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Author Topic: RG-8X Power Handling Capacity?  (Read 36709 times)
AA4HA
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« Reply #30 on: November 10, 2014, 12:47:22 PM »

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.
The corona effect at high altitude where breakdown at high voltage first became an issue when aircraft were equipped with radios. What worked great at ground level would suddenly begin to arc and spark at 20,000 feet.

A worse problem was that things would just not cool off very well. Even if it was -15 F at altitude, if you did not force massive amounts of icy air into the radio those components would just burn up. The partial atmospheric pressure was acting as a thermal insulator.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W8JX
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« Reply #31 on: November 10, 2014, 01:39:44 PM »

The partial atmospheric pressure was acting as a thermal insulator.

Not really a insulator but rather the thinner air lacked thermal mass. It requires a higher flow rate to make up for reduced air mass. It is not a big problem.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W9FIB
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« Reply #32 on: November 10, 2014, 05:00:49 PM »

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.

Not even close to a fair comparison. Coax being pushed to its limits is far different then a home appliance or electronic device that run normally does not approach its operating limits.

What a clown!
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73, Stan
Wisdom is knowledge you gain after you know it all.
N6AJR
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« Reply #33 on: November 14, 2014, 01:26:06 PM »

On B 52 G's we used ram air at several hundred MPH to cool the gear. large, aerodynamic scoops on the side of the bird leading directly to the radiators on the gear.  works except when they did low level flights and got birds and grass and such in the scoops.  I hated them doing low level approaches. Gear was electronic countermeasures type, all classified, but air cooled.
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W8JX
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« Reply #34 on: November 14, 2014, 02:02:50 PM »

On B 52 G's we used ram air at several hundred MPH to cool the gear. large, aerodynamic scoops on the side of the bird leading directly to the radiators on the gear.  works except when they did low level flights and got birds and grass and such in the scoops.  I hated them doing low level approaches. Gear was electronic countermeasures type, all classified, but air cooled.

You know when they first took 52's low level, the damage a few from speed as they found they could not go as fast as they thought. As I recall they wanted to do 425 but had to limit it to 400.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
WN2C
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« Reply #35 on: November 17, 2014, 10:06:12 AM »

I have witnessed a ham using RG8X on a perfectly matched system, dipole 75 feet high tuned, on 75M with 2500 Watts CARRIER. The cable was a little warm. He was using the AM mode back in 1994. So figure 2500 Watts times 4 to give you P.E.P.
Fred


So your friend was violating part 97 by running 2500 watts carrier on AM?  Does that mean also that he was running 10,000 watts PEP on SSB?

Just curious

Rick  WN2C
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W8JX
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« Reply #36 on: November 17, 2014, 10:13:58 AM »

I have witnessed a ham using RG8X on a perfectly matched system, dipole 75 feet high tuned, on 75M with 2500 Watts CARRIER. The cable was a little warm. He was using the AM mode back in 1994. So figure 2500 Watts times 4 to give you P.E.P.
Fred


So your friend was violating part 97 by running 2500 watts carrier on AM?  Does that mean also that he was running 10,000 watts PEP on SSB?

Just curious

Rick  WN2C

As much as I like 8x for some things, I think that tail is a bit tall. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
PA1ZP
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« Reply #37 on: November 17, 2014, 10:20:04 AM »

Hi

I ran 1KW on my groundplane for 20 mtrs for years with RG58 both CW and SSB.
ran the same type RG58 to my full size vertical for 80 and GP for 40 mtrs.
Never a problem with RG58 and 1KW.

Problems only are there with higher bands as 20 mtrs and high SWR on cable.
With full size monobands never had a problem.

73 Jos
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W8JX
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« Reply #38 on: November 17, 2014, 10:22:45 AM »

Hi

I ran 1KW on my groundplane for 20 mtrs for years with RG58 both CW and SSB.
ran the same type RG58 to my full size vertical for 80 and GP for 40 mtrs.
Never a problem with RG58 and 1KW.

