Losses in the former
Hi Adam, Eddie, all,
You had a question about secondary former losses. I posted this a while ago,
so here is a copy:
Fact: The E-field goes from every turn of the winding to the base (or ground)
of the secondary.
Given the second fact that the material is about 5mm thick and the E-Field
lines are somewhere around 500mm "long", only about 1% of the E-field is
contained within the plastic itself. Virtually all TC components are going to
have loss factors of way more than 1% esp. the spark gap.
What would be of more concern to me is the surface resistance. Esp. as the
material gets older. (i.e. the surface resistance will go down).
It is quite clear that building a cap with PVC (as dielectricum) would be less
than desirable because in this case the 95% of the E-field will be contained
in the plastic sheet and only about 5% will be present in the oil and / or
So the only thing you should worry about are the aging effects (effects on
surface conductivity) of the material.
Cardboard tube has two real disadvantages:
a.) It absorbs water, so you will have to prevent it from doing this by
coating it somehow.
b.) A cardboard former will burn (literally) up pretty fast, if you overcouple
your coil. If this happens you will not be able to repair it. A plastic former
is a little stronger (and more forgiving) than cardboard tubes. This means be
careful when increasing the coupling during tuning.
A small 2.5" coil I built a while ago overcoupled (with only 200W input). The
cardboard charred within about 5 secs run time. I was able to cheat and
continue using the coil (it was a test coil anyway) by simple removing the
windings from the top to a little underneath the charred area, but you
wouldn´t want to do domething like this on a show coil.
Coiler greets from germany,