Mike Coslo wrote:
On 10/18/10 6:23 PM, Cecil Moore wrote:
On Oct 18, 11:20 am, Jim wrote:
You can see that for this kind of coil, the coupling from turn to turn
must be pretty low.

For an average air-core coil, the delay through the coil seems to be
in the ballpark of half of the coil wire stretched into a straight
line, i.e. the VF of the coil is about double what is the VF of the
straight wire used to wind the coil. The turn to turn coupling exists
but turn to far away turn coupling is very low.

This seems to be the most accurate inductance calculator that I have
seen and includes the characteristic impedance and axial propagation
factor.


So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.


Given the relatively few turns on a typical bugcatcher, and the low
coupling of flux, the voltage rise would be negligible, and the fact
that you're not having a high voltage across the coil in the first place
(compared to turn spacing), I don't think it's an issue to leave the end
free.

Practical experience: On most tesla coils, the primary is a 10-20 turn
coil with turn to turn spacing of 1/4" to 1/2" or so and runs at a peak
voltage around 20kV. One adjusts the tapping point to adjust the
primary L to bring the system into resonance, and it usually winds up
being tapped about 70% of the way into the coil. Almost never do you
get arcing/corona from the free end of the coil, which is potentially at
as much as twice the voltage, and if it were in free space, you'd start
to see corona from the relatively small radius of curvature.