[TowerTalk] Coil loss

[Michael Tope]

Jim/Tom,

If the portion of the coil inside the tube isn't used, couldn't you just
add extra finger stock contacts at regular intervals along the inside
length of the tube to short out the unused turns?

Mike, W4EF...........................


>
> You're talking about the circuit formed by the unused turns (inside the
> lower tubing) and the C to the surrounding tube? I would think that the C
> is going to be pretty big (the tube is close to the coil).  Say, if you've
> got 30cm inside the pipe, with the coil being 5cm in diameter and the tube
> being 6cm, I get 92 pF. Assuming 100 turns for the coil inside the pipe
> (about 8 TPI), I get 77 uH using Wheeler's formula.  These could be off by
> quite a bit because there is an interaction between the coil and the
tubing
> wall. But anyway, the resonance would be around 1.9 MHz, which is close
> enough to be of concern (that is, if the L or C were off by a factor of 4,
> you'd be resonant at 3.8)
>