Vic Rosenthal wrote:
>Robert & Linda McGraw K4TAX wrote:
>> I seem to recall that RG-58 is rated at 600 watts at frequencies below 30
>> MHz. This presumes a matched load i.e VSWR at 1:1 which is what should
>> exist between the amp and the tuner except during "tune up" which should be
>> done at reduced power.. I'd expect that RG-58 to be OK in as much as the
>> typical SB-200 will output about 500 to 600 watts.
>
>The limitation in this case is current, not voltage breakdown, so the
>duty cycle is relevant. I believe the 600 watt limit refers to 100
>percent duty cycle, so assuming a CW duty cycle of 50 percent (SSB is
>less), it should be good for 1200 watts. Even this is conservative for
>non-contest operations. The symptom of trouble would be that the coax
>gets warm!
>
What really gets warm is the center conductor - you don't feel it from
the outside, but it softens the polyethylene. If there is a permanent
sharp bend in the cable, the problem is a very slow migration of the
center conductor towards the inside or outside of the bend. Years later,
probably during CQWW, the jumper suddenly goes short-circuit.
I'm not saying it's common, but it can happen.
For example, at a British TV transmitter a few years ago they were using
a flat coil of RG213 to dump some unwanted driver power at 800MHz. It
operated almost continuously for a few years, just "comfortably warm"...
and then mysteriously went short. The cable still looked completely
normal - to find the fault, they had to cut it up into short sections.
Semi-airspaced cables like 9913 are particularly bad for this, because
the center conductor moves as soon as you bend it, and then there isn't
much further to travel through the thin sleeve. When I tried to use 9913
to carry 1.5kW at 432MHz around a rotator, it worked fine for a few
hours and then went short (not hard to find where - it blew a hole in
the cable). The solution was to change to Andrew FSJ4 Superflex, which
has a slightly larger center conductor (so runs cooler) but more
importantly it has semi-rigid PTFE foam insulation which is not prone to
creepage.
However, I wouldn't be so confident about the soft foam polyethylene in
typical RG8-X. If that gets over-warm, it could collapse very quickly.
If you want to put lots of power through it, especially at higher
frequencies, then avoid sharp bends.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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