tenth of a dB or so with measurements made with a Bird wattmeter. Agreed
that the Bird isn't terribly accurate, but it's not too bad for
comparative readings where the differences aren't great.

Assuming that the measurement is made with an SWR meter, I agree that
the result won't be accurate if the line impedance is different from the
SWR meter's. However, for the normal variations one sees in "50 ohm"
cable, the results are accurate enough to be quite useful. The method
certainly shouldn't be used for lines whose nominal characteristic
impedance isn't the same as the meter's. I apologize for not pointing
this out, and thank Reg for doing so.

Contrary to (1) below, no slotted line or voltmeter is necessary to use
this equation.

I do agree that this wouldn't be a good method to measure very small
amounts of loss. But for the original purpose of deciding whether old
cable is usable, it should be fine. If the loss is that small, there's
no reason not to use the line.

There are many occasions where a casual measurement is better than no
measurement at all -- always with the caveat that you realize the
limitations of the measurement's accuracy and don't try to draw
conclusions from it that aren't justified. This is certainly a method
that gives results which, under a number of circumstances, is quite useful.

Roy Lewallen
Certifed Reg's Old Wife, Nit-picker, and Busy-body

Reg Edwards wrote:
"Roy Lewallen" wrote

You can also determine the loss by measuring the input impedance or SWR
of an open or short circuited piece of cable -- the better the SWR, the
higher the loss. Loss in dB = 10 * log[(SWR + 1) / (SWR - 1)].


===============================

The above formula may be correct but it's not of any practical use.

(1) It applies only when SWR is measured by running alongside a slotted
line with a voltmeter, stopping to make a note of the voltmeter readings at
the max and min points, and then calculating Vmax/Vmin. It is of academic
interest and is entirely unrelated to line Zo.

(2) The error in application arises from the confusion caused by the
old-wives incorrect assumption that the TLI (Transmitter Loading Indicator)
actually measures SWR. It doesn't.

(3) If the ordinary TLI did measure SWR then the measurement would apply
only to 50-ohm lines. On any other line, such as old coax when Zo is
unknown, the measurement would be meaningless.

(4) If the cable to be checked was really 50 ohms and the so-called SWR
meter was standardised on 75 ohms then the measurement would be equally
meaningless.

(5) If the shack 50-ohm SWR meter can be relied upon (and most of them
can't) then the ONLY information to be gleaned is to measure both the
short-circuit and open-circuit SWRs. If they are in the same ball park then
the unknown cable is probably not far from 50 ohms.

(6) Line attenuation is a very small quantity. It is the most difficult of
all line parameters to measure accurately. The last instrument to choose is
the SWR meter in the shack. The most attrocious mechanical damage can be
caused over a short length without hardly any effect on attenuation over a
longer length. To detect the presence of other than catastrophic cable
faults is not possible by attenuation tests. There are far more sensitive
ways of detecting and locating faults.

(7) To check an old length of line just test for DC continuity and look at
the ends. If all is bright and clean there's nothing wrong with it.
----
Reg, G4FGQ