Determining inductance and turns for unknown cores is a problem without
instruments. The aircoil will do it right but as you said, it is too
large.

You can buy off the shelf 300uH inductors. They are ready for use.

To determine a toroid or other core's parameters you need a RLC bridge or
at least an inductance meter. Even so you will not know for sure if it's a
filter (dispersive) core or a good high Q core, and what it will do when
it will have dc bias through it.

A simple RL bridge can be improvised like so:

12Vac -(A)---Lx--(B)--R--(C)- 12Vac'

R = 10 Ohms 20W (but you will use it for a short time).

For L~= 300uH you would measure about 0.135Vac between A and B. The 12V
comes from an ordinary transformer (110->12V ac, 1.5A). The coil must
withstand 1.5A current. For high Al toroids this could not be so. Higher
voltage = higher inductance, lower voltage the other way around.

hope this helps,

Peter

(The formula used was Zl = 2*pi*f*L and Zcircuit = sqrt(R^2 + Zl^2) where
R was chosen ~= 100 times Zl to reduce errors)

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