[TowerTalk] CM chokes

Earl Morse kz8e at wt.net
Thu Nov 7 12:54:40 EST 2013

The 31 material will give you more common mode impedance at the lower frequencies than the 43 material.  Depending on the core size and how high in frequency you are going you might be able to get away with just the 31 material chokes and have enough impedance to cover the entire range of frequencies desired.  The only disadvantage to putting both mixes on the cable would be having 10 cores instead of 5 hanging on the cable but other than that there really isn't going to be any other detrimental effect in mixing the beads on the cable. 

We could also have a discussion about whether it would be better to put the 31 mix at the feed point and then the 43 mix or vice versa or have the two mixes interspersed at the feed point but I doubt you could tell the difference at HF .  I would just put them all as close to the feedpoint as possible.

You didn't mention what frequencies that this antenna covers.  Depending on the frequency you might be able to attain the desired attenuation with multiple turns/multiple beads of #43 material if the #31 material wasn't available. 


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?

Dan -- N3ND


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