On Mon, Oct 11, 2021 at 09:21 AM, Andrew Kurtz wrote:
R is quite low and may vary between -2 and 5 ohms.
That "-2" suggests to me that you're not actually reading 'Resistance' there. Resistance should be positive, but Reactance can be in negative ohms.
Since high Q _requires_ low R, it's going to be difficult to get good, low resistance connections to your coil - do you solder the cable from the NanoVNA directly to the coil leads/terminals? Any mechanical variability in the metal-to-metal contact there will certainly affect the Q value.
says
"The Q, or quality, factor of a resonant circuit is a measure of the ¡°goodness¡± or quality of a resonant circuit. A higher value for this figure of merit corresponds to a more narrow bandwidth, which is desirable in many applications. More formally, Q is the ratio of power stored to power dissipated in the circuit reactance and resistance, respectively:
Q = Pstored/Pdissipated = I2X/I2R Q = X/R where: X = Capacitive or Inductive reactance at resonance R = Series resistance.
This formula is applicable to series resonant circuits, and also parallel resonant circuits if the resistance is in series with the inductor. This is the case in practical applications, as we are mostly concerned with the resistance of the inductor limiting the Q."
Therefore, you could try measuring a resonant circuit, with, as has been mentioned, a high quality capacitor like an air-insulated variable or a mica capacitor to make the resonant circuit. It might be worthwhile to do some research on the Q or quality of various types of capacitors, to ensure that you get a good one.
This is an interesting question/'problem and I've often wondered myself about determining the Q of a component or circuit. I've had lots of training and experience as an electronic technician (and ham radio operator), but none in the advanced types of math needed for engineering.
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Doug, K8RFT
