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* Nano VNA overview by K3EUI Barry *

Each time I give a "talk" on the Nano VNA I learn something new. I've given five talks now, and from the questions from listeners, I know where the hurdles are.

1) Calibration of the VNA - where you do the calibrations matters. Why?

2) Impedance varies with coax feed line length... but why?
And (mostly) SWR does NOT change with small feed line changes - why?
This is a key concept, not easily seen by most of us.
The "reflection coefficient" is established right at the antenna-feed line junction.

3) SWR vs. RETURN LOSS
They are both saying the same thing, with different units (SWR has no units, RL is in dB)

4) The magic of the Smith Chart: can we get over the anxiety? Think of it like a DART BOARD
An antenna can have some RESISTANCE and some REACTANCE.
The Smith Chart just shows you R and X, for each frequency you plot.
Of course, the Smith Chart does not show you POWER output, but does show SWR indirectly by how far
your data are from the Bull's Eye (if standardized at 50 ohm).
I included a few slides to illustrate this concept - maybe lost my audience here.

5) Resonance - when measured at the END of your coax, if not 1/2 wavelength, or multiple of 1/2 wavelength
When the reactance is zero, this is resonance.
But the resistance may not be close to your coax's 50 ohm. That sounds simple enough.
But resonance is NOT (necessarily) where the SWR will be lowest as measured in your shack.
A one-wave dipole, center fed, won't match to 50 ohm coax, but it might be "resonant".

6) What happens when you add an "Antenna Tuner" in your shack? The rig sees a SWR of 1:1
but meanwhile, what is happening on your feed line to the antenna?
Is the "forward power" really greater than the power emerging from the rig? Yikes... magic?

Comments on my latest (but not last) PPT will be appreciated by me.
I now have some experience with the S21 (two port) measurements of filters.
My next project: investigate the isolation of a 2m/70cm diplexer (Comet) with nano VNA

TU for all of the feeback off list
Feel free to share this pdf,

de k3eui Barry

Hi Barry,

It's getting more beautiful every new version, two thumbs up.


About the Smith Chart and losing the audience.

If I explain the Smith Chart to my children (read: listeners) I start telling them that the Smith chart shows the reflection of the sent signal. So how big is the reflection (0-100% compared to the sent signal) and under what phase (+/-?? 0-180dgs) did we measure it. Then what happens if the same load was a bit further away; what would happen to the phaseangle. This is a first eye-opener.

Next: The reflection percentage easily transforms to SWR as a next step. What SWR is 100% reflection, what is SWR at 0% reflection, and at 50% reflection? Where is that on the chart?
And then the formula about relation of SWR and reflection coefficient can be demonstrated with simple ohmic loads leading to SWR=1, 2, and 3

Next comes the part: What Impedance caused that reflection that was measured in the first place.
First introduce simple complex numbers R+jX, what they mean and show some examples. Only after understanding this, I introduce the different Smith chart impedance circles slowly.
Then the forst demo: measure a potmeter on 10MHz and change its value, same for variable inductor and variable capacitor.

This way of introcing the Smith chart is something that a normal ham can use to build on his knowledge he or she already has. Most hams are interested in the nano for SWR measurements and antenna tuning in the first place and getting an impedance of 50 ohms with a tuner.


I've given several presentations about the Smith Chart only. That takes me about 1,5 hours from the start to finish. Last part of the presentations is about? how tuning networks ( like L, pi, of T) behave in the Smith chart and why. Of course I use the nanoVNA as measuring equipment for the demo's.
I've done this live (before COVID) and also online. Everybody had a good understanding of the Chart and could make use of it afterwards.


So.... starting with a Smith Chart with impedance circles is a too big leap. Also folding the rectangular system onto a circle. Might be so, but that kind of math does not help a ham.


My two cents worth. Hope I gave you new ideas.

73 to all and stay safe.

Arie PA3A


Op 14-2-2021 om 17:07 schreef Barry Feierman:

Hi Barry,

I agree with Arie above.

I wonder if you have seen the slides from my 4 hour presentation on the NanoVNA (some stuff is probably outdated now).


(they are also available in the Files section here)

If you can use anything from those slides, feel free to "steal" anything you want.

73,
Luc ON7DQ / KF0CR

Excellent presentation thanks for sharing! :-)

73!
Greg / W4GAP

Hello,

Has it been "YouTubed"?

Thanks, larry

Hello,

Has your presentation been "YouTubed"?

Thanks, larry