I just finished measuring the resonance frequency of my secondary by observing
the null due to the series resonance of the 1/4 wavelength
coil. The toroid on
top of the coil was constructed out of 4", "semi-rigid flexible
aluminum duct",
bought in Home Depot. The ID is 3.5", OD 11.5" and the center of the toroid is
hollow so that it slides over the PVC coil form. The coil is 9" x 3.5" wound
with # 24 wire and resonates at 500kHz.
I noticed that when the toroid was about 3" above the top of the secondary
winding the resonance notch was very deep, but when the toroid was moved close
to the top of the winding, the notch was not very steep. Clearly the toroid
was affecting the Q of the coil, probably by acting as a shorted turn. I have
not come across any information about how far the toroid should be placed from
the top of the coil. Anybody have any information about this? Should I place
a slit through the toroid?
Also the surface of the semi-rigid flexible duct looks like an accordion with
1/4" deep ridges spaced about 1/4" apart. Will this surface effect the
streamers? This is a low power Tesla coil that I hope will produce at least
10" streamers.