[TowerTalk] balun design summary and data

Tom Rauch W8JI@contesting.com
Sat, 17 Jul 1999 11:19:29 -0400

> Incidentally, the reason I don't think I had a heating failure is that I
> understand these will happen on the end of the bead balun nearest the
> antenna, and my blowout was in the middle of a 50-bead string- two beads
> gone completely, a big hole in the tape wrap, a gouge in the teflon coax
> outer jacket, some braid visible, and 3-4 beads on either side cracked but
> still contained within the tape.

That's an untrue tale Pete.

Current is the same through the entire string. All the beads have 
the same power applied. The end of the balun that "collects" the 
most heat is where the beads go.

If you hang the balun vertically, the upper beads go first. If you lay 
it down flat, the end beads go last.

The beads nearest the connector are generally "heat-sunk" by the 

But all beads, if the same type in series, all dissipate the same 
power. Uneven heating with the same beads is a thermal problem, 
not electrical.

When some bead types get too hot, their loss tangent shoots up. 
That means they get hotter and go into thermal runaway faster than 
cooler beads in the string. Everything works to make heating 
uneven, and the tiniest unpredictable influence like physical 
position determines where the string heats fastest. 

Another thing to remember is the beads can permanently loose 
permeability if overheated, and their loss characteristics will also 
change. Even if you don't shatter the beads, they can be damaged.

73, Tom W8JI

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