[TowerTalk] 4:1 balun questions KT-34XA

[Jim Brown]

On 10/28/2014 11:11 AM, Paul wrote:
> Further reading...
>
> Looking at Fair-Rite catalog of toroids, mix 61 which seems to be very
> popular and recommended for HF baluns, has a mu of 125, and Al (nH/T2) of
> 170 for a 2.4" dia core. The Arnold MPP core, which is Nickel-Iron-Moly has
> same 125u and Al of 155 for a 2" core.  I'm not sure what else would make
> one better than the other...there must be more to it though.
>
> Help

There's a tutorial discussion that may help you understand ferrite 
materials and how to choose them in k9yc.com/RFI-Ham.pdf  Most of the 
tutorial is aimed at using ferrites to build common mode chokes, but 
that's important stuff to understand.  What you need to feed that 
antenna appears to be a TRANSFORMER, not a common mode choke, and 
Steve's warnings about a pair of chokes to form a Guanella balun are on 
target.  A so-called "current balun" is really a common mode choke, and 
is NOT a transformer.

Also, study the online Fair-Rite catalog, which is quite extensive. I 
emphasize the word "study" in the context of both the tutorial and the 
catalog.   Zero in on the first section of the catalog, which shows 
fundamental properties for the various core materials (called "mixes), 
especially looking at the graphs of u' and u''  To understand those 
graphs, study my tutorial.

There are two reasons for using a transformer or choke to feed an 
antenna. The transformer performs the function of impedance matching. A 
common mode choke performs the function of keeping antenna current off 
the outside of the coax. That current would fill in the nulls of the 
antenna's pattern, increasing RX noise and degrading front-to-back and 
front-to-side.

73, Jim K9YC