[Elecraft] Toroids - Effect of Winding Spacing on Inductance andDistributed Capacitance

[Jack Smith]

At the moment, I cannot reach his sub-pages either, and can only see 
Andy's main page http://g4oep.atspace.com/index.htm from my locally 
cached copy. I've had trouble in the past with connectivity on his page, 
so I suggest trying again later.


As far as cores being non-conductive, that is true for powdered iron 
(the typical red and yellow cores) but it is not necessarily true for 
ferrite cores. The bulk resistivity of ferrite materials varies widely 
from 1E9 down to 50 ohm-cm, and not all (few, actually in my limited 
experience) ferrite  cores are coated with a insulating surface. In 
addition, many ferrite materials have an extremely high dielectric 
constant, which suggests the need for a bit of spacing between the core 
and the winding to reduce distributed capacitance arising from paths 
through the core (obviously more important when the core is more 
conductive.) Teflon tape has been used for this purpose. We should not, 
of course, associate the term "toroid" with a particular core material, 
as there is a world of difference between powdered iron and ferrite 
materials, both of which are commonly available in a toroidal form.

There's also some degree of unit-to-unit variation in the core material, 
both powdered iron and ferrite. I wound 140 4uH inductors last October 
on T50-2 cores and measured every one for Q and inductance with an 
HP4342A Q-meter. I found about a 10% difference in Q from the best to 
the worst and around a 5% difference in inductance, to the point where I 
had to wind more turns than the target and then measure and remove turns 
to achieve my goal of 4uH +/- 2% with a minimum Q of 225 at 7.9 MHz. I 
recall FairRite specs a 20% tolerance on permeability for its ferrite 
materials, but my measurements suggest they actually are quite a bit 
better than that. And, the majority of ferrite material is used in 
applications where permeability tolerances are not critical, such as 
broadband transformers and RFI suppression.

As far as following the instructions in the manual, I could not agree 
more. If you follow the instructions it will work, absent the rare 
defective component.

Jack K8ZOA


Ron D'Eau Claire wrote:
> The link doesn't work for me either.  
>
> There is a reason the Elecraft manuals clearly show the placement and
> spacing of turns on the inductors. Unless the text says otherwise (such as
> optimizing the output filter in the KX1) they should be wound just as shown:
> spacing and number of turns. But it's not high precision work. If the coils
> *look* like the pictures with about the right amount of core exposed in the
> gap, and you have *exactly* the number of turns (counting the number of
> times the wire passes through the center hole), the coil will work FB. 
>
> One mistake many builders make is in thinking the wire must lie tight
> against the toroid core all the way around in each wrap, or turn. That's not
> so. A small gap between the wire and core is fine. Indeed, it's preferable
> to pulling the wire so tightly the enamel gets scrubbed off or the wire
> breaks! 
>
> The second mistake is worrying too much about small breaks in the enamel
> coating. The core is *not* conductive! Bare wire touching it is not a
> problem. The problem is when two adjacent turns short out, usually where the
> wire passes through the center hole, because bare copper on adjacent turns
> touches. Not only can that cause trouble from the start, if it's a poor
> contact between turns it can cause an intermittent that is very frustrating
> to locate later. Again, don't be too aggressive about pulling the wire too
> tight around the cores. Allow a natural, small, gap to form between the wire
> and the core as you wind with moderate pressure on the wire.  
>
> Ron AC7AC
>
> -----Original Message-----
> Jack,
>
> Sounds interesting but the link in your e-mail 
> (http://g4oep.atspace.com/coil/coil.htm) does not work for me.
>
> Tom KG3V
>
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