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Retuning with bigger cap?



I had those small caps in series a while back, I got them tuned to a bit
better performance. The chain is 13 caps long, for 7.7 nF; the primary is
about 10 turns of 0.75mm2 stranded on a 18 cm coil form, and the secondary
is the same, 10.7 cm form with 1260 turns 0.315mm enameled wire. And the top
terminal is just a 5.25" hard disk platter.

So, in tune, ground strike stayed about the same, 15-17 cm, but air discharge
got noticeably thicker, noisier and longer at 10-12 cm. I even measured
the waveform on an analog oscilloscope, I was standing about two meters away
from the coil and holding on to the probe tip. Voila - a cheap antenna :-)
But, it seemed to oscillate at about 310 kHz, double the secondary resonant
frequency. Strange.

Then I reasoned that the primary resonant frequency should exactly double, if
I exactly quadruple the capacitance, so I made three more strings of 13 caps,
for a total of 30.8 nF for all four strings. But now the air discharge, or
what's left of it, is about 1-2 cm and ground strike is 5 cm. No matter what
I do with the primary, 4 to 15 turns, it doesn't get any better than that.

The strangest thing is that when I measure the waveform with the bigger cap
and 10 turns in the primary (original setup), it seems to be a superposition
of 155 and 310 kHz signals a bit out of phase in relation to each other, 
I would have expected a cleanish 155 kHz decaying sine if it were in tune.

What on earth is happening here?