[Elecraft] Toroid Balun Cores

[John Ragle]

An interesting discussion...much is usually made of the "self-shielding" 
property of ferrite toroids, and we see this in practice in interstage 
transformers as well as in "baluns [sic]". (I am talking here about the 
"balun" type also called a "choke balun.")

However, it is also a fact that the magnetic field outside a cylindrical 
solenoid tends to zero as the solenoid tends to "infinite" length. So, 
for example a choke balun wound on a cylindrical form rather than as a 
clump of scramble-wound turns is also more or less self-shielding.

Parenthetically, it is this self-shielding nature of solenoids that 
plays an important role in experimental verification of the Ahranov-Bohm 
effect (see recent QEX  article by Robert Zimmerman and Feynman Lectures 
Vol. II Chapter 15, sections 15.4 and 15.5).

I agree with the contributor(s) who said that the cracked toroid is 
basically junk. It has lost its utility because the magnetic circuit is 
broken at the crack. It is unlikely that superglue will restore it.

John Ragle -- W1ZI

=====

On 10/16/2011 4:41 PM, Ian White GM3SEK wrote:
> Ron D'Eau Claire wrote:
>>> They also didn't do much except make us feel better. Ferrite core chokes
>>> actually DO something useful.  The cracked balun was poorly designed, so
>>> it overheated.  That's a DESIGN mistake, not a "defect" of ferrite cores.
>>>
>>> 73, Jim K9YC
>>>
>> Why would you think that?  Properly designed, an air core balun and a
>> ferrite core balun are electrically identical.
>>
> That simply isn't true. Air-cored chokes are reactive in impedance,
> narrowband and easily detuned. A properly designed ferrite core choke is
> predominantly resistive and broadband, which makes it a much more
> dependable solution.
>
> Steve Hunt G3TXQ has measured a wide range of different chokes and
> plotted the results on a graphic that shows this point very clearly.
>
> <http://www.karinya.net/g3txq/chokes/>
>
> In Steve's graphic, an effective choke needs the largest possible
> bandwidth of dark green, preferably with the black underline denoting
> predominantly resistive impedance. This can only be achieved with the
> help of ferrite - and even then, only with the right ferrite core and
> the right kind of winding.
>
> Air-cored chokes are shown to be relatively ineffective, narrowband
> devices... and then there's worse. Steve has also shown that the series
> reactances of the feedline and the wrong kind of air-cored choke can
> sometimes cancel, leading to a higher level of common mode current than
> before. This cannot happen with a ferrite choke that is predominantly
> resistive.
>
> For further information read Steve's page followed perhaps by my own
> article, at:
>
> <http://www.ifwtech.co.uk/g3sek/in-prac/index.htm#1005>
>
> That in turn will lead you on to K9YC's much longer papers. Steve, Jim
> and I have different styles of presentation but we're absolutely agreed
> on the one key point: an air-core choke and a ferrite-core choke are
> very different indeed.
>
>
>

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