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Re: NanoVNA-Saver 0.2.0

 

Thanks for the additional explanation, Herb. Sorry if I was too unclear.

It would perhaps be possible for the software to have an 'auto' mode that
started at a low frequency and increased it until the reflection came
conveniently close to the limit of the range for best accuracy. But I don't
really like auto procedures that hide what's really going on : what if you
just wanted to look for partial reflections due to stubs or joints ?

The critical thing is that you obviously felt you should test it at the
working frequency of the system, and I can see why you'd start there. But
as Herb says, what you actually need is to test at a frequency where the
wavelength is comparable with the cable length. Presumably if you tried to
display the results of something more than a wavelength away, you'd get
multiple responses (repeated one wavelength apart).

On Fri, Nov 15, 2019 at 3:55 PM hwalker <herbwalker2476@...> wrote:

On Fri, Nov 15, 2019 at 05:36 AM, Barry Jackson wrote:

"If a particular frequency span setting, sweep mode etc. is needed for
TDR measurements, should not the software set those modes temporarily when
the TDR button is clicked and then revert back to the previous settings on
leaving the TDR screen?"

-----------------------------------------------------------------------------------------------
Barry,
Expanding a bit on what Adrian Godwin responded. This is one of those
situations where its better to think of frequency like a ham in meters
instead of Hz. That helped me to wrap my head around the fact that when
measuring meters of cable I needed to change my stop frequency to a value
that was equally long in meters for the TDR function to yield good data.
If you have firmware installed that has the TDR menu option, you will run
into the same thing using the NanoVNA in the field.

Also, as Adrian said, neither the NanoVNA or any software program knows
what length of cable you are attempting to measure, so just as you need to
manually enter the velocity factor, you may also need to manually change
your stop frequency for accurate results.

- Herb
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