[TowerTalk] One off balun question

[Steve Hunt]

Just to develop this a little further ....

I referred earlier to a spreadsheet I had produced that allows me to 
specify the number of turns, the number of stacked cores and the stray C 
of a choke using #31, #43, #52 and #61 ferrite mixes; it will then plot 
the complex impedance of the choke between 1 and 30MHz. It takes full 
account of the variation of u' and u'' with frequency.

This afternoon I took a look at Jim's (K9YC) measured choke data shown 
in Fig 41 of his paper. By adjusting the value of C in my spreadsheet I 
can get good agreement with any of Jim's curves in that chart. For 
example, 7 turns on 5 stacked FT240-31 cores with 8pF parallel 
capacitance is predicted by the spreadsheet to give resonance at 3MHz 
(Z=7750 Ohms), Z at 2MHz=6050 Ohms and Z at 10MHz=2600 Ohms; for comparison, 
Jim's measured figures were: resonance at 3MHz (Z=8000 Ohms), 
Z at 2MHz=6500 Ohms and Z at 10MHz=2900 Ohms.

Repeating that exercise for different numbers of turns on the same 5 
stacked cores required the following values of C for best match with 
Jim's measured figures:

4 turns on 5 cores: 2pF
5 turns on 5 cores: 3pF
6 turns on 5 cores: 4pF
7 turns on 5 cores: 8pF

Looking at those figures it seems counter-intuitive that adding an extra 
turn going from 6 turns to 7 turns would double the stray capacitance. 
It's not a problem with Jim's data - I get exactly the same sort of 
anomalies using my own measured data!

Note that in this case the ferrite mix and the stack size didn't change 
- only the number of turns. Initial thoughts are that a simple "parallel 
C" model is inadequate to describe what is happening and something more 
complex is required.

In the case of large conventional loading coils we have to move from a 
"lumped component" model to a transmission line analysis to get accurate 
predictions; but I wouldn't have thought that necessary with these 
chokes that are physically small compared to a wavelength.

Any thoughts?

Steve G3TXQ