[TowerTalk] One off balun question

[Steve Hunt]

I have a spreadsheet which does pretty much what Jim just described.

One thing that interested me when I started looking at the "best fit" 
equivalent-circuit values was that the value of C varied depending on 
the core material. In other words, you couldn't assume that 10 turns of 
a particular coax on a "240-size" core, say, would always give the same 
value of equivalent capacitance - it varies considerably depending on 
the core material.

I've not yet yet found a mechanism that will predict the variation in C 
to my satisfaction, but clearly a simple model based on inter-winding 
capacitance doesn't get close.

If we had a good way to predict C, predicting the CM impedance of a 
particular choke design would be relatively straightforward.

Steve G3TXQ



On 30/10/2014 23:48, Jim Brown wrote:
> 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.
>