[TowerTalk] Ferrites 31 vs. 77 material

[Jim Brown]

On 11/9/2019 7:45 AM, jimlux wrote:
> Big cores (high mu) with lots of turns (big N^2) is the way to go..
> (Except that at high frequencies, the capacitance between the turns 
> short circuits the impedance of the windings - so there *is* a happy 
> medium)

Jim,

It's FAR more complex than that. Common mode chokes work by using the 
RESISTANCE coupled from the core, in the region where the choke is 
resonant. The ideal core material has a very low Q, providing a broad 
resonance and thus a wide effective bandwidth. #31 is a well engineered 
material for HF -- it is MnZn material that combines the circuit 
resonance (the turns) with it's dimensional resonance, which serves to 
broaden the resonance in the HF spectrum.

BTW -- because R is coupled from the core, it also increases as N 
squared, and C increases with N.

These concepts are developed in k9yc.com/RFI-Ham.pdf  which is based on 
work I published in an AES Paper in 2005. I learned of dimensional 
resonance from a classic work by Snelling, which is referenced in the 
paper and the tutorial. A colleague who was on faculty at the Univ of 
Chicago at the time found it in the sub-basement of their engineering 
library and loaned it to me. :)  That AES paper is also on my website -- 
scroll way down.

There are a number of MnZn materials, but #31 is the only one that is 
useful from 160-6M. #75 is useful for transmitting chokes on 630M, and 
smaller #75 cores are useful for RX chokes.

#77 and #78 materials are NOT useful for chokes because their 
dimensional resonance in the range of 1 MHz is VERY high Q -- in other 
words, it does NOT combine in a useful manner with the circuit resonance.

73, Jim K9YC