[TowerTalk] OCF dipole and "good" balun

[K4SAV]

..."All the 4:1 baluns I know about are TRANSFORMERS. They pass all the
transmitter power through the core of balun (transformer). Maybe you
know something I don't know. "....

A good commercially available 4 to 1 current mode balun will have two 
cores.  One to perform the 4 to 1 function and one to perform the choke 
function.

If you are trying to correct my usage of the words "current mode balun", 
it isn't my term.  I got it from the manufacturer of the product.  If 
someone goes looking for a 4 to 1 common mode choke they aren't going to 
find it.

Jerry

Jim Brown wrote:

>On Tue, 30 Oct 2007 22:55:31 -0500, K4SAV wrote:
>
>  
>
>>The balun you need for this antenna is a 4 to 1 current mode balun.
>>    
>>
>
>All the 4:1 baluns I know about are TRANSFORMERS. They pass all the 
>transmitter power through the core of balun (transformer). Maybe you 
>know something I don't know. :)
>
>A current mode balun is a COMMON MODE CHOKE. It works by adding a 
>large lossy impedance in series with the common mode. It works best 
>if the impedance is very large and resistive (lossy). Choke baluns 
>work by reducing the common mode current to a very small value 
>(thanks to their high common mode impedance). If they don't, they're 
>much more likely to burn up. 
>
>Off-center fed antennas are inherently unbalanced, thus they will 
>generate considerable common mode voltage. To work, the choke balun 
>must have a VERY HIGH common mode impedance to reduce the current to 
>a sufficiently small value that it doesn't overheat and fail. 
>
>Choke baluns work best if they're wound with coax. Choke baluns 
>wound with parallel conductors put roughly 30% of the transmitter 
>power in the core as leakage flux. That power will cause heating in 
>a lossy core (and the choke won't work very well unless the core is 
>lossy, so that it can reduce the common mode current to near zero). 
>Low loss cores (#61) solve the heating problem for low power (100 
>watts), but their impedance is mostly inductive on the lower HF 
>bands, and the Q is so large that they're very narrow on the higher 
>bands. Inductive chokes are sensitive to line length. 
>
>Bottom line -- OCF antennas need very high resistance choke baluns 
>wound on coax if you want to take the feedline out of the equation. 
>
>See http://audiosystemsgroup.com/RFI-Ham.pdf
>
>73,
>
>Jim Brown K9YC
>
>
>
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