Following-up my previous message...
Over its "wideband" region, a ferrite bead becomes increasingly
resistive as well as inductive. The resistive losses in each bead will
be I-squared*R and these will of course heat up the bead.
However, the "I" that we're talking about is the surface current on the
coax, that the balun is trying to suppress. If you use enough of the
right beads, you will create so much R in series with the current path
that I will be reduced to a very low value, so the heating effect in
each bead will be very small.
The only times you can expect serious heating in a bead balun are if at
least two of the following factors apply:
1. Defective balun (not enough beads, wrong core material, broken
beads, incorrect construction)
2. Diabolically unbalanced antenna (so fix the antenna already)
3. High power.
There is good information, including estimates of power dissipation for
a range of commercial bead baluns, at:
http://www.w8ji.com/Baluns/balun_test.htm
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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