[TowerTalk] Toroid Core Material Selection

[Tom Rauch]

> up a lot of my long-standing RFI problems with them. My
question. Ignoring the
> economics, as I have both types available, what would be
the results of
> stacking one each of type 43 and 77 toroids?

There is a good bit on nonsense circulating about this, like
adding a few beads of some type at the end of a string of
other beads when building a balun. I guess the thought is
the current "vanishes" along the string of beads. The idea
is since less current exists at the end of the string you
can use a more resistive bead, and a less resistive (lossy)
bead at the start of the string.

The current can change, but it a function of distributed
capacitance to the outside world. If the physical area
occupied is small the current is basically uniform. I have
some measurements of loading coils that show this effect.

http://www.w8ji.com/mobile_and_loaded_antenna.htm

Now what does this mean to your question? Probaly your
string of beads will be a tiny fraction of a wavelength long
and current will be uniform throughout the string.  At any
frequency the impedance of the combination is a product of
the two complex impedance presented by the beads, so you
won't really have two independent chokes.

Something similar happens in RF plate chokes, where a choke
with a series resonance on a particular frequency cannot be
corrected by adding a good choke at that frequency in series
with it. The actual result is more complex than you might
think.

Would this make a toroid with the
> combined bandwidth of both and the combined choking
impedance of both?

 Sure, but one may have a capacitive reactance and the other
an inductive reactance and that may actually reduce
impedance. The result will be something that looks different
and not necessarily better. That isn't saying it won't be
better on some ranges, but it almost certainly will get
worse on some ranges also.

And would
> this result be the same with the same two toroids in
series? Type 31 material
> seems like the ultimate weapon but doesn't seem to be
readily available yet.

They are around.

http://www.dxengineering.com/Products.asp?ID=116&SecID=57&DeptID=11

73 Tom