Topband: Core for K9AY loop transformer

[Jim Brown]

>For a common mode suppressor, you simply want enough inductive reactance to 
>impede current flow. 

Not true. The loss component is of far greater use. Consider a suppressor on 
a wire shorter than a quarter wave. The choke and the line are a series 
circuit, and the line will be capacitive.  An inductive reactance will 
combine with it to reduce the total impedance, and thus increase the 
current. Exactly the opposite of the desired result. 

A common mode choke is a parallel resonant circuit. One wound on an air core 
has fairly high Q, while one wound on a ferrite core may have a much lower Q 
(and a much higher impedance). Above resonance, the choke will look 
capacitive, so it will combine with the inductive reactance of a wire longer 
than a quarter wave to reduce the total impedance, thus increasing the 
current. Again, not the desired result. The loss component, however, ALWAYS 
reduces the current, thus the choke should generally have the highest 
practical equivalent series resistance over the frequency range where 
suppression is needed. 

>The type of core does not matter -- air, low mu or hi 
>mu materials -- as long as the result is at least a few hundred ohms 
>reactance. 

A major shortcoming of most choke baluns is that the designer believes that 
a few hundred ohms choking impedance is sufficient. For most ham 
applications, a value on the order of 3-5K ohms is a worthy design 
objective.  

>10 turns through 43 material is roughly 1000 ohms at 160M, which 
>should work well.

10 turns through a 2.4 inch o.d. toroid (FT-240 in ham-speak) of #31 
material will give you about 3x that value. See the measured data in 
Appendix #1 of my tutorial. 

http://audiosystemsgroup.com/RFI-Ham.pdf

73,

Jim Brown K9YC