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Author Topic: RG-8X Power Handling Capacity?  (Read 36696 times)
KG9SF
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Posts: 282




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« on: November 07, 2014, 12:39:03 PM »

I use a vertical on 40M, fed with RG-8X.  I've never before run more than 100 watts through it.
Now I have an amp, and I'm curious how much power I can put through this coax safely.

I'll appreciate any remarks or suggestions.

Thanks, 73,
Scott. KG9SF
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W1BR
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Posts: 4179




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« Reply #1 on: November 07, 2014, 12:47:52 PM »

So long as the SWR is reasonable, it should be fine... W8JI once commented that all the test jig cables at Ameritron used RG-8X. High SWR might lead to destructive current or voltage levels, however.

Pete
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KG9SF
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« Reply #2 on: November 07, 2014, 12:51:46 PM »

Thanks Pete.  My amp has an automatic antenna tuner onboard, so SWR shouldn't be a problem (fingers crossed).
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KG4NEL
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« Reply #3 on: November 07, 2014, 12:54:45 PM »

Thanks Pete.  My amp has an automatic antenna tuner onboard, so SWR shouldn't be a problem (fingers crossed).

Are you just using the tuner to get CW or SSB subband coverage, or trying to go multiband with it?

The former shouldn't be a problem, but the latter might be Smiley
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AA4PB
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« Reply #4 on: November 07, 2014, 12:58:12 PM »

Thanks Pete.  My amp has an automatic antenna tuner onboard, so SWR shouldn't be a problem (fingers crossed).

The tuner won't lower the SWR on the feed line. It only lowers the SWR on whatever coax is between the tuner and the amp. If the SWR was too high on the RG8X feed line before, it will still be high with the tuner.

Here's the Belden spec:
1KW at 10 MHz with 1:1 SWR
370W at 50 MHz with 1:1 SWR
Maximum voltage = 300V RMS
« Last Edit: November 07, 2014, 01:03:51 PM by AA4PB » Logged

Bob  AA4PB
Garrisonville, VA
W8JX
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Posts: 13268




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« Reply #5 on: November 07, 2014, 01:40:04 PM »

Thanks Pete.  My amp has an automatic antenna tuner onboard, so SWR shouldn't be a problem (fingers crossed).

That tuner only matching amp to feed line and does not lower SWR on coax itself. As far as power. I would not run over a KW on RTTY long term but for SSB 1 kw or a bit more is not a problem at all on HF.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
N3DT
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« Reply #6 on: November 07, 2014, 02:23:29 PM »

I run my AL80B on RG8X all the time, but the swr is  well below 2:1. I've even used the 80B with RG6 and F connectors but the swr is real low on that one too. As said, the tuner won't change the swr on the feed line, it only makes it look good to the TX feed point. Voltage can be calculated with SWR and P.
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KC4MOP
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« Reply #7 on: November 07, 2014, 04:50:12 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
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KG4NEL
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« Reply #8 on: November 07, 2014, 05:43:36 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


Hence why you don't see 7/16" DIN adapters for RG-8X all that much  Tongue
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GM3SEK
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Posts: 99




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« Reply #9 on: November 08, 2014, 12:33:51 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

Average power is a better measure than PEP because the loss comes almost entirely from heating of the center conductor. Softening of the foam dielectric can then lead to an internal short.

The reason why SWR is important is that it creates current maxima along the line where the internal heating problems will be far worse (because heat release is proportional to I-squared). On the other hand, voltage breakdown due to elevated SWR is almost never a problem.

But the biggest problem of all about "RG8X" is the huge variation in quality between different brands. The size of the center conductor should be fairly dependable, but the quality and stability of the foam, density of braid coverage, ruggedness of the outer jacket and all-around mechanical quality can all be highly variable.

Much as I would like to say "Find a good brand and stick to it", even that isn't possible because even reputable vendors have to contend with the same problems from their  own suppliers.

73 from Ian GM3SEK
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W1BR
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« Reply #10 on: November 08, 2014, 08:21:26 AM »

Here's an earlier post by W8JI:

W8JI
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a question of coax size?
« Reply #11 on: October 28, 2004, 02:42:18 AM »
Reply with quoteQuote
I've been very sucessful with RG8X, although I've never used it at 1500 watts on six meters or on runs where loss is important. 

http://www.w8ji.com/my_shack.htm

I use hundreds of feet of RG8X as downleads on my low band antennas. I almost always run 1.5kW CW.

We use it as jumpers in testing amplifiers also, at 1.5kW and more. Some of those cables are almost 20 years old.

I use it for HV leads in the test lab, and it breaks down at over 8kV.

73 Tom

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AA4PB
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« Reply #11 on: November 08, 2014, 08:31:18 AM »

W8JI says over 8KV. Belden says 300V. Take your pick.
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Bob  AA4PB
Garrisonville, VA
W1BR
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« Reply #12 on: November 08, 2014, 08:40:14 AM »

I also think 8 kV is a bit over the top... I use a lot of RG-8X for jumpers inside the shack. Once the RF hits the conduit going outside, it becomes 1/2" and 3/4" hardline to the antenna rotator. 

I've damaged RG-213 when accidentally running high power into the wrong antenna.
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N6AJR
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« Reply #13 on: November 08, 2014, 08:43:38 AM »

Rule of thumb, 800 watts intermittent, typical use,
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W8JX
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Posts: 13268




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« Reply #14 on: November 08, 2014, 11:35:08 AM »

Rule of thumb, 800 watts intermittent, typical use,

RG 58 will handle 800 intermittent. A KW or a bit on 8x on HF on SSB is really not a problem at all on 15m and below.   
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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