Jamie wrote:

I've tried to use one of those things from the back of a monitor (presumably
to prevent RF interference) as one of the coils and another adjustable coil
from another device, to no avail. It has no markings and it appears to be
a ferrite device in the shape of a ring.

Anyone know an easy way to figure out how many turns to get (presumably 300mh?)
from one of these? Can I measure it with a ruler and some how get a formula
for the number of turns / mH ? Are these really bad deviced to use? (the thought
occurs I must admit, that if it was meant to BLOCK rf, using would be illogical)

Yes, I would expect they would be really bad ! If you knew the
permeability of the material used then measuring with a ruler and
applying some math (I don't know the formulas) would in theory get you
an iductance calculation. But the materials used for these EMI chokes
generally has too high a permeability to make inductors with convenient
numbers of turns for the HF bands, and generally is quite lossy so the
resulting inductors will have low Q. It is extremely unlikely to work.

Since you don't want to buy any parts, I would recommend once you
figure out the required inductance from the Amidon formulas, just wind
air core coils (well, some sort of low loss former like a piece of
plastic tubing is probably needed) with the same inductance.
Inductance formulas for air core coils are available everywhere in
handbooks and on the web. You might have to play with the number of
turns in the feedback link though, as the coupling between it and the
main coil will not be as tight in an air core coil as in the toroid. I
would start with the same ratio of turns or maybe add a bit to the
feedback coil.

73,
Steve VE3SMA