Re: magnetic insulation and aspect ratio
> Original Poster: Eddie Burwell <eburwell-at-HIWAAY-dot-net>
> I've been mulling over the issues of designing a secondary coil and have
> looked at a number of factors that come into play. First I noticed that the
> maximum L for a given length of wire occurs at a H/D aspect ratio of .46.
> Then I noticed that the minimum self C occurs at an aspect ratio around 3.
Good one. I went through the same exercise a few years ago and
found the same things. A couple of years later, I built a number of
coils of the same diameter and winding pitch, spacewound and
closewound. I found that maximum Q was hit in the spacewound one
for an h/d of 1 (well it should shouldn't it? Everyone from Terman
down says so ;) Guess what h/d Tesla used in his extra coil. The
closewound coil Q peaked at a somewhat higher h/d which is good
news for smaller coils.
> Now I'm wondering about the effects of magnetic insulation i.e. the magnetic
> field of the resonator will deter charged particles from traveling
> perpendicular to the axis of the coil by deflecting them tangentially around
> the coil. This effect would be most significant inside of the secondary and
> at the exterior surface of the coil. Now the tricky part is that peak
> voltage on the secondary occurs at the same time as zero current and peak
> current occurs at the point of zero voltage. This would mean that the field
> would not effect streamer formation (if it occurred at peak voltage) but
> could influence the lingering arc channel.
> If the effects of magnetic insulation play a significant role then a
> longer aspect ratio may be preferred so that the inductance of the secondary
> is reduced and the resonant current in the secondary is increased. Perhaps
> conductive shunt plates at the ends of the coil could also be used to
> squeeze the magnetic field around the secondary to some benefit (as well as
> lowering the L).
> Any thoughts?
> Eddie Burwell
A most interesting set of thoughts. Worth following up.