On Wed, 29 Mar 2006 18:50:04 GMT, rocky wrote:

I run up to a KW, dipole and could be using an antenna tuner to stretch
the bandwidth. I was just thinking if it could be made even more
effective by using other bead material. Looks like #75 offers the best
attenuation at 75 M. but if I am getting this right, it will heat up
more under high power and off resonance.

Hi OM,

Think of the beads as series 1W resistors. Ask yourself how much of a
common mode load they will present to the antenna "as a source" (this
means that the dipole will look like a KW source with complex
impedance).

You then have to judge the potential that source will present in
Common Mode to the beads. Elsewhere in this thread I've done a
simplified analysis that suggests 50 #75 beads will get a tad hot
under matched conditions because they are dissipating roughly 1.5W
each. "Hot" is of little consequence until we shatter the ceramic
because of thermal stress (the sintered product contains bulk
irregularities). That stress could be arguably applied through a rain
drop hitting a bead. Then we get into new issues of balance and
symmetry because one part of the bead is cooler than the other. ;-)

If, instead for this matched load, you replace the 50 #75 beads with
80 #73 or #77 beads; then you have roughly the same total resistance,
but the heat dissipation (still 1W suggested load for each) is spread
out more. Again, moving off resonance demands another analysis.

73's
Richard Clark, KB7QHC