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Re: [TowerTalk] Measuring Ferrite

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring Ferrite
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 10 Apr 2014 14:38:30 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 4/10/2014 1:58 PM, Earl Morse wrote:
I confess, I am spoiled.  I have access to a plethora of RF devices such as 
network and impedance analyzers so I wouldn't have to make do with making this 
measurement with a rigged up setup.

Actually, measuring ferrite chokes is not nearly as easy as it looks. The only stand alone instrument I would trust the the HP impedance analyzer. All reflection-based analyzers have the limitation of greatly reduced accuracy as the impedance is much more than 1/5 or 5x the circuit Z. There are HP applications notes that address this issue.

The equivalent circuit of a ferrite choke is series R and L in parallel with C (the interwinding capacitance), which simplifies to parallel RLC near resonance. Effective suppression is provided by the resistance, not the reactance (because the reactance can resonate with the line it's trying to choke). Ferrite materials that provide effective suppression are very low Q (on the order of 0.5 for an effective suppression material) where they are resonant.

What makes the Z measurement so difficult is that C in that parallel resonant circuit for an effective choke is on the order of 2-10 pF and the desired R is on the order of 5K.

73, Jim K9YC
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