dave wrote:
Well, I wound 6 turns of #26 enamel magnet wire onto on of the surplus
toroids. It measures an whopping 714uH on my Almost All Digital Electronics
LC meter.

The toroid measures a bit less than 1 inch OD but definitely more than
0.94". I don't have a set of calipers here but it looks to be about 0.97"
based on the ruler I have. The ID is about 0.50 inches. The thickness is
about 0.34". These dimensions don't seem to be standard. By the way, here
is the listing for these toroids:

http://www.goldmine-elec-products.co...p?number=G6683

This inductance seems way too high - either I'm getting a false reading or
else this not a good material to make RF transformers / inductors with?
. . .

There isn't a single material that's good for both broadband
transformers and high-Q inductors. For broadband transformers, you want
a winding impedance that's at least several times (5 - 10 is a usual
rule of thumb) the level of impedance the winding is connected to.
Unless you're dealing with a lot of power, the sign of the impedance
doesn't matter, except that resistive impedance is desirable because it
stays relatively constant with frequency and doesn't contribute to
undesirable resonances. So high-permeability ("low frequency") ferrite
cores are best for this at HF, since they give the highest impedance per
turn squared. These cores are also good for EMI suppression because
they're free from resonance effects over the HF range and beyond, and
they maintain a relatively constant resistive impedance over many
decades of frequency. See the Fair-Rite web site for typical values for
various materials.

But the impedance of those high-permeability ferrite cores is typically
resistive above a few MHz, and the Q is no more than a few over the
entire HF range. So they're not suitable for use as high-Q inductors or
tuned transformers. For that purpose, you're almost always better off
using powdered iron cores. There are some high-frequency ferrites, but
they tend to have a high temperature coefficient and have to be used
with care if any DC bias is present, since they saturate much more
easily than powdered iron. Also, it's often difficult to achieve as good
Q with them as with powdered iron. So I generally use high-permeability
ferrite for transformers, and powdered iron for high-Q inductors. There
are, incidentally, some low frequency powdered irons which aren't
suitable for this application at HF and above. I generally use
MicroMetals type 6 material, and occasionally type 2.

What you really need to know is the type of ferrite or powdered iron
you're dealing with. You can infer this by measuring the low frequency
inductance with an inductance meter, then using that information along
with the core dimensions to calculate the material permeability. From
there, you can find a likely ferrite or powdered iron type and from a
manufacturer's web site get the impedance at the frequency of interest.
A much more direct way is to simply measure the complex impedance of a
one- or few-turn inductor at the frequency of interest with an antenna
analyzer or impedance bridge if you have either of those instruments.

Roy Lewallen, W7EL