[TowerTalk] 75 ohm cable power handling ?

[jimlux]

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
> 
> https://catalog.belden.com/techdata/EN/9104_techdata.pdf
> 
> 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?