Comparing inductance predictions: femm vs standard formula

["Robert Macy" <macy@xxxxxxxxxxxxxx>]

Not having good results when trying to predict inductance of a multi layer
solenoid coil.

dimensions are
inside radius 0.25 in
length 1 in
thickness of coil 0.44 in

Did femm analysis using asymetric style in a 10 inch diameter sphere with
over 14k nodes. Then assuming that inductance is 2 times the magnetic field
energy divided by the total current squared resulted in an inductance
prediction of 7.638 nH per turn squared.

However using the formula to predict the inductance of a multilayer solenoid
from ITT Reference Handbook for Radio Engineers 6th ed which is

L = 0.8 *(r*N)*r*N)/(6*r+9*length+10*thick)

produces a prediction of 3.356 nH per turn squared


Why the huge disparity?

- Robert -