Jim,
## #61 ferrite is UHF material and semi useless for an HF balun.
Not right. 61 is good for absorption in the UHF range, and for
relatively low loss applications on HF. It works great as balun, for
example in the form of an FT-240-61 core with 12 turns of RG400 on it.
Instead it's not good for a bead balun, because its permeability is too
low for that.
#77 ferrite is out of vogue these days for HF baluns.....even though
Hy-hain still uses it on their BN-4000 balun. Both have been
replaced with type 31 ferrite.
77 is low frequency, high flux density ferrite for power supply
applications. It can be pressed into HF balun applications, but is far
from optimal. Many people have used old TV flyback transformer cores and
even TV yokes for winding baluns - that's much the same material.
31 is formulated for absorption in the HF range, not for transformers.
It can be used in baluns only if enough turns (or beads) are used to
provide a really high impedance, because the impedance it provides is
almost purely resistive. It makes for a VERY lossy balun if used to
provide a moderate impedance.
61 provides a lower impedance than 31 or 77, but it's mainly inductive
in the low and mid HF range. So its far less lossy. If used in a way
that provides too little impedance, though, it can detune an antenna.
So, it all boils down to selecting the material best suited for the
balun on intends to build, and then design it correctly. 31 cannot fully
replace 61 nor 77, nor can 61 fully replace 31 nor 77, etc. Each has its
applications.
Some companies offer several dozen different ferrite mixes, so that
designers can select the optimal material for a given application.
Manfred
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