[TowerTalk] Balun Overheating?

Jim Brown jim at audiosystemsgroup.com
Mon Apr 21 12:47:22 EDT 2014

On 4/21/2014 5:32 AM, Jim Lux wrote:
> The real sticky issue when trying to calculate stuff is "what is the 
> voltage".  And that's something that is hard to determine in a simple 
> way.  One can do a numerical model (using NEC, for instance), but in 
> practice it's highly installation dependent. 

YES! NEC will at least get you in the ballpark if the model you enter is 
close to your reality (like feedline length). But antenna imbalance 
increases the common mode voltage (and thus the common mode current), 
and those imbalances can be hard to model, because they come from things 
like ground slope, variations in soil under the antenna, surrounding 
metal objects (wiring in your home, adjacent antennas and even adjacent 

All of which are reasons why I've always taken a brute force "more is 
better" approach to designing for high power. If you buy them right (in 
quantity from vendors who specialize in Fair-Rite products, you should 
be paying no more than about $5 for a #31 2.4-in o.d. toroid, tax and 
shipping included). And all it takes to buy them right is a group 
purchase with one or more local ham clubs. I recently did a very 
successful group buy with Dexter Magnetics, near Chicago, and I've seen 
good reports about Lodestone Pacific.

73, Jim K9YC

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