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Getting started with the nanoVNA guide

HexAndFlex
 

https://hexandflex.com/2019/08/31/getting-started-with-the-nanovna-part-1/

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.

SPACE OPQA
 

Thank you very much!

Highly appreciated your help and work!

73 de Luis CT2FZI

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

https://hexandflex.com/2019/08/31/getting-started-with-the-nanovna-part-1/

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.



VE3ELQ
 

Very helpful, thanks. LF to part 2. Nigel VE3ELQ

BruceN
 

Very helpful. Just what I was looking for in my long post. When you get done with Part 2,
can you post PDF versions in the FILES folder too? With the docs already there, a good
foundation for a library can be formed.

Bruce, K4TQL

HexAndFlex
 

I should be able to do that.

hwalker
 

I'm sure HexAndFlex will be getting a lot of "hits" on its getting started guides. Considering the fact that the nanoVNA has not been in mass production that long, the amount of chatter and development that it has already generated is a testament to its usefulness to the target DIY/hobbyist crowd. A year or so ago, I bought a KC901S 10kHz-3GHz network analyzer for over $1000 to support my ham radio hobby. Even though it has been in production a few years, it still has nowhere near the documentation and software support that the nanoVNA has. I now find myself using the nanoVNA more than the KC901S as I am much less afraid of destroying a $50 item during experimentation than a $1000 item. After showing the nanoVNA off at my ham club, almost every member has purchased one and found it to be more educational and cheaper than purchasing a hardcover vector network analyzer book. With this being an open source project, I'll be following closely the software, and hopefully hardware, developments that piggyback on it over the following months. I know Hugen has already said he will be selling his nanoVNA with a protective case fairly soon. I'm sure the other clone manufacturers will be following suite.

Jerry Gaffke
 

Yes, a good start.
I definitely learned some stuff about the user interface on this thing.

I especially liked these few words about the Smith chart:
This is just a polar plot with our received signal plotted as an amplitude and phase.
The outside circle represents a situation where we have received a signal of equal amplitude
to the transmitted signal. The centre of the plot shows where the received signal is near zero.
Short and concise. A very good beginning.
Add a few more paragraphs about pure resistance, inductive and capacitive reactance,
what happens over frequency, how to use it assuming a 50 ohm environment.
It's a subject that is seldom presented in a way understandable to somebody
who does not know what a Smith chart is.

I'd assume 90% of the market for a $50 VNA would be looking for
a quick start on how to plot the SWR of an antenna.
Should be possible to do that in the first few paragraphs, maybe
using a dummy antenna of a cap and resistor to demonstrate
in a way that the reader can duplicate with their own measurements.
Then start talking about how much more information you get when
it is plotted as a complex impedance (and what a complex impedance is).
How to add a series cap or coil to make the antenna a pure resistance.
How to measure a cap or coil across frequency, seeing what
happens at self resonance and beyond.
How to design a matching network to transform any load to 50 ohms.
How to perform a 2 port measurement to see the passband of a filter.

Only then start talking about Smith charts and S parameters
and reflection coefficients and exactly what the heck a reflection is.
I doubt many readers who need an explanation of S paramenters
will ever get around to using them in a formal manner.
But that material should be included, it's impossible to make sense of
any other documentation on VNA's without knowing about S parameters.
Or to make sense of this groups.io forum, for that matter.

Regarding this passage:
Connect your cables/Define your reference plane.
Unfortunately, it never says what a reference plane is.
Perhaps start by saying that you should calibrate using the same cables that you plan
to perform the test with. And then a few pictures of S11 and S21 test setups
with arrows pointing to this reference plane.

Maybe I'll write my own nanoVNA introduction someday,
but first I'll have to figure out how to use it.
I've got much to learn, and your document helps considerably.

Jerry, KE7ER


On Mon, Sep 2, 2019 at 05:52 AM, BruceN wrote:
Very helpful. Just what I was looking for in my long post. When you get done
with Part 2,
can you post PDF versions in the FILES folder too? With the docs already
there, a good
foundation for a library can be formed.

Dr. David Kirkby from Kirkby Microwave Ltd
 

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

https://hexandflex.com/2019/08/31/getting-started-with-the-nanovna-part-1/

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

https://www.kirkbymicrowave.co.uk/Support/Guide-to-vector-network-analyzers/

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.

https://www.keysight.com/en/pdx-2812784-pn-N5227B/pna-microwave-network-analyzer-67-ghz

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
$10,000.

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
defintions.

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 <https://zs1sci.com/blog/nanovna-tdr/> . 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
personally.






