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:
> ## 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.
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