I measured Q of couple of coils, and noticed that Q curve or trace is very uneven. It has sharp peaks and valleys.
Today my V2 Plus2 or 4 has a bad day, I cannot get a good screen capture, and NanoVNA app shows only a line.

That is kind of funny, I had a PC software which could take working screen capture, but I can't find it today.
Now I see 4 NanoVna programs or desktop icons for Win 7 and there was a couple more for Win10, and none takes a good screen capture (Today)

You are measuring very small resistances and "noise" is inevitable. If you parallel your inductor with a good quality capacitor - I use a 2% silver mica but polystyrene will do. Then look at resonance you can zoom in and get the. -3dB bandwidth and divide that into the resonant frequency to get the Q(at that frequency ) Much more accurate than measuring A few score or hundreds of milliohms. I put the parallel LC between the first port and ground.

Cheers, Ian McCrum MI5AFL

__

Q of large reactances or even moderately high Q's are very tough to
measure on any instrument, even my new $30K ENA or old HP impedance
measurement test sets have flyers and erratic spots in data.

This is because the impedance is far away from the bridge null being
used as a reference. It is at the edge of detector range in bridge type
instruments.

When I want to measure high Q inductors or capacitors I use a variety of
copper box test fixtures. Some even have vacuum capacitors with Q>10,000
and some are cavities. Q is really tough to measure unless Q and
reactance are both low. I pretty much have to do it in a special fixture
and often with a resonant circuit.

Sometimes I even measure heat or voltage with high RF current. It
doesn't take a lot of power, but it can take high current or voltage.

73 Tom

On 1/26/2025 7:22 PM, Leif M via groups.io wrote:
> I measured Q of couple of coils, and noticed that Q curve or trace is
> very uneven. It has sharp peaks and valleys.
> Today my V2 Plus2 or 4 has a bad day, I cannot get a good screen
> capture, and NanoVNA app shows only a line.
> That is kind of funny, I had a PC software which could take working
> screen capture, but I can't find it today.
> Now I see 4 NanoVna programs or desktop icons for Win 7 and there was
> a couple more for Win10, and none takes a good screen capture (Today)
>

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Yes, noise is a prime suspect.
I think I have to plot both resistance and reactance to see if resistance value has noise.

The resistance plot was noisy. I'll check my test setting and calibration, because resistance was quite high. But  that is an other story.

I think there were specialized Q-meters, I saw one tube type made by TESLA. Its output impedance was really small, I estimated few ohms made by a wide stripline with a teflon isolation covered with a sheet. Low output impedance was closer to the measured impedance and there was a high Q variable capacitor to make a resonant circuit together. I think that it measured the peak voltage in the middle of series LC circuit.
As far as you are sure interested how your L  works in filter on certain frequency, I will build a simple LC circuit losely coupled to Nanovna and estimate the Q from the resonance curve as it somebody proposed here.