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Re: Scoping coils

I've been using the base fed scheme here, with scope probe
well isolated from the coil, and the coil as far as possible
from anything (not very far in my basement). I am confident
I'm making valid measurements of the order of 200 to 300, for
3"OD coils wound on the much-despised cardboard mailing tubes,
dried in the oven for several hours and well shellacked.  
(Shellaced?)  After burning one up due to careless, I've
had pretty good luck.  
These were forms I made up several years ago and have had sitting
in the rafters.  Point is, good plastic is certainly much better,
but using what you have on hand is a good place to start, and maybe
much cheaper.  For a few cents worth of shellac or varnish, and
a couple of dollar's worth of wire, together with an hour at the
bench, you can make a tolerable small coil which is fun to play
with.  My problem, regard to measuring Q, is the signal generator
drift.  A better method, which I haven't tried lately, is to measure
the series resistance at resonance by a substitution method.  Feed
from the signal generator through a hundred ohms or so, and measure
the voltage across the coil (to ground, of course) at resonance.  
Then substitute a resistor box (non-inductive) and adjust it to get
the same voltage.  This will give the equivalent series resistance
at resonance, from which the Q can be calculated.  This is actually
easier than the delta f over f method, and you can use a bag of 
carbon resistors instead of the variable R.  By using series and
parallel combinations, then measuring the final value with a n
ohmmeter (preferably digital) you can get results that are good
enough for "government work".
Ed Phillips