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Re: [TowerTalk] One off balun question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] One off balun question
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 30 Oct 2014 16:48:27 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 10/30/2014 3:22 AM, Jim Thomson wrote:
  I realize the uh, measured at low freqs tells me little about the CM Z of a 
choke.  I was just interested to know how much uh you folks are seeing on these 
bigger  CM chokes, where 5-15 turns of coax are used
on 4-8 cores, that type of scenario.

Hi Jim,

Values of parallel R, L, and C can be computed from the plots of choking Z using conventional curve fitting. Rp is the measured Z at resonance (the peak of the curve), L can computed from the slope of Z curve at low frequencies, C can be computed as the value that resonates at the peak frequency with L. I go one step further and put those values in a spreadsheet, plot the impedance of that network on the scale as the measured data, and tweak values for best fit.

This method is based on materials like #43, which have only one resonance. #31, which has two resonances, requires more reliance on curve-fitting. #31 with a lot or turns is like a double tuned IF. I discuss this in the tutorial).

Thus, C is that which matches curve on the higher frequency side of resonance, the value of L near resonance is that which resonates with C, and the value of L at low frequencies is that which matches the slope of the curve well below resonance. Values obtained are a fair approximation of what might be obtained from more detailed curve fitting.

73, Jim K9YC

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