Re: Getting started with the nanoVNA guide

Dr. David Kirkby from Kirkby Microwave Ltd <drkirkby@...>

On Mon, 2 Sep 2019 at 06:28, HexAndFlex via Groups.Io <iain_haggis=> wrote:

I have been working on what I hope is an easy to follow introduction to
the nanoVNA. It's fairly basic and aimed at users who have probably used
other test test equipment such as oscilloscope or maybe even a spectrum
analyser, but are unsure why they might benefit from a VNA.

This is a work in progress and I will aim to take any feedback given here
to improve the content.

Part 2 is in the works and I hope to have it uploaded this time next week.
Thank you - that will be most helpful to me. Here are a few comments, which
I hope are constructive - they are not meant to be critical.

1) I'm not really convinced an RF engineer would find a VNA more useful
than a multimeter. 😀😀 I think a VNA is second only to a multimeter.

2) I don't believe the analogy of the train in the tunnel is as good as the
diagram here

which I stole from an Agilent Application note. I think the transmitted and
reflected signal off a lens is a bit less confusing than the train in the
tunnel. If you want to bring different frequencies into it, then this
could be expanded with different colour LEDs.

3) A high-end VNA will cost as much as a house *nowadays* - it is not just
years ago.

has a starting price of $176,888 for a basic 2-port model. With 4 ports and
a second source raising that cost rises to $358,749. Add in a few options,
and it will easily go beyond $500,000, and it would not surprise me if it
goes over $1,000,000 USD.

4) The graph of S11 is not the reflection coefficient, but the magnitude of
the reflection coefficient. I think a diagram showing the phase of the
reflection coefficient would be easier to understand than the Smith Chart.
I believe the Smith Chart overcomplicates things.

5) The comment "The cal kit and cables that come with the kit are pretty
cheap and nasty. That said, many pro VNAs don’t include cables or cal kits
at all. These generally cost extra, and aren’t cheap." I'm unaware of any
professional VNA that comes with cables or calibration kits, so at least
saying some(perhaps all) don't come with calibration kits is useful. I
would point out that they cost in excess of $1000 and many in excess of

6) The comment about missing features being "No de-embedding." does not
seem appropriate to me in a beginners guide. If someone does not know what
a VNA is, they are unlikely to know about de-embedding.

7) I believe the fact the VNA assumes ideal standards for opens and shorts
is a *serious* problem - far more than deembedding. Neither my 8753ES or
8720 support deembedding, but both have good support for calibration kit

8) The comment "f you wish to do S21 measurements, you will probably need
to connect a cable to both ports", whilst accurate, I believe you should
say it's essential use both ports, and normally cables would be needed on
both ports. I would also reitteratee that S21 measured are "S21
(transmission) measurements.

9) More accurate calibrations will result if the open standard is not used.
The VNA assumes an ideal open, but the open standard just adds capacitance,
making the calibration less accurate. I posted a plot of the phase of both
an untermnated port and one with the open standard. The one with the open
standard was poorer.

10) The comment "Load: – Smith chart should be grouped around the centre of
the plot. LOGMAG Plot should be showing a low number (-50dB or below)" may
not be true at high frequency. I would check that, but I doubt it would be
worth checking at 900 MHz.

11) The comment "Time Domain Reflectometry. You can do this in
post-processing <> . Lots of high end
VNAs do not have this feature." is I feel untrue. I'm not aware of any
high-end VNA that does not have this feature, although it is sometimes a
software option that has to be paid for.

12) Somewhere (I forget where), the reference plane is mentioned. I would
at that this where the outer conductors of a pair of connector mate, and it
is often inside the connectors.

AS I say, these are meant to be constructive, so don't take them

Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
Tel 01621-680100 / +44 1621-680100

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