Re: Coil forms, Q, and instruments

Jim Sheldon

Ha Ha! Yup, for amateur use, spending 1000+ on a spectrum analyzer vs $65 on a Nano VNA or even maybe a bit more on a "Tiny" spectrum analyzer is pretty huge "overkill" LOL.

I have both the Nano VNA and the tiny Spectrum Analyzer and have gotten super good results from both of them. I sold my expensive spectrum analyzer. The only thing the two little ones don't have is a "tracking generator" and I never found the TG to be super necessary for most of my normal SA measurements.

Jim, W0EB

------ Original Message ------
From: "Jim, N5IB" <>
Sent: 7/22/2021 2:25:26 PM
Subject: [4SQRP] Coil forms, Q, and instruments

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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