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Roger

I have seen the article. Return loss measurements of 40 dB or greater turn
to mush on most instruments that cost less than $10,000. No surprise the
same would apply to the NanoVNA. But a couple of points:

1) Use of the NanoVNA or even calibration at a 900 MHz span is nonsense
and meaningless. No one would use a 900 MHz span for anything.

2) Characterizing the noise floor and the resulting possible dynamic range
based on such a span is also nonsense. The resolution bandwidth at that
span is 9 MHz! Useful for nothing.

3) Reducing the span to something useful and reasonable will put even your
White Salamander into spec or very near to it for the 600-900 MHz range.
The general spec is 50 dB of dynamic range from 600-900, 60 dB from
300-600, and 70 dB from 0-300. My White Salamander easily meets that spec
at 720 MHz when the span is reduced to something useful like 5 or even 50
MHz.

Attached is my White Salamander at the full 900 MHz span and with Center
at 720 MHz with a 50 and then a 5 MHz span. Worst case, allowing your
generous 10 dB of margin from the noise floor, it can easily resolve and
display return losses down to -35 dB and that is plenty good enough for
99.9% of the folks here who would use the instrument.

Comparing instruments based on a 900 MHz span noise floor is not very
informative. And no one else, including you, have published comparisons of
reasonable RL measurements made at reasonable spans and RBW.

WA8TOD

On Sep 2, 2019, at 5:45 PM, Roger Henderson <hendorog@...> wrote:

Hi Warren,
I don't know if you have seen this before. See the chart labelled
"Reflected Power Measurement Errors for Various Load Return Loss Values"
in this PDF:




In reality the trace we are talking about is the effective directivity of
the device. This is the error corrected directivity and can be compared to
the directivity of a directional coupler attached to a Spec An or a Scalar
Network Analyser.
The Vector part of the VNA allows this error correction to improve upon the
directivity of the bridge which is in the device. Interestingly the real
physical directivity of my white unit is slightly better than the hugen
unit.
It is the stability which is worse.

You can attach an antenna and see a spike down below this 'floor'. There is
an example of this in one of the online nanovna reviews IIRC.
The reviewer even questions it and says something like 'it shouldn't be
able to measure that'. It sounded to me that he didn't realise that the
error can go in both directions either. I can't find that review it at the
moment though.

I agree with you, in that if the effective directivity is 30dB, you can
measure down to 20dB plus or minus about 3dB - so your measurement is
within 6dB of whatever it really is.
If that is all you need, then that is fine. Absolutely no issue.

Anyway, I did a couple more tests - one up to 6MHz to shine a light on the
low freq performance.
The other was full span.
See the bottom of the page:




The full span test shows them about the same.
The low freq test shows an obvious difference between the units. The hugen
units looks incredibly good here. Maybe I have done something wrong!
Whether it is material to you Warren, or anyone else, is not for me to
decide. I don't know what you want to use it for. For me, they are about
the same price. I know which one I would rather have.

Before these tests, I did wrap some aluminum tape around both units. There
are still plenty of holes to let the heat out - Its pretty rough - I'm not
a surgeon - and it took about 2 minutes to do.
I can't say if the tape made any difference though.

Its not worth me spending any more time on this. Hopefully someone else can
pick this up and let us know if blocking up the sides, or putting it in a
bigger box as mentioned by Jerry, does help or not.

Roger.