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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. [edit: this sentence is wrong]

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 to buy bunches of cores and measure them, but it would seem to be more efficient if I could somehow estimate an ideal geometry and use that as a starting point. For example, if the limit is due to core volume, maybe lighter cores would be the preferred starting point. Or if it is somehow related to diameter or circumference, then perhaps a smaller, wider core would be better.

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The K9YC paper you mention gives data up to 1 GHz on page 49:

enter image description here

These data show for these core types the maximum number of turns is just one or two. Furthermore, it's quite difficult to achieve a choking impedance above 1 kΩ. That's not very much, and for many applications you'd want more.

To get a higher choking impedance will require a larger core, and so ferrite chokes can become quite expensive at higher frequencies. For this reason it's often worth considering other techniques to supplement or replace them.

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  • $\begingroup$ As much as I like this answer --especially correcting my misinformation-- I think there is more to it. I get the impression that smaller cores, in addition to making more efficient use of ferrite, have higher resonant frequencies. So to get a higher choking impedance, maybe it is sometimes advisable to use a smaller core along with a smaller wire (and 5-10 turns as per Laird). There was something along these lines in one of Snelling's books (either Ferrites for Inductors and Transformers or Soft Ferrites, Properties and Appications).but it did not sem important at the time. $\endgroup$ – Chris K8NVH Apr 15 '18 at 21:08
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    $\begingroup$ As a postscript... I bought some small (0.125 inch ID) ferrite beads in #61 and #43 to see if the number of turns might be higher for any given frequency. They were not. Curves were close to what was posted here. $\endgroup$ – Chris K8NVH Jul 26 '18 at 2:04
  • $\begingroup$ A single turn is max? Is this why they frequently stack the beads on the wire? $\endgroup$ – user10489 Jul 26 '18 at 11:26
  • $\begingroup$ @user10489 A single turn is the max at VHF, so stacking a lot of beads is the only way to get more impedance. But when it's done at HF, I guess some people just have too much money. Or possibly they need (or think they need) a very wideband balun. $\endgroup$ – Phil Frost - W8II Jul 26 '18 at 11:50
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The primary issues with coax wound common mode chokes are;

  1. The inductive reactance of the choke can cancel out the capacitive reactance associated with the feedline shield thereby worsening the CM situation
  2. The resistive component of the choke is not sufficient such that the core undergoes an undesirable temperature rise
  3. The interwinding capacitance forms a self resonant circuit which reduces the choking action

At VHF and UHF frequencies these pitfalls can be even more exasperating as windings become a significant fraction of a wavelength and the selection of suitable core materials diminishes.

You may wish to examine other methods of common mode suppression such as 1/4 wavelength sleeve baluns or transmission line baluns before attempting the coax on a ferrite choke approach. But if you care to pursue some lab work with ferrites you may find the VNA measurement method used by Steve G3TXQ to be helpful. His published work is for HF baluns but the measurement technique should be extensible to the higher bands.

What you are mostly likely to find, however, is that there is no number (minimum or maximum) of coaxial turns on a ferrite core that will make an adequate CM choke at frequencies above 100 MHz.

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  • $\begingroup$ This does not seem to answer the question, which was specifically about the usable number of turns, not what happens when that number was exceeded. $\endgroup$ – Kevin Reid AG6YO Apr 8 '18 at 15:09
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    $\begingroup$ @KevinReidAG6YO Edited to be more explicit. $\endgroup$ – Glenn W9IQ Apr 8 '18 at 20:29

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