# Re: Primary calculations

```Mark says

"Ed,

I'm really surprised you find Wheeler's formula in error.  I have found it is

quite accurate even when pushed to weird dimensions and aspects which you are
not
doing.  Are you sure you are using the radius of your coil and not the
diameter
in the formula:

L(uH) = (r^2) * (N^2) / (9*r + 10*h)

where:
r = coil RADIUS in inches
N = number of turns
h = coil height in inches

If I'm being obvious and insulting, please ignore this message, but
occasionally
even the best of us misses something.

Zap,
Mark"

Mark,

What I'm having a problem with is my existing primary is 3/8" copper tubing
wound on a saucer shaped form at 32 degrees.  There is 14 turns using 3/8"
spacing between turns.  This primary is about 85 uh.  Calculating a new
primary for a different project, using a 6" pvc pipe for a form and 14 turns,
3/8" spacing, I only get 44uh.  I guess this is correct and due to the fact
that this new primary would only use about 22 feet of tubing and the existing
one (that I'm trying to compare it to) uses about 80 feet of tubing.

Ed Sonderman

```