--
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
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100

Jerry Gaffke
 

I don't feel I know something until I touch bottom with physical reality,
assuming such a thing exists.

In the case of the S parameters, that means describing them as a vector ratios
of two vector voltages, using complex number arithmetic:
https://en.wikipedia.org/wiki/Complex_number
https://en.wikipedia.org/wiki/Electrical_impedance
https://en.wikipedia.org/wiki/Reflection_coefficient
https://en.wikipedia.org/wiki/Scattering_parameters

Bottom paragraph of post https://groups.io/g/nanovna-users/message/1318
shows that I got hung up on that for awhile. Many sources did not
explicitly state S11 (for example) was a voltage ratio. I assumed it dealt with power,
like SWR. (Historically, some "S parameter" models did deal with power ratios.)

Baier's paper on correction factors has some great stuff, but does not touch bottom:
http://sdr-kits.net/DG8SAQ/VNWA/12_term_dg8saq.pdf
He never bothers to mention that all his variables are vector quantities.
Never describes what physical properties they measure.
Figure 1 shows a two terminal load ZL connected between the
lines representing the incident and reflected voltages (wrong!).
The TX-Oscillator port of Figure 1 is ignored in the math.

Maybe I should let HexAndFlex write that guide to the nanoVNA,
and I'll write a guide to help non-RF people like me navigate Baier's paper.

Jerry, KE7ER

Larry Rothman
 

Looks great!
In the calibration section, you should get the user to calibrate the touchscreen as well. That will go a long way in making the use of the touchscreen a pleasant experience.
One of my earlier Aug posts has the instructions listed.

Regards
Larry

Jeff Anderson
 

On Mon, Sep 2, 2019 at 09:18 AM, Dr. David Kirkby from Kirkby Microwave Ltd wrote:


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.
Does the software really assume an "ideal" open? In a different thread (https://groups.io/g/nanovna-users/message/1187), Roger Henderson mentioned that the software had programmed into it a C0 value of 50 femtofarads for the open
standard. Would this be enough to compensate for the phase difference you measured?

- Jeff, k6jca

Jerry Gaffke
 

I guess it's traditional. Section 3 figure 2 of the HP document from 1967
that W5DXP pointed to in post 1668 also shows a two terminal load placed
across the incident and reflected signals.
Unlike Baier, they also did this with the source on the left side.

From my rather limited understanding of what's going on, this is simply wrong.
The load should have both a2 and b2 signals from the s-box going into a single port.

We can't really assume a solid ground plane, since we are often dealing
with transmission lines here. So that single port into the load
represents the electrical return path as well.

Am I missing something?

Jerry, KE7ER

On Mon, Sep 2, 2019 at 10:36 AM, Jerry Gaffke wrote:
Figure 1 shows a two terminal load ZL connected between the
lines representing the incident and reflected voltages (wrong!).

Buck Calabro, KC2HIZ
 

On Mon, 2 Sep 2019 at 13:36, Jerry Gaffke via Groups.Io
<jgaffke=yahoo.com@groups.io> wrote:

Maybe I should let HexAndFlex write that guide to the nanoVNA,
and I'll write a guide to help non-RF people like me navigate Baier's paper.
This is exactly the point of open source.
The community members add to the store of knowledge as they are able.
Put differently, we individuals don't wait for someone else to do the
work; we do it ourselves.

The wiki is a good place for a central repository of knowledge, and
will serve dual duty as a FAQ.

--buck

HexAndFlex
 

Thanks everyone. Particularly Dr Kirby. Lots of great advice there. I need to work out the best way to collate that together and what changes need to be in part 1 Vs part 2 or even 3.

I have found that trying to write a short beginers guide is actually a pretty difficult balancing act. You don't want to go too deep, but if you keep it too basic, you can actually mislead people.

Regarding the 'high-end’ VNA comments. I think I picked my words poorly. I wasn't really trying to refer to really high end units like the PNA. I was thinking more of the regular professional units that such as Anristsu shockline or R&S ZVL. High end with respect to the nanoVNA. Good enough that it comes with traceable calibration. I will change my wording slightly.

Thanks again

Joe St. Clair
 

Hello. I like what you've started here. It's helped me already.

A suggestion about technical writing is to decide who your audience is and keep the material at that level. In this case I would suggest technician level radio hams would be a good audience to address. Keep up the good work.

HexAndFlex
 

I have jsut updated my post to include some of the useful feedback received here.

http://hexandflex.com/2019/08/31/getting-started-with-the-nanovna-part-1

Many thanks