Distributed capacitance effects inductor Q?

In a low-Q capacitor, energy is lost in the dielectric e.g. plastic insulation. I imagine your external tuning capacitor is a high-quality air-spaced type.

circulating currents between the turns of the inductor. Sounds reasonable, although the math to calculate the losses must be fairly complex.

Apparently, the air capacitance between each turn of the inductor sets up a LC circuit that causes current to flow that wouldn't normally happen if there was no interwinding capacitance. The total of all these little currents adds up to increased I^2R losses in the larger LC circuit and reduces efficiency. Sort of the same idea as using laminations to reduce circulating currents in the core of a power transformer, except in this case the idea is to use a larger spacing between turns to reduce capacitance and parisitic circulating currents in the windings. This is probably why the Q of a self resonant inductor is lower than the Q of the same inductor at a lower frequency. Too many extraneous currents generating heat in the windings.

-Bill

I've never noticed it. As the Q meter was on the bench I tested an AM ferrite radio coil. Value measured beforehand on two LC meters as 480uH, (one at 1000Hz to preclude self capacitance issues). (wire was Cotton covered, Litz style, 3 stranded).

600pF 300kHz Q=110 200pF 559kHz Q=140 100pF 800kHz Q=160 70pF 900kHz Q=200 30pF 1350kHz Q=220 10pF 2200kHz Q=200 I didn't extract out the self capacitance but would have thought the measured Q values would degrade pro rata with reducing external capacitances.

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