[TowerTalk] Toroid Core Material Selection

[Tom Rauch]

> Tom, I'm a little confused by the above. Take for instance
the SB-200.
> I have modified several for 160 meter use, and besides
fixed inductors
> and extra capacitance, the only addition mod was to place
a closely
> identical RF choke in series with the existing one.

I'm sorry, I was speaking of the series-resonant point.
That's the frequency where a section of the choke has
extreme voltages and the choke acts like back-to-back L
networks. This causes the choke to have very low series
impedance and produce very high voltages and currents
compared to applied voltage and current. When you add
another choke in series with that choke, it only shifts the
series resonance lower. It doesn't make a good broadband
choke.

Ferrite or iron chokes can be similar in behavior. If they
are both inductive at the desired frequency and the system
is far from resonance things are OK, just like at low
frequencies your chokes are OK. As frequency increases
adding an additional series inductor moves unwanted high
frequency series resonances lower. You might find a choke
that worked very well on ten and fifteen meters quits
working when you add an additional series inductance to
extend the LF range.

Since they were talking about multiturn toroids and
impedance, the effects are very similar.

73 Tom