[Elecraft] Toroids - Effect of Winding Spacing on InductanceandDistributed Capacitance

[Jack Smith]

David:

I have another message lost in the aether on the subject. I strongly 
suspect the Drake tuner has a ferrite core balun. Ferrites can be 
conductive, with a bulk resistivity ranging from 1E9 down to 50 ohm-cm, 
depending on the material.

Hence if the balun is handling any degree of power, there may be a 
significant turn-to-core voltage that causes arc-over. The voltage 
turn-to-turn is less, of course, as the total voltage is more or less 
evenly divided amongst all turns. But if the core has even moderate 
conductivity, then the total voltage across the first and last turns 
will appear between the first and last turns, insulated only by the wire 
insulation.

Hence, spacing or wrapping the core is useful.

Jack K8ZOA


David Cutter wrote:
> Your comment about spacing off the core is interesting: my Drake AMU 
> has an attached balun which is constructed with the wire wound on a 
> former which stands the turns off the core.  The wire is 1mm diameter 
> bare wire and does not touch at any point.  I have never seen this 
> anywhere else.  The core is also bare, ie no visible coating.  I do 
> not know its material.
>
> David
> G3UNA
>
> ----- Original Message ----- From: "Jack Smith" 
> <jack.smith at cliftonlaboratories.com>
> To: "Ron D'Eau Claire" <rondec at easystreet.com>
> Cc: <elecraft at mailman.qth.net>
> Sent: Monday, February 26, 2007 6:17 PM
> Subject: Re: [Elecraft] Toroids - Effect of Winding Spacing on 
> InductanceandDistributed Capacitance
>>
>> 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 
>
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft   
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>