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Author Topic: RG-8X Power Handling Capacity?  (Read 36691 times)
AA4HA
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Posts: 2630




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« Reply #15 on: November 08, 2014, 12:21:38 PM »

Looking at a thermograph (picture taken in the infrared) of coax is really interesting. Particularly at higher frequencies where the wavelengths are shorter and the coaxial losses are higher.

If I had access to a multi-kilowatt 450 MHz amplifier I would redo the setup and put it up on Youtube.

With a very close impedance match from amp to feedline and to load you could see the warm spots on the coax. It looked like a striped snake.

The warm spots were due to the natural insertion loss of the coax cable. We did not use RG-8x but if we had that would of been 7.85 dB/100'. Those losses are converted into thermal energy (heat) and a little bit of radiation (due to shielding efficiency).

When we mismatched the cable (we used a coaxial tee and put two loads on the cable so it was seeing around 25 ohms impedance) the SWR high. The thermograph also went a little crazy too. Hot spots appeared everywhere.

We stopped our experiment because the amplifier did not like it and it was messing with the fluorescent lighting in the building.

The heating due to attenuation and the heating due to the SWR mismatch made the coax act like a imperfect heating element.

I could see that on systems where you have a really significant mismatch and a kilowatt that you could get some heating that might damage the cable or cause arching.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AA4PB
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Posts: 15022




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« Reply #16 on: November 08, 2014, 12:45:12 PM »

Remember, there are a number of variables that affect the power handling ability of coax cable such as the operating frequency, the SWR, the air circulation, the ambient temperature of the air, heating of the coax exposed to the sun, how the connectors are installed, and manufacturing differences. Just because one person has gotten away with running 1.5KW to RG8x doesn't guarantee that your particular installation will be successful. For example, the person with RG8x exposed to Alaska outside temperatures can likely get away with a lot more power than someone who has it exposed to the hot summer Arizona sun or someone who has it run inside of insulated walls in his house.

300W is probably pretty safe up to 30MHz given all the possible variables. RG-213 isn't that expensive when compared to the cost of repairing an amp because the coax shorted out.

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Bob  AA4PB
Garrisonville, VA
WB6BYU
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Posts: 18389




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« Reply #17 on: November 08, 2014, 03:00:33 PM »

And don't forget the mechanical issues with foam core coax.
When it gets warm (in the sun or due to power dissipation) the
center conductor can drift through the softened insulation,
especially at bends. Wind a feedline choke on too small of a
diameter form and it may develop a short after handling high
power for months. Physical damage from being crimped or a
zip tie cinched down too tight can also create a weak spot -
whether that is a problem depends on the standing wave
distribution on each band.

That doesn't mean you can't use it, just that it requires a bit
more attention to detail.  At least with jumper cables it is easier
to do a physical inspection and they aren't tied too tight to tower
legs out in the hot sun.
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K8AXW
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Posts: 7036




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« Reply #18 on: November 08, 2014, 04:54:53 PM »

I think this is the where the phrase, "Ignorance is bliss" applies!  Whatever happened to the old days when we used what we had?  If it fails, go bigger.

I recall the tail of a buddy getting started in ham radio an winding a coil on a Mother's Oats cardboard box.  It stated smoking so he determined that he needed something else to wind that particular coil on.

With ham radio and intermittent (on/off) type emissions, use the coax, whatever it is and see what happens!  Especially if it was all I had and couldn't afford bigger coax.

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A Pessimist is Never Disappointed!
KI6LZ
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Posts: 738




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« Reply #19 on: November 08, 2014, 05:16:14 PM »

I have RG-8X feeding my 40 meter antenna using 1300 watts pep on ssb and cw. No problems. SWR is mostly less than 1.5:1 going up to 2:1 at 7.3 Mhz. Many years ago used RG-59 on 10, 15, 20. Same power, no problems. Just my experience.
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SWL2002
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Posts: 895




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« Reply #20 on: November 08, 2014, 06:18:15 PM »

W8JI says over 8KV. Belden says 300V. Take your pick.


I'd go with W8JI if he is talking about 8 kV DC.  My Slaughter Hipot tester shows that the RG8X I have here can withstand over 9 kV.  I just tried it. 
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KH6AQ
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Posts: 7785




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« Reply #21 on: November 08, 2014, 06:56:26 PM »

Solid dielectric coax is used as HV cable; RG-58 to 15 kV and RG-8 to 70 kV. RG-8X has foamed dielectric and may experience partial discharge in the gas bubbles, leading to early failure.
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GM3SEK
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Posts: 99




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« Reply #22 on: November 09, 2014, 01:01:15 AM »

Solid dielectric coax is used as HV cable; RG-58 to 15 kV and RG-8 to 70 kV.

