Re: Scoping coils
Subject: Re: Scoping coils
From: ed-at-alumni.caltech.edu (Edward V. Phillips)
Date: Thu, 14 Dec 1995 20:48:21 -0800
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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".