[TowerTalk] Balun Recommendation

[Steve Hunt]

On 16/04/2012 12:11, Jim Thomson wrote:
> ## Steve, does this mean 8 turns of RG- 58 on a single core.  Will a single core even handle
> 1.5 kw CCS  CXR + SWR.    RG-58 is junk, so it would have to be wound with RG-303 teflon coax
> or similar.
Yes, 8 turns on a single core.

I don't agree that RG58 is "junk"; it's rated for 380W continuous 
average power at 28MHz into a matched load - that's adequate for UK 
power levels. If you want to go higher, yes you need a teflon coax.

You can't determine what power a core will handle without knowing the CM 
stresses the balun is subjected to. On something like a beam, with close 
to 50 Ohm feedpoint impedance and reasonable load balance, plus the 
_worst possible_ CM path impedance, the single FT240-31 core wound with 
8 turns of RG-303 should handle _continuous average powers_ of:

20m: 1.7kW
15m: 950W
10m: 500W


>
>
> ##  what about using .25 or .75  or  1.25 wavelength of coax, with the shield bonded to the top of the tower.
> I mean a physical .25 or .75   or 1.25 wavelength...   minus 2%.   The Z at the feed point would be sky high.
> Then no requirement for a conventional balun  at all.
I don't understand why a quarter wavelength of coax bonded to the top of 
the tower would necessarily create a high CM path impedance - it could 
just as easily be a very low CM path impedance depending on the 
impedance of the tower path to ground (which is in series with it). 
Perhaps I've misunderstood what you are proposing! The Pawsey Stub would 
work in this application, but that's not what you have described.

73,
Steve G3TXQ