Distance To Fault (was Low Cost VNA)
Do any of these VNAs genuinely do "Time Domain
Reflectometry"? Genuine TDR involves sending a large,
sharp-edged pulse up the line and measuring the amplitude and
timing of the reflected signal, all of which would require
totally different hardware from what's inside a VNA.
Surely what we are really talking about is
"Distance to Fault" - which is something quite different, vastly
superior to traditional TDR and new to amateur radio. DTF is the
output from a wide-range sweep of return loss, with all the
vector data Fourier-transformed into the time domain.
Multiplying by the propagation velocity, values of time are
converted into distance along the line.
The DTF plot then shows a "local return loss" at
any distance along the line. Even small impedance bumps along
the line due to connectors, bends etc can be clearly seen - and
most importantly, located in terms of distance. The difference
between these expected small fluctuations and any larger
problems is very clear to see. I believe it was the Mitsubishi
Sitemaster that first brought "Distance To Fault" to the market,
so tower climbers can pretty much know
where the fault is before leaving the ground. For amateurs, we
can now make DTF sweeps of the entire feedline and antenna
system from the comfort of the shack, and then save the whole
display for future comparison so we can check every little
detail down the line in one go.
All of this places Distance To Fault in a totally
different league from traditional TDR. By persisting in using
the wrong name, we are actually underselling one of *the* most
useful things that a low-cost VNA can do.
Obviously this all depends on the support
software, and on us learning how to use it, but the necessary
hardware is already in our hands.
73 from Ian GM3SEK
On 30/09/2020 23:38, Tim, VK2XAX wrote: