[Elecraft] RE: OT: Effect of Compression and Expansionon the Inductance of Toroids?

[Cortland Richmond]

Ron AC7AC   wrote:
> ... I'm confident that the added capacitance by squeezing the turns
together
> is not what is causing my L-meter to show increased inductance. If
anything,
> the capacitance would tend to cause the L-meter to show lower inductance. 

Inductance is a result of the magnetic field intercepted by each turn.  An
ideal inductor has each turn intercepting the same (entire) field.  In a
solenoid much longer than it is wide, however, this does not happen, and
the inductance realized is lower than the ideal case, sometimes much lower.
While a ferrite core concentrates the field, and a toroid insures almost
all of it remains within the core, the winding is after all halfway
surrounded by air and some of the field is still not shared by all turns.
Compressing it reduces this effect.   A Bug Catcher coil comes closer to L
changing as the square of turns than a Hamstick(r)!

That's my take, anyway. 

One way to test this would be to cover the winding with ferrite. A
babushkoroid! Or try a pot core. If my surmise from High School physics is
correct, the more the field is in ferrite, rather than air, the less effect
compressing the turns will have.  Indirectly, we might also try surrounding
a toroid with a solenoid to see if coupling to that winding decreases as
toroid turns are compressed, but leakage coupling will depend on the angle
each part of the toroid winding has to the solenoid -- and the sum of all
the fields on a completely occupied cores at a surrounding solenoid should
be zero regardless of leakage, I think. Perhaps instead of a solenoid, an
external toroid wider than the inductor under test might be used.   But I
like the babushkaroid. 

Remember, you saw it here first!


Cortland
KA5S