[Elecraft] Balun Questions

[Jim Brown]

On Fri, 23 Jul 2010 11:53:33 -0700, Brett Howard wrote:

>Interesting enough his high power one uses two cores and appears to be
>made the same as the "budget" one that is wound on one core...  Seems
>like stacking them on top of one another is going to just couple them
>and hinder your isolation but maybe I'm wrong there....

No, the only coupling between chokes wound on different ferrite cores 
that provide a complete path for current (like a toroid) is the stray 
capacitance that results from their physical proximity. Virtually ALL 
of the flux stays in the ferrite core -- there is virtually NO leakage 
flux (that is, coupling between coils wound on different toroids). 

This is generally true of ALL coils wound on closed ferrite paths. 
Leakage flux (and thus coupling outside the core) occurs when the 
ferrite does NOT complete the path (for example, a coil wound on a 
ferrite bar, also known as a loopstick receiving antenna). 

If you want a very good 4:1 impedance tranforming balun for less than 
about 500 watts, consider building one from two of the 12-turn or 14-
turn THHN chokes shown in my tutorial. Each choke is wound on a #31 
2.4-inch o.d. core. Total cost about $20 (two cores plus some wire and 
connectors). 

73, Jim Brown K9YC