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Author Topic: Coax  (Read 5052 times)
N9TLU
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Posts: 35




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« on: April 05, 2012, 11:14:31 AM »

Setting up my station and need to get some new coax.  Obviously, I can get a much better price buying a spool.  I need some opinions on the TYPE of coax to use.

I'm looking for cable that is flexible enough to be used on my beam for HF and will not have a huge loss on VHF/UHF.

Does this elusive product exist?

Scott
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W8JX
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« Reply #1 on: April 05, 2012, 01:10:07 PM »

What is lossy and what is not is also dependent on length of runs involved.
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WX7G
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« Reply #2 on: April 05, 2012, 04:58:54 PM »

RG-8 or RG-213.

Don't be fooled by RG-8X. It is neither loss loss nor can it handle much power.
« Last Edit: April 10, 2012, 03:29:50 PM by WX7G » Logged
K9KJM
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« Reply #3 on: April 05, 2012, 10:39:27 PM »

As pointed out, The "right" type of coax depends on several things, Including overall length, Frequency, And power level.

For HF use at reasonable lengths of 100 or so feet, Good quality RG 213 like from Belden is considered the top type to use. (Super tough stuff!)

For VHF and UHF, lengths up to 75 or so feet, TIMES LMR 400 is a top pick nowadays.

Something like LMR 400 UF (Ultra flex) could be used for everything, But it is nowhere near as tough as a solid center dielectric like RG 213, Plus is quite expensive.

In the end, It us usually simpler and cheaper to just use several types of coax.
"Plain" solid center conductor LMR 400 (Low cost) for longer runs at VHF/UHF, Short jumpers of LMR 400UF if needed around rotors, RG 213 for higher power HF, And even some quality RG8X for shorter lower power runs.
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W8JX
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« Reply #4 on: April 06, 2012, 08:39:56 AM »

RG8x will easily handle a KW on shorter runs on HF below 25mhz or so.
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WX7G
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« Reply #5 on: April 06, 2012, 02:22:04 PM »

RG-8X can handle power inside where it's cool and there is no solar loading (heating due to sunlight) but outside be careful. Outdoors at 28 MHz RG-8X can handle only 150 watts.

Here's the data:

The Belden power rating for RG-8X at 10 MHz is 1000 watts and it's 370 watts at 50 MHz. The power rating at 28 MHz is close to 500 watts. I could not find the ambient temperature for which this is valid so let's run a thermal analysis. RG-8X uses polyethylene insulation and the melting point is just about 100 deg C.

The operating temperature range of RG-8X is -40 to +80 deg C. By my calculations the jacket heat rise at 1.5 kW at 28 MHz is 35 deg C (the center conductor will be hotter). A black coaxial cable illuminated by noon sun can reach a surface temperature of 170 deg F when the air temperature is 100 deg F. 170 deg F is 77 deg C. So, RG-8X cable outside under these conditions can handle RF power such that the temperature rises by 4 deg C. That is 150 watts average at 28 MHz.

At 1.5 kW the heat rise of the cable jacket will be 40 deg C over the solar loading temperature of 77 deg C giving a jacket temperature of 117 deg C. The inner conductor will be hotter (I can calculate this but don't need to for this illustration) and will melt the polyethylene and may physically migrate toward and touch the shield, shorting out the cable.

« Last Edit: April 06, 2012, 02:35:48 PM by WX7G » Logged
W8JX
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« Reply #6 on: April 06, 2012, 03:31:08 PM »

RG-8X can handle power inside where it's cool and there is no solar loading (heating due to sunlight) but outside be careful. Outdoors at 28 MHz RG-8X can handle only 150 watts.

Here's the data:

The Belden power rating for RG-8X at 10 MHz is 1000 watts and it's 370 watts at 50 MHz. The power rating at 28 MHz is close to 500 watts. I could not find the ambient temperature for which this is valid so let's run a thermal analysis. RG-8X uses polyethylene insulation and the melting point is just about 100 deg C.

The operating temperature range of RG-8X is -40 to +80 deg C. By my calculations the jacket heat rise at 1.5 kW at 28 MHz is 35 deg C (the center conductor will be hotter). A black coaxial cable illuminated by noon sun can reach a surface temperature of 170 deg F when the air temperature is 100 deg F. 170 deg F is 77 deg C. So, RG-8X cable outside under these conditions can handle RF power such that the temperature rises by 4 deg C. That is 150 watts average at 28 MHz.

At 1.5 kW the heat rise of the cable jacket will be 40 deg C over the solar loading temperature of 77 deg C giving a jacket temperature of 117 deg C. The inner conductor will be hotter (I can calculate this but don't need to for this illustration) and will melt the polyethylene and may physically migrate toward and touch the shield, shorting out the cable.



