[HomeBrew] Coil linearity?

[N2EY@aol.com]

In a message dated 12/29/02 6:51:51 PM Eastern Standard Time, 
catman351@yahoo.com writes:

> Just wanted to know this. Is coil inductance linear with respect to turns? 

No.

> For example, if I wound a coil evenly at 10 turns and it is valued at 1 uH, 
> can I assume that each turn is 0.1 H and that winding 5 more turns would 
make 
> the coil value 1.5 uH?

No. Inductance in a single-layer solenoid of constant dimension varies as the 
square of the turns.

L = (a * a * n * n)/([9 * a] + [10 * b])

where (all dimensions in inches)

L = uH
a = coil radius
b = coil length
n =  number of turns

> I'm trying to wind a 30 meter coil for that AC-1 
> replica that I just built. The 40 meter coil is about 15.5 tpi and the 
plate 
> of the 6V6 dips at about 10 % of the plate cap value and the antenna load 
cap 
> doesn't affect the power out that much into a dummy load (50 ohms). I 
> guesstimated the plate cap value to be about 36.5 uuF (10% of 365 uuf) and 
> reverse engineered the coil to determine a value of the 40 meter coil 
(wound 
> with #22 magnet wire at 15.5 tpi). Of course, I used the formula 
freq=1/(2*pi*
> SQRT(L*C)). Calculating the final turns per inch for a coil with the plate 
> cap at 25% (or 91.25 uuF) yielded a coil at 7 tpi #22 magnet wire 
resonating 
> at 10.115 MHz. 
>  Let me know your thoughts on this and thanks. Cal, N6KYR. 

Useful resonance formula:

LC = 25,330/(f * f) 

The AC-1 uses a pi-net, whose design is covered in Handbooks and on a few 
websites.

 73 de Jim, N2EY