[TowerTalk] Common mode choke

Jim Brown jim at audiosystemsgroup.com
Thu Nov 7 13:04:02 EST 2013

On 11/7/2013 8:14 AM, Dan Atchison wrote:
> With my second floor shack, I have common mode RF issues that have 
> been well mitigated with common mode chokes of 5 FT240-43 cores, but 
> at 40 and below frequencies the choking could be improved.  I ordered 
> several FT240-31 mix cores and was going to replace the 43 mix, but 
> thought I might have good luck if I just added the 31 mix with the 43 
> mix, i.e. 10 ferrite cores in a choke as opposed to five. Are there 
> any disadvantages to this or perhaps is it overkill?


Nothing wrong with using #31 and #43 together, but the more important 
issue is the number of turns. Without winding enough turns, the 
resonance is too high be be useful.

More to the point though -- exactly what "common mode issues" are you 
having? Perhaps the real issues are equipment with Pin One Problems, 
and/or antenna problems.  I suggest that you study 
http://k9yc.com/RFI-Ham.pdf to first understand the Pin One Problem and 
RFI in general, and second for specific suggestions of how to identify 
and fix the problems.

73, Jim K9YC

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