Sure, there is no problem in using those cables at extremely high voltages, provided that the ends are properly terminated in a HV connector with a long overlap of the center insulation.

The much lower figures that we sometimes see are either for DC breakdown across a straight-cut end, or else they may be RF voltages that have been back-calculated from the RF power rating.

Quote
RG-8X has foamed dielectric and may experience partial discharge in the gas bubbles, leading to early failure.
True, but the main problem with "RG-8X" is the huge variation in the quality of the foam, ranging from a stiff, dense foam with good mechanical properties to something that has the look and feel of marshmallow.


73 from Ian GM3SEK
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W8JX
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Posts: 13268




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« Reply #23 on: November 09, 2014, 04:46:47 AM »

Solid dielectric coax is used as HV cable; RG-58 to 15 kV and RG-8 to 70 kV. RG-8X has foamed dielectric and may experience partial discharge in the gas bubbles, leading to early failure.

Many years ago when I used to have horses I had a electric fence charger (horses have a lot of respect for them) that was mounted in barn and I used RG58 to feed it out of barn and to fence. Not sure of voltage but I think it was over 15kv as it was good for several miles of fence and it would knock you silly if you touched it and would kill grass growing into it most of the time. Never had a issue with 58 arcing thru. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KH6AQ
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Posts: 7785




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« Reply #24 on: November 09, 2014, 06:23:45 AM »

The failure mode for solid dielectric used at HV is inner dielectric damage starting from the shield inwards. This is caused by partial discharges (air breakdown) in the air gaps between the shield and the outside of the inner dielectric.
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G3RZP
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Posts: 1225




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« Reply #25 on: November 10, 2014, 02:48:59 AM »

WX7G has it. This is why HV power cables (11, 33, 66 kV over here, 3 phase plus neutral aluminium sheath), are jelly - used to be oil - filled.
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NO9E
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Posts: 882




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« Reply #26 on: November 10, 2014, 06:19:23 AM »

I use RG8X at legal limit and sometimes high SWR. I also loop the cable around a ferrite for a cheap balun by the feedpoint, creating stresses.  The only failures were at connectors. With RG8X my only worries are losses, not reliability.
Ignacy
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N3QE
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Posts: 5576




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« Reply #27 on: November 10, 2014, 07:37:39 AM »

RG-8X is great for matched loads, it will handle legal limit on the low bands just fine.

But for HV ratings... I used some -8X as HV jumpers in an antenna tuner of mine. Not even as coaxial transmission line, just as high voltage jumpers. In short order I experienced both the RG-8X inner dielectric and outer jacket arcing over at just a few hundred W (which could've well been several thousand volts easy, given the arbitrary load mismatch.) The -8X insulation performance was just horrible, arcing over at voltages that regular old house wire would've handled with ease.

I replaced the -8X jumpers with house wire and RG-58, no problem.

Tim N3QE
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WB2WIK
Member

Posts: 21836




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« Reply #28 on: November 10, 2014, 10:29:25 AM »

Remember, there are a number of variables that affect the power handling ability of coax cable such as the operating frequency, the SWR, the air circulation, the ambient temperature of the air, heating of the coax exposed to the sun, how the connectors are installed, and manufacturing differences.

Power handling is also effected by barometric pressure (altitude above sea level).  Cable that can handle a kW at sea level won't necessarily do that at 5,000 feet and a lot won't do that at 10,000 feet.

Also, big difference between indoors and outdoors, and depends where the "outdoors" is.  Here in the Southwest with 300+ intensely sunny days a year, stuff made of any kind of plastic generally doesn't last as long.  I had a 160m "choke type" current balun (it was about 70 turns of Belden RG8X wound on a glass pickle jar about 6" in diameter then taped to hold its form -- pickles removed first!) just laying on the ground in the yard near the cable entry point to the shack, to minimize common mode on the coax coming in, and it failed within one year, never running more than 1 kW CW or SSB.  The cable was literally melted.  I don't think the power did that, it was the combination of the incredibly hot summer sun + power, maybe mostly the sun.  Black cable doesn't reflect well and anything colored black, outside, in July through October, can easily reach 150 degrees.
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W8JX
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Posts: 13268




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« Reply #29 on: November 10, 2014, 11:36:39 AM »

Power handling is also effected by barometric pressure (altitude above sea level).  Cable that can handle a kW at sea level won't necessarily do that at 5,000 feet and a lot won't do that at 10,000 feet.

Generally you do not see any noticeable derating until you get above 8000 feet or so. Most home electronic devices are good to about 10,000 feet or so.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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