Topband: Effect of DC on Ferrite Toroids?

[Jim Brown]

On Mon, 8 Jan 2007 23:41:46 -0500, Jeff Maass wrote:

>My concern is the +12Vdc on the feedline to a preamplifier, which
>will pass through a choke of 8 turns of RG-8X through four #31 
>2.4-inch toroids. The current is low.

You have two very good reasons NOT to worry. First, the toroids see 
only the DIFFERENCE between the current on the shield and the 
current in the center conductor, which is zero. Second, the current 
is low, but it could be large and not affect operation of the 
ferrites. 

Fair-Rite publishes extensive data for their materials and products, 
including data for changes in permeability with temperature and DC 
bias. 

If you haven't done so already, take at look at 

http://audiosystemsgroup.com/RFI-Ham.pdf

which includes a long discussion about how ferrites work. I may need 
to expand it a bit to address questions like yours. I covered it in 
a piece that I wrote for the pro audio world. 

Jim Brown K9YC