Re: Distance To Fault (was Low Cost VNA)

Stephen Tompsett

I used to have a HP140+HP1415A TDR system, a real TDR...

I think it used a  tunnel diode to generate a fast pulse and a diode sampling gate and nuvistors in the sampling section . A superb piece of equipment, but not very portable. I think it had enough resolution to explore impedence discontinuities in coaxial adapters.

The 'TDR' mode of the modern handheld VNA devices is much  convenient to use outside the lab.

On 01/10/2020 11:51, Gordon REASON via wrote:

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 <gm3sek@...> wrote:

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:

Hi Mark,

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 wrote:
Hi all

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




Stephen Tompsett (G8LYB)
Tel: 01788 578940
Mob: 07956 855816

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