[Elecraft] Toroid Balun Cores

[Ian White GM3SEK]

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.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek