More on toroids/1 of 2

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From: Mike Czuhajewski (Mike.Czuhajewski%hambbs@wb3ffv.ampr.org)
Date: Sat Jun 25 1994 - 08:39:56 EDT


In a recent message on winding toroids, Chuck Adams said,
                          
> "If you have a choice on the wire size and a larger wire size,
>i.e. next smaller wire number, will work, by all means do so. It'll
>increase the Q and reduce the bandwidth. I'd bet that in most cases
>you might have to reduce the turn count by one or two due to
>increased distributed capacitance within the coil. I can do this
>calculation, but I have bigger fish to fry. :-)
>
> dit dit Chuck Adams K5FO CP-60 adams@sgi.com"
                       
    I'll counter his bet with my own, based on actual experiments:
For most of the toroidal coils that we typically use at HF, I'll bet
you will NOT need to reduce the turn count by one or two due to
increased distributed capacitance. It does increase, but I contend
that it's not enough to worry about in the real world, and that you
can make a larger change by varying the spacing of the turns instead.
                         
                         
   I did some very carefully controlled experiments a while back
along these lines, and saw only insignificant changes in apparent
inductance due to changes in wire size. (It was detailed in both the
QRP Quarterly, long form, and in Technical Correspondence in QST,
condensed.) If you have a number of different cores which are
nominally identical, such as a half dozen T50-2's, and wind all of
them with the same number of turns but different sizes of wire (and
insure that turn spacing is identical for all coils--a critical
variable), you may see some noticeable inductance differences. But
the cores are probably NOT identical, and the actual permeability can
(and does) vary from core to core; the inductance variations may
appear to come from different wire sizes but actually be due to a
combination of that and core-to-core permeability variations. (If
you can get a Micrometals catalog for their powdered irons, look up
front--they cite inductance tolerances of plus/minus 5%.)
                                   
 In my experiments I carefully notched a T68-2 core around the edge
with a knife (each turn had its very own notch) so I could wind many
different coils on it with different wire sizes and be sure the turn
spacing was identical each time. (It's well known that turn spacing
has a significant effect on inductance.) I used only one core to
eliminate the core to core permeability variation. I also eliminated
the variable of turn spacing with the notches, so any inductance
change would (theoretically) be due solely due to wire size. That
would include, among any other effects, the differences in
distributed capacitance due to different diameters. I repeated this
with a couple different core sizes, and variation due to wire size
changes was on the order of 2 or 3%. (On the T68-2 core with 16
turns, measured at 7.9 MHz, going from #18 to #28 caused an increase
from 1.47 uH to 1.50 uH, or 1.7%.)
                            
 My test equipment was a Boonton 260A Q meter. While they are
getting pretty old now, unlike Rodney Dangerfield they DO get
respect, at least from the "old timers." (They were very well
respected in their day, as well as high priced. I have a 1964 ad for
them, and the price was exactly one half what my parents paid for a
brand new Chevrolet the previous year!) Hardly new and modern, no
bells and whistles, full of vacuum tubes, but still good units and
lots of fun to play with. The basic principle is a signal generator
feeding a tuned circuit, which consists of a well calibrated variable
capacitor and unknown inductance which you connect. Tune for
resonance and read the Q off the meter, and inductance off a scale on
the capacitor dial if you're using certain test frequencies, or else
plug the capacitance and frequency into the standard formula to get
inductance.
                            
 I also wound coils on many different T37-6 cores, all with the same
number of turns, to try to chart the variations in permeability
between cores. (Remember, Micrometals, who makes those powdered
irons that Amidon sells, cites a plus/minus 5% inductance tolerance.)
I couldn't do the notching trick, of course, but wound the coils as
identically as possible. How did I take care of the turn to turn
spacing? I wound each one with the same number of turns (15), same
wire size, carefully chosen to just fill the coil completely. I used
every last T37-6 in the house, eventually borrowed every T37-6 I
could from every local ham I knew, and by the time I was done I had
wound coils on 58 of them. (Yes, it cost me a lot of wire, and yes,
it got pretty old after a while!)
                            
The inductances were measured by resonating the coils at 14.0 MHz on
the Boonton, reading the capacitance off the dial and plugging that
into the standard formulas. The inductances ranged from a low of
0.688 uH to a high of 0.745 uH. The mean value was 0.722 uH and most
were clustered around it, although the two extremes are close to the
+/- 5% tolerance cited by Micrometals. The moral here is that you
can have a number of cores which are nominally identical and yet
produce coils of somewhat different inductances.
                    
   References:
      QST, Technical Correspondence for June 1993, "Getting to the
Core of the Problem", WA8MCQ
      QST, April 1983, page 39, "Choosing Wire Size for Toroidal
Inductors," W7EL
      QRP Quarterly, Jan and Jul 1993, "Effect of Wire Size on Toroid
Inductance," parts 1 and 2, WA8MCQ.
                              
End of part one; part 2 follows. DE WA8MCQ/24 June 1994

--
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
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