[TowerTalk] 75 ohm cable power handling ?
jimlux at earthlink.net
Sat Jan 6 07:51:26 EST 2018
On 1/6/18 2:09 AM, Jim Brown wrote:
> On 1/6/2018 1:41 AM, Martin Sole wrote:
>> Anyone have an idea of the 30MHz power handling capability for RG6A/U
>> and Belden 9104?
> RG-numbers are essentially meaningless -- the tell us nothing more than
> approximate size and Zo. The Belden data sheet
> tells us that 9104 is a CATV cable, CCS center, shield is Al braid and
> Al foil. Your guess is as good as mine for power handling. It's designed
> for low power CATV systems where the signals are mostly UHF. It tells us
> that DCR (resistance at DC) of the center is 4.45 ohms/100 ft, and 1.2
> ohms for the shield. These values will increase (a lot) with frequency.
> My WAG (Wild Assed Guess) is that these cables will be OK for 100W into
> matched loads, but I'd be cautious with high power or poorly matched
> loads. And remember -- the match that matters is that between the
> antenna and the line.
0.9 dB/100ft, so 0.009 dB/ft - that's about 0.2% 1000W * 0.002 is 2W/ft
As Jim noted, that's in a matched system - Think of it as a series of
little resistors - 2 W/Ft is about 0.2 watts/inch. I think that will
get noticeably warm.
Another way to look at it is that the center conductor is 20 gauge
(copper coated steel). 1 kW at 75 ohms is 4 amps rms. Are you
comfortable running 4A through a steel 20 gauge wire with 1/8" of
polyethylene foam insulation?
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