More on toroids

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From: Mike Czuhajewski (Mike.Czuhajewski%hambbs@wb3ffv.ampr.org)
Date: Sun Jun 26 1994 - 19:08:29 EDT


In reply to one of my recent 'grams, Jack, W5TFB, asked what happened
to the Q of the coil on the T68-2 when I compressed the turns from a
30 gap all the way to a 270 degree gap. I hadn't bothered to record
it, so I dug it out and checked it again. The answer is, "not much."
Here are the results; since the original spacings are impossible to
duplicate exactly, the values are somewhat different.
                                   
Core: T68-2, 16 turns #18 wire, measured at 7.9 uH.
    
30 degree gap between ends of the coil: 1.641 uH "apparent
inductance", Q 221
     
90 degrees, 1.737 uH, Q 224
       
180 degrees, 1.962 uH, Q 217
       
270 degrees, 2.425 uH, Q 217
                  
I'd take these Q figures with a quarter of a grain of salt, since
this is read off an analog meter scale (albeit a rather large one)
and thus subject to a bit of interpretation. The main point, though,
is that there was not much change in Q when the turns are compressed.
(All the gaps shown were eyeballed and are not exact.)
              
By the way, as with all previously reported experiments, this was
done on a Boonton 260A Q meter. Although I show values to three
decimal places, such "precision" is going to be virtually
unattainable, unrepeatable, and unnecessary in actual practice. For
each measurement the coil was resonated with the variable capacitor
in the 260A, the value (to a tenth of a picofarad) read off dials of
the main and fine tuning caps and plugged into a calculator. (The
fine tuning cap dial is calibrated directly to 0.1 pF points, the
main dial to 1 or 5 pF, depending which end you're on. And no matter
how carefully you set the main dial to the nearest 5 pF mark, you
have to be just a tad suspicious of the reading on the fine tuning
dial!)
                  
Calculators are neat devices--they give you answers to a gazillion
decimal places, even when more than 2 or 3 are utterly unnecessary
and, practically speaking, meaningless. I arbitrarily truncated the
answers at 3 decimal places, but in the real world one place would be
more appropriate. (After all, you can very easily get a 10% or
larger change in inductance by varying the spacing of the turns.)
These figures imply a precision in the coils that does not exist;
they are simply a reflection of the exact value at that instant, and
may never be seen again.
                
As Zack Lau said in Tech Correspondence in the June 93 QST,
"...home-brewed inductors are rarely precision components. Why
specify their inductance to three significant digits? Sometimes,
even two significant figures is a bit absurd." And I heartily agree
with all of that! While it's OK and educational to do it in a
tightly controlled experimental setting, in the real world you'll
never be able to duplicate the inductance value to that
precision--and there is rarely any need to. (Circuits often contain
variable components which can compensate for slight variations in
inductance, and the inductors themselves can often be varied.)
              
When I first started studying inductors several years ago (when I got
the Boonton) I asked one of the truly brilliant (in both theory and
practice) electrical engineers in my ham club some detailed questions
about inductance and designing inductors. He told me the simplest
thing was to forget much of the detailed theory, variables,
microscopic permutations and oddball physical effects; just wind a
roughly designed coil that was in the ball park and trim it until it
worked as desired in the circuit. At the time I thought he was being
a wise guy, but over the years I've come to realize he's right
(except now I know why, and have a much better understanding and
appreciation of the identity and effect of all those factors). There
are so many variables in both the inductor and the circuit that you
can drive yourself crazy trying to account for all of them.
         
73 and Queue Our Pea DE WA8MCQ
             

--
Mike Czuhajewski, user of the UniBoard System @ wb3ffv.ampr.org
E-Mail: Mike.Czuhajewski%hambbs@wb3ffv.ampr.org
The WB3FFV Amateur Radio BBS - Located in Baltimore, Maryland USA
Supporting the Amateur Radio Hobby, and TCP/IP InterNetworking


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