Problems only are there with higher bands as 20 mtrs and high SWR on cable.
With full size monobands never had a problem.

73 Jos

They make a RG58 sized cable that will handle 5kw on HF. Its called RG142 with a Teflon dielectric. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W0BTU
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« Reply #39 on: November 17, 2014, 01:05:19 PM »

I have witnessed a ham using RG8X on a perfectly matched system, dipole 75 feet high tuned, on 75M with 2500 Watts CARRIER. The cable was a little warm. He was using the AM mode back in 1994. So figure 2500 Watts times 4 to give you P.E.P.
Fred

As much as I like 8x for some things, I think that tail is a bit tall.  

Maybe not. I saw 10,000 watts on CW through RG-8 (it was slightly warm, but not hot at all). It depends on the band and the ambient temperature.

Remember that manufacturer's coax power ratings assume 100% duty cycle, and that the coax might be outside in the hot afternoon sun. Indoors where it's cool, the power handling ability goes up quite a bit.

And I think that small PTFE coax is good for more than 5 kW. More like 9 kW, depending on the band. Reason being, PTFE will withstand 500-600 degrees F.
« Last Edit: November 17, 2014, 01:11:39 PM by W0BTU » Logged

AA4HA
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« Reply #40 on: November 17, 2014, 01:21:43 PM »

It is a moot point. You may have neat dielectric on the cable but a PL-259 connector is only rated to 500 volts (peak), 1000 volts RMS.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W8JX
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« Reply #41 on: November 17, 2014, 04:29:34 PM »

It is a moot point. You may have neat dielectric on the cable but a PL-259 connector is only rated to 500 volts (peak), 1000 volts RMS.

Well 500 volts is 5kw in 50 ohms  and 1000 volts is 20 kw.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
WA7PRC
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« Reply #42 on: November 17, 2014, 05:27:44 PM »

I have witnessed a ham using RG8X on a perfectly matched system, dipole 75 feet high tuned, on 75M with 2500 Watts CARRIER. The cable was a little warm. He was using the AM mode back in 1994. So figure 2500 Watts times 4 to give you P.E.P.
Fred

As much as I like 8x for some things, I think that tail is a bit tall.  

Maybe not. I saw 10,000 watts on CW through RG-8 (it was slightly warm, but not hot at all). It depends on the band and the ambient temperature.

Remember that manufacturer's coax power ratings assume 100% duty cycle, and that the coax might be outside in the hot afternoon sun. Indoors where it's cool, the power handling ability goes up quite a bit.

And I think that small PTFE coax is good for more than 5 kW. More like 9 kW, depending on the band. Reason being, PTFE will withstand 500-600 degrees F.
Duty cycle is a good point. I've run 1KW from my SB-220 thru RG-58C/U (Mil-C-17, solid PE dielectric) at 14 MHz into a Cushcraft A3S on SSB and CW (about 75 feet). IIRC, that cable is capable of handling about 250W AVERAGE power at up to 30 MHz. Since human voice and Morse are around 25% D/C, running 1000W at 25% D/C (barely) works. In fact, after running a frequency in a contest, the feedline seemed warm, and it never failed. However, because there was NO margin, I don't recommend it.  Wink

Unless the OP intends to run high power using RTTY, FM (or other high duty cycle mode), and/or loads nowhere near 50Ω j0 and/or very long runs and/or higher frequencies, RG8X likely will be fine.

vy 73,
Bryan WA7PRC
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KH6DC
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« Reply #43 on: November 21, 2014, 03:13:52 PM »

It can easily handle legal limit as I have several times.
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73 and Aloha,
de Delwyn, KH6DC
W4VR
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« Reply #44 on: November 22, 2014, 06:27:56 PM »

You sound like a newbie.  If you have a high SWR on the transmission line feeding the antenna you can't run much power.  However if the SWR is low then you can run the legal power limit into the coax.  Using a tuner at the radio shack end has nothing to do with the coax power handling capability.
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