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
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