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Author Topic: Power Capacity of Coax  (Read 3113 times)
VE3QN
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Posts: 7




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« on: September 15, 2004, 07:53:32 PM »

I am considering adding an amplifier.

I currently run my TS-940 barefoot into my antennas using RG-8X for one and RG-59/U for the other.

Can I keep these transmission lines if I were to run 800 watts PEP? If not, what would be the most practical replacement coax cables?

Thanks in advance.

Bryan  VE3QN
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WA9SVD
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Posts: 2198




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« Reply #1 on: September 15, 2004, 08:21:30 PM »

    RG-213/U cable would be a much better choice for the 800 Watt power level.  (Or RG-11/U for 75 Ohm systems.)  The power handling ability of the smaller cables will vary with frequency, and power handling charts will only show the capability in a well matched system.  If you experience a mismatch between the Amp. and antenna, you could exceed the breakdown voltage of the smaller size cables, especially foam insulated types such as RG-8X.  I think it would be prudent to replace your feedline with more substantial cable.  It's a small investment compared to the price of the amp.  Good luck.
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VE3QN
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Posts: 7




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« Reply #2 on: September 16, 2004, 09:18:21 AM »

Just the answer I was looking for. I'll upgrade the transmission lines first.

Thanks and 73,

Bryan  VE3QN
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WB2WIK
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Posts: 21837




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« Reply #3 on: September 16, 2004, 12:39:44 PM »

Others will disagree, but I do agree that neither RG8X nor much of the "RG59" around will handle a kilowatt under some conditions.

RG59/U mil-spec stuff with a solid poly dielectric will handle a kW pretty well through the HF bands.  RG8X has larger conductors and looks as though it *should* handle a kW easily, except a lot of it doesn't.  Depends on how you handle it, and if you're using it under mismatch conditions or not.

I can demonstrate (and have, many times) that I can absolutely, positively make any brand RG8X fail in less than an hour with 1 kW applied power into a 50 Ohm dummy load (perfect match), if I wind it up into a tight balun and leave it laying out in the southern California sunshine on a hot day.  I've never found any kind of RG8X that could take this.

The same line rolled out straight, or in the shade, survives just fine.

I'd switch to RG213/U in your case, just because.  The cables you have may survive many years with the new amplifier, but Murphy's Law dictates that when one finally fails, it will blow up your amplifier and also occur after an ice storm, or on a day when there's 70 mph winds.

Why tempt Murphy?

WB2WIK/6
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