On 11/9/19 10:32 AM, Jim Brown wrote:
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.
Yes, it is a gross oversimplification, but I think it *is* a short
explanation why many turns on a bigger core is better than a string of
beads, particularly at lower frequencies.
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.
yes, that Snelling book is hard to come by. I got a copy at work, where
they borrowed it from Armstrong Flight Research Center.
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.
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