NEC-LIST: Loading Coils

I can see no reason to pursue air-core toroidal inductors. The potential gain
over a solenoidal inductor is minimal, decreasing as the length of the
solenoid increaes.

When used with core materials of high permittivity the situation is very
different. If a hi-mu core is used in an open magnetic path, the uncompensated
poles at the end produce a serious demagnetization field, which in turn leads to
a proportionate decrease in inductance and, therefor, Q. The main purpose of
the toroid is to close the magnetic path with the minimum possible reluctance.
Also, the high mu does a good job of concentrating the flux to the inside of
the coil, where it is needed.

Although I have never bothered to wind one, I am confident that an air-core
toroid would show only a slightly higher Q than an equivalent solenoid (one
with the same length as the toroidal circumferance). That is most likely the
reason you never see it done. In my 40+ years of experience in electronics, I have
NEVER seen a single air-core toroidal RF coil.

Eric von Valtier K8LV

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