I looked up some likely ferrite beads in the Amidon catalog, and it looks
like mix 43 has the right properties (used in many snap on split beads for
RF interference, as well as standard toroids which could be threaded on
before the hardware.
Any of FT-50B-43 beads (.312 inch hole), 2X-43-251 split cores with 1/4 inch
hole or 2X-43-151 splits with 1/2 inch holes all result in about 8 to 11
microhenries per foot of beads/cores (the 50B is 1/2 inch long, the others
1.125 inch long). At 25 MHz, Amidon says Z=56 ohms for one FT-50B-43, Z=171
for one 2X-43-251 and Z=159 ohms for one 2X43-151.
At the higher HF frequencies, there is clearly enough inductance to pretty
effectively break up the segments with a foot of beads. At lower frequencies
the reactance starts to fall into the range of values suitable for loading
coils for shortened antennas, which might lead to some surprise resonances on
40m and down.
With enough experimentation on placement and quantity of beads I suppose it
might possible to do some interesting things with guys loaded to resonance
on 80 or even 160 meters but which tune out on the higher frequencies, but
it would be a real killer changing the number of beads half way up a guy.
Darrel AK6I
darrel@dii.com
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