[TowerTalk] Ferrites 31 vs. 77 material

[Jim Brown]

On 10/27/2019 1:31 PM, David Gilbert wrote:
> I'm sure K9YC, who knows at least 20 dB more on the subject that I do, 
> will have a more thorough explanation but basically the reason is that 
> Type 31 material has desirable loss at HF frequencies instead of only 
> contributing inductance.  When relying only on inductance for common 
> mode chokes you can get undesirable resonances that actually subvert the 
> purpose of using them, whereas the loss in the Type 31 material gives 
> you the low Q performance you want and it does so across a wide 
> frequency range.

Yes. Common mode chokes work by forming low Q parallel resonances in the 
common mode circuit by winding multiple turns around a suitable 
material. #31 is the only suitable material for transmitting chokes at 
HF. 77 is not, and #43 is not. See http://k9yc.com/2018Cookbook.pdf and 
study k9yc.com/RFI-Ham.pdf for more discussion of the the theoretical 
concepts.

73, Jim K9YC