Re: Scoping coils

Fellow Coilers,

I'd just like to add an amen to everything Malcolm has said regarding 
secondary coil parameter measuring technique.  Here are some of my 
procedures that have worked quite well.

The closest I have come to getting an accurate reading of secondary 
unloaded Q goes like this.  Ground the base of the coil and clear away 
EVERYTHING around it for a radius of at least 6 feet.  Make an 
excitation coil by scramble winding about 30 turns of hookup wire 
around a gallon paint can.  Slide the wire off the can and wire-tie or 
tape it tightly together.  You should now have what looks like an old 
color TV de-gaussing coil.  Connect this coil to your signal generator 
and place it near the base of your secondary.  I have found that a 
distance of 3 feet away from the base works well.

Now for the scope.  Usually, just a piece of wire for an antenna-type 
pickup works well, but since we are trying to be as uncoupled from the 
coil as possible with our drive source, we have less than the usual 
amount of signal and 60Hz and radio stations become a problem.  I 
constructed a little 3-pole passive high-pass filter from 3 R's and 3 
C's, followed by a 1-pole low-pass.  This completely eliminates the 
interference problem and you can watch clean waveforms on your scope 
with the pickup antenna several feet away from the coil.  Also, suspend 
the coax going from the antenna to the scope horizontally in a 
direction pointing radially away from the coil (really, no kidding).

Using this technique, I have measured Q's upwards of 200.  In fact the 
Q's are so high, that short-term signal generator drift becomes a 
problem during the measurement.  Even the swaying of nearby trees and 
any body movements (even 6 feet away) were clearly visible making it 
necessary to stand dead still and wait for the winds to die down before 
we could make a good measurement!  Of course a frequency counter is a 
must, and several measurements of f0 and the 3db-down points should be 
taken and statistical methods applied (throw out the lowest and highest 
readings, etc.).

BTW, the secondary used in this experiment was a 2 foot diameter 
polyethylene barrel wound with 22awg teflon stranded wire 3 feet high.  
It resonates at 80KHz with a small toroid.  This is the secondary for 
the FET driven system.

All this may sound like a lot of work, but it's really not and it will 
yield very accurate readings.  If you still get a Q of 32 from your 
secondary after all this, throw it away.