We have been down this road before. I have used RG8x for many years on and off and a kw is NO PROBLEM. (I have never loaded amp on 10m and do only do barefoot there) Your Belden ratings are way off. 
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WX7G
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« Reply #7 on: April 06, 2012, 07:08:11 PM »

I did some digging and found a plot of coaxial cable power derating vs. temperature.

http://www.timesmicrowave.com/products/tl14/downloads/71.pdf

For Foam Polyethylene cable (like RG-8X) the power derates to 37% at 77 deg C. Being that Belden rates RG-8X for 500 watts at 28 MHz, when it is at 77 deg C the rating drops to 185 watts. That is pretty close to the 150 watts I calculated using engineering thermal calculations (RF-to-thermal energy, free air convection cooling and solar loading).  

One kW OOK CW or SSB is within the manufacturer's max rating up to 28 MHz when the cable is in a room temperature environment. I use RG-8X in the shack at 1200 watts. But try to use it outdoors at these power levels and you risk failed cable.

Here is another interesting applications note on coaxial cable temperature and solar loading derating:

http://www.rfsworld.com/userfiles/pdf/Transmission_Line_Tech_Section.pdf
« Last Edit: April 06, 2012, 07:45:08 PM by WX7G » Logged
W8JX
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« Reply #8 on: April 06, 2012, 07:36:42 PM »

I did some digging and found a plot of coaxial cable power derating vs. temperature.

http://www.timesmicrowave.com/products/tl14/downloads/71.pdf

For Foam Polyethylene cable (like RG-8X) the power derates to 37% at 77 deg C. Being that Belden rates RG-8X for 500 watts at 28 MHz, when it is at 77 deg C the rating drops to 185 watts. That is pretty close to the 150 watts I calculated using engineering thermal calculations (RF-to-thermal energy, free air convection cooling and solar loading).  

One kW OOK CW or SSB is within the manufacturer's max rating up to 28 MHz when the cable is in a room temperature environment. I use RG-8X in the shack at 1200 watts. But try to use it outdoors at these power levels and you risk failed cable.

If this was really the case I would have melted some years ago. These rating remind me of wire ratings for wire in bundles or conduits when are much lower than single wire/cable runs.
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WX7G
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« Reply #9 on: April 07, 2012, 08:54:25 AM »

I think what allows hams to run RG-8X so hard is the number of hours we put on it. Time is needed to damage cable.

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W8JX
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« Reply #10 on: April 07, 2012, 10:57:47 AM »

I think what allows hams to run RG-8X so hard is the number of hours we put on it. Time is needed to damage cable.



RG8x is more "HD" power wise than 58 and 58 will take some abuse too. I not sure were Belden is getting their numbers but they are way low for that cable.
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WX7G
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« Reply #11 on: April 10, 2012, 08:06:57 AM »

Belden rates RG-58A/U for higher power than RG-8X


Belden rates RG-58A/U for 1937 watts at 10 MHz and 782 watts at 50 MHz.  
http://www.dxengineering.com/pdf/Belden%20RG8X%20Date%209258.pdf


Belden rates RG-8X for for 1000 watts at 10 MHz and 370 watts at 50 MHz.
http://www.dxengineering.com/pdf/Belden%20RG8X%20Date%209258.pdf


Why is RG-8X rated for lower power when it has less RF loss? The path for heat flow from the center conductor to the cable jacket is through the dielectric. RG-8X has a foamed PE dielectric while RG-58A/U has a solid PE dielectric and solid PE has 3 times the thermal conductivity. Everything else being equal, at HF, foam coax has 1/3 the power handling ability. Given that at 10 MHz RG-8X has 0.8X the loss of RG-58A/U and RG-58A/U has 3X the thermal conductivity RG-58A/U can handle 3 X 0.8 = 2.4X the power. Now let's account for the thermal conductivity to air. RG-58 has 0.8X that of RG-8X (due to the smaller diameter). Now the power handling factor becomes 3 X 0.8 X 0.8 = 1.9. This agrees with the Belden rating of 1.9X the power for RG-58A/U.


« Last Edit: April 10, 2012, 03:27:17 PM by WX7G » Logged
K8GU
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« Reply #12 on: April 10, 2012, 09:06:22 AM »

There is a lot of junk coax out there.  Everything I can tell you will be in this document:

http://www.audiosystemsgroup.com/Coax-Stubs.pdf

I have a lot of The Wireman's CQ118 in service, but you probably want something like BurryFlex from Davis RF to get lower loss at VHF.  I use a Belden RG-8 for rotator loops and LMR-600 and 1/2-inch Heliax on VHF.
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