Are you aware , Ian , of the
simple TDR , using an 74AC14 circuit ......... works well , but
needs a good modern oscilloscope , for accurate time response
Look on youtube .........
On 01 October 2020 at 11:36 Ian White
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.
On 30/09/2020 23:38, Tim, VK2XAX wrote:
Thanks for your analysis, While I have instruments to do TDR
& RL from 10kHz to 6GHz I'm primarily interested in its
other functions. Its good to know that its at least reasonable
and not telling pork pies! But then, that also assumes the
operator knows what he's doing too :)
On 30/9/20 6:44 pm, Mark GM4ISM via groups.io wrote:
I have not done direct comparison tests ( I rarely get my
eldery 8754A VNA out) but a few publications have indicated
that the results are broadly similar.
I do however us VNAs extensively and know their limitation
and pitfalls in their use.
The Nano VNAs stack up moderately well overall and on most
circumstances are more than adequate