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Re: [TowerTalk] Ferrites 31 vs. 77 material

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Ferrites 31 vs. 77 material
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sat, 9 Nov 2019 10:32:23 -0800
List-post: <mailto:towertalk@contesting.com>
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
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