[TowerTalk] Tuners

[jimlux]

Kevin Normoyle wrote:

> His reasoning: The Q of a air coil goes down when you have a small number of turns
> (uses example of Q just = 20 to 30 on 10M with small number of turns).
> So if Q increases with N (turns), you should use the most N.
> 

I'm not sure that Q goes down with small N...

For long air core coils, inductance is roughly proportional to number of 
turns (i.e. the turn to turn coupling is small).

I would expect that R is also proportional to number of turns, so, if L 
and R are both proportional to turns, Q would be reasonably constant.

If the coil is more strongly internally coupled, then L is going to be 
more proportional to N^2.. but R is still going to be linear with N.  So 
Q would get bigger as N gets bigger.

But.. use Wheeler's equation for inductance..
(http://home.earthlink.net/~jimlux/hv/wheeler.htm)

L (uH) = r^2 * n^2 / (9 * r + 10 * l)

where

r = coil radius in inches
l = coil length in inches
n = number of turns

For common values of R and N and L...
Say 1" in diameter, and 5 turns/inch

L= 1 * N^2/(9+10*N/5) = N^2/(9+2*N)...
for N greater than say, 5, it starts to be proportional to N