What is the maximum number of turns in a ferrite core used as a common mode choke?

2018-04-06 23:38:25

Is there a way to estimate the maximum number of turns which can be used with a ferrite core used as a common mode choke?

It is known that the impedance of such a choke is proportional to the square of the number of turns and so there is a strong incentive to use multiple turns. But it is also known that there is a limit due to what looks like a sort of resonance phenomenon and, once that limit is exceeded, increasing the number of turns has either no influence or negative influence.

There is an excellent paper on the subject by Jim Brown (K9YC) ( http://audiosystemsgroup.com/RFI-Ham.pdf ). Unfortunately, his maximum frequency of interest is about 100 MHz and my application is VHF and UHF.

The ferrite core manufacturers provide excellent information for impedance of a single turn at different frequencies. This can be used to estimate the impedance when using multiple turns, so long as the limit is not reached; that limit could be as low as 1 turn!

I am willing t