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[TowerTalk] Bead baluns

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Subject: [TowerTalk] Bead baluns
From: (Tom Rauch)
Date: Thu, 7 Feb 2002 08:31:30 -0500
> For 80-10, 50 beads of 73 mat'l. (FB-73-2401) will do fine. (all the
> way down to 160M actually)    40 beads of 73 mat'l. plus an additional
> 10 beads of 43 mat'l. at the dipole end would do slightly better if
> you expect either a significant mismatch, or are going to run high
> power on the higher frequency bands.

If we have a perfect dipole element mounted on a tower having low 
common-mode impedance, the bead bead string or balun will have 
half the feedpoint voltage impressed across the beads.

We can simulate that condition on the test bench by reversing the 
grounding at opposite ends of a choke-balun that is feeding a 
dummy load, and running 1/4th rated power (half voltage).

When I tested several common bead balun designs that way, they 
all over-heated after a few minutes of operation on lower bands. 

I carefully investigated baluns several years ago, and concluded the 
cheapest and most effective way to make a balun for a given 
amount of power was to just wind coax in an air-core solenoid coil. 
The next step down was to use a stack of cores and use multiple 
turns through the stack, and the least effective in terms of cost-for-
power-rating was a string of beads. The multi-turn core fell 
someplace in between those two cases.

I also found that stacking an assortment of cores didn't help, and 
usually made things worse.

73, Tom W8JI 

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