Yes, that was my thought also. A TDR is pretty much a frequency sweep
converted to time domain (and therefore to length assuming you know the
velocity factor of the line)) via an inverse FFT. In fact, I was
originally going to take the measurements at different frequencies and
either try to have my math major son throw some math at it, or I was
going to load the data into EXCEL and try to do an inverse FFT on it
that way (not sure how many data points Excel needs, though ...).
But then it occurred to me that I should first see what TLW would tell
me, and that turned out to be a usable approach.
And yes, I was worried that even the very low power of my FA-VA5 vector
impedance analyzer might be enough to alter the fault characteristics
and make the measurements useless. At a couple of frequencies when I
was taking measurements from the tower end of the feedline (the long
side of the fault) there was some jitter in the readings and I thought
"oh crap", but luckily I had done enough damage that nothing
substantially changed. ;)
73,
Dave AB7E
On 11/8/2019 6:48 PM, jimlux wrote:
On 11/8/19 4:00 PM, David Gilbert wrote:
By the way, I was wrong when I said earlier that TLW was telling me I
had a dead short. The graph only made it look like that, and a more
careful look at the axis dimensions said that the dips for each
frequency were always about 28 ohms resistive at the low power of my
analyzer ... which makes more sense than a short for a fault caused
by a high power RF arc over. I find that even more impressive (the
data, not my error), since it means that the location of even
non-absolute anomalies (neither open nor short) could potentially be
found by this method.
It's essentially like doing the FFT of a frequency sweep to do time
domain reflectometry.. And it's very, very sensitive. The one thing
that will cause troubles (with either approach sweeps or TDR) is some
sort of repetitive partial fault - because of ambiguities.
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