Using Mathcad, the coil mentioned in this thread has the following
properties:

1) From 0 to 20 turns the inductance is 0.038 times n squared microhrenies
(square law).

2) From 20 to 100 turns the inductance is (1.49 n - 16) microhenries (very
linear).

For the first 20 turns the coil has the square law property because all of
the turns are tightly coupled to each other. This is not the case for the
other 80 turns of a long solenoid.

A PDF of the Mathcad calculation and graph are available by email from W0IYH
at QRZ.COM).

Bill W0IYH

TURNS. INDUCTANCE.
100 132uH
90 120uH
80 103uH
70 89uH
60 73uH
50 58uH
40 43uH
30 29uH
20 15.8uH
10 4.7uH
0 0.12uH