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Author Topic: The RG-8X myth  (Read 7060 times)
WX7G
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« on: April 11, 2012, 01:36:15 PM »

RG-8X power handling; real or myth?


Belden rates RG-58A/U for 1937 watts at 10 MHz and 782 watts at 50 MHz.  
http://www.belden.com/techdatas/english/88240.pdf

Belden rates RG-8X for for 1000 watts at 10 MHz and 370 watts at 50 MHz.
http://www.belden.com/techdatas/metric/9258.pdf

If this data is correct it means that RG-58A/U can handle higher power than RG-8X.

I found out what's going on and it is explained in a later post.

Belden's power rating method is found here: http://www.belden.com/pdfs/Techpprs/CoaxialCablesandApplications.pdf

"The power rating is based upon the permissible rise in temperature above ambient, and is mostly dependent upon the temperature properties of the dielectric material. The values typically assume a low RL or VSWR value and 20C ambient temperature in air. Burying cables, or other installation variances, will alter this capability."

The power handling of a cable is inversely proportional to the VSWR. Operate a cable that is rated for 1000 watts becomes into a 2:1 VSWR and it becomes 500 watt cable. Run the cable at a high ambient temperature (outside in the sun) and the power handling drops again.

« Last Edit: April 12, 2012, 11:03:53 AM by WX7G » Logged
W8JX
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« Reply #1 on: April 11, 2012, 01:55:50 PM »

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

These power ratings drop with increasing ambient temperature or with solar illumination.

Why the difference? RG-58A/U has a solid polyethylene (PE) inner dielectric while RG-8X has foamed PE and solid PE has higher thermal conductivity.


If there was any real merit to this, there would be a LOT of melted RG8x out there. Your link in the ONLY place I found that rates that cable that low.
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WX7G
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« Reply #2 on: April 11, 2012, 02:14:49 PM »

I compared RG-8X from Belden against RG-58A/U from Belden because I assume both cable types were rated using the same method.

I would like to see data from other companies that supply a power rating for both cable types.

We can calculate the temperature of the dielectric using this:

http://drajput.com/notes/electronic_properties/coaxial_cable_thermal_conductivity.php

RG-8X has foamed PE which has a low thermal conductivity than RG-58 which has solid PE.
« Last Edit: April 11, 2012, 03:59:49 PM by WX7G » Logged
W8JX
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« Reply #3 on: April 11, 2012, 06:36:37 PM »

RG-8X has foamed PE which has a low thermal conductivity than RG-58 which has solid PE.

But, 8x also has a slight larger inner conductor diameter as well as a somewhat larger outer diameter to maintain proper impedance (same as RG59) Which means it can carry more current before it develops same amount of internal heating as 58 does.
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N3QE
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« Reply #4 on: April 12, 2012, 05:31:51 AM »

Note that in part they quote UL compliance, and there are multiple different UL ratings for any cable depending on a large number of factors. Single cable in free air, single cable in some size conduit, multiple cable in some size conduit, multiple cable in cable tray, etc. You are perfectly correct, without knowing the exact paragraph from the rating agency, you can never tell which one you're dealing with.

Specific RG-58 and RG-59 type cables do have UL ratings for plenum I know. It's not a generic catch-all value, it varies from brand to brand and between different varieties in each brand. I'm sure Belden sells a dozen different varieties of RG-58 with different UL and plenum ratings for example.

If there is a myth, it's that RG-8X is RG-8X is RG-8X. For any given popular coax "type" there are dozens of different detailed formulations within that type. In some cases the differences can be very extreme (e.g. so many different kinds of RG-6 out there today which do not even share external dimensions, it is perverse and often boggling.)
« Last Edit: April 12, 2012, 05:33:50 AM by N3QE » Logged
WX7G
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« Reply #5 on: April 12, 2012, 05:49:51 AM »

RG-8X has foamed PE which has a low thermal conductivity than RG-58 which has solid PE.

But, 8x also has a slight larger inner conductor diameter as well as a somewhat larger outer diameter to maintain proper impedance (same as RG59) Which means it can carry more current before it develops same amount of internal heating as 58 does.

