Coil forms, Q, and instruments

Jim, N5IB

With too much free time for my own good, I did an experiment with 3D printing some spiral grooved coil forms.
I was curious if the PLA plastic material used to print would degrade the Q of the coil.
16 turns AWG24, about 1" diameter, about 1" long on a hollow, grooved PLA tube

Short answer, no noticeable Q degradation when I inserted a rod of the PLA material into the center of the coil.
The effective inductance of a coil wound on the form is a few percent lower than the "air wound" formulas predict.
prediction: 5.6 uH, measured: 4.8 uH
But that's to be expected because of the increased inter-winding capacitance due to the dielectric constant of the form's PLA material (about 3). We see the same effect with toroids when spreading/squeezing turns - changing the spacing changes the inter-winding capacitance.

Made a series trap with the test coil and a 200 pF dipped mica cap, and measured the 3 dB frequencies.
The interesting finding is that when I did the Q measurement with a $1000+ spectrum analyzer, I got Q=185.
The on a whim, repeated the measurement with a $65 Nano VNA , and got Q=180
Darn ! 2.7% low !

I think there is a lesson hiding here LOL

Jim, N5IB

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