The datasheets do seem to defy common sense. I ran more detailed thermal calculations and they say that RG-8X can handle higher power. For the thermal conductivity of foamed PE I am estimating it from solid PE times 1/3. Is one of the Belden datasheets wrong? The power dissipated by RG-8X is 2/3 that of RG-58A/U. And as you point out the outer diameter is 20% larger thereby raising the cable-to-air thermal conductivity by 20%. If the datasheets are correct the difference in power handling lies in the dielectric thermal properties.

I think I'll have to fire up both cable types at DC and run some measurements. Using the "change of resistance" method I can measure the temperature of the inner conductor which is the same temperature as the PE touching it. I'll call Belden and see if I can get ahold of an aps engineer and ask him/her about this.

« Last Edit: April 12, 2012, 05:56:15 AM by WX7G » Logged
WX7G
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« Reply #6 on: April 12, 2012, 10:02:42 AM »

I took a careful look at several Belden datasheets. Belden makes many types of RG-58 and a few types of RG-8X. The differences are the inner conductor size, dielectric type, braid coverage, and jacket material. Here's some data condensed with the power ratings at 50 MHz.

I compared the RG-8X that DX Engineering sells. It is Belden 9258 and is a rather low (370 W at 50 MHz) power rated RG-8X, apparently because the dielectric is FPE and not HDPE. HDPE has a higher melting temperature. HD is high density PE.

I had inadvertently listed the power rating for Belden 82240 RG-58 which has a solid FEP Teflon dielectric. This can operate at high temperatures hence the high power rating.

The foam HDPE dielectric RG-58 coaxial cables all handle higher power than the RG-8X sold by DX Engineering.


Belden 82240, RG-58A/U, 782W, #20 conductor, solid FEP Teflon

Belden 7807A, RG-58, 813W, #17 conductor, Foam HDPE (High Density Polyethylene)

Belden 7806A, RG-58, 694W, #19 conductor, Foam HDPE

Belden 9258, RG-8X, 370W, #16 conductor, Foam FPE

Belden 7808R, RG-8X, 1186W, #15 conductor, Foam HDPE
« Last Edit: April 12, 2012, 10:10:20 AM by WX7G » Logged
W8JX
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« Reply #7 on: April 12, 2012, 10:15:14 AM »

Want to see some small 58 sized cable with a very high power rating? Check out RG142. We used it a lot in flight test for many years and also used RG393 as well. It is a difficult cable to terminate at times but easier with practice and both are VERY tuff cables and nearly indestructible in normal use. 
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WX7G
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« Reply #8 on: April 12, 2012, 10:54:27 AM »

That RG-142 looks like great stuff.

I use RG-174 in the car at 100 watts and the DX Engineering RG-8X at home at 1200 watts and it all has held up. 

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W8JX
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« Reply #9 on: April 12, 2012, 11:57:23 AM »

That RG-142 looks like great stuff.

We used it up to a few ghz for shorter runs. We used 174 on BNC patch panels and frequencies below 100mhz.
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N8TI
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« Reply #10 on: April 14, 2012, 04:56:25 AM »

I imagine that with CW, you could exceed the rated wattage because of the intermittent power flow.

Joe
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W8JX
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« Reply #11 on: April 14, 2012, 07:27:21 AM »

I imagine that with CW, you could exceed the rated wattage because of the intermittent power flow.

Joe

Not likely as it is not a fuse. it takes time of constant power being applied to raise temperature of cable due to resistance losses.
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N3QE
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« Reply #12 on: April 15, 2012, 04:22:07 AM »

Want to see some small 58 sized cable with a very high power rating? Check out RG142. We used it a lot in flight test for many years and also used RG393 as well. It is a difficult cable to terminate at times but easier with practice and both are VERY tuff cables and nearly indestructible in normal use. 
And their dielectric? Teflon! They handle about 10 times the power of foam and poly dielectric coax, and cost about 10 times as much per foot.

What I like about the Teflon cables is that the dielectric never melts when exposed to soldering temperatures for long long periods, and even away from the temperature extremes it has a natural slipperiness and concentricity that foam and PE don't have, and even after 50 years teflon jacketed cables never are showing any deterioration. The reason I love teflon is not necessarily the RF properties but its utilitarian toughness.

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WX7G
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« Reply #13 on: April 15, 2012, 06:29:52 PM »

And cooking with Teflon coax is great because it's non-stick.
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W8JX
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« Reply #14 on: April 15, 2012, 07:51:09 PM »

RG 393 would be the coax to bury in yard. 50 years direct burial would be nothing for that cable
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