[TowerTalk] RG-8X power handling. Re: re coax jumpers help

[Jim Lux]

At 10:27 AM 10/20/2006, Terry Dunlap wrote:
>What kind of power handling capability does RG8X have?
>
>73 de Terry KK6T

I would imagine the limit is thermal dissipation in the center 
conductor.  I would guess that the center conductor is approximately 
the same size as RG-58 (being roughly the same physical size) or 
about AWG 20, perhaps slightly larger (foam dielectric, and all)

You can approach the power handling problem two ways: look at the 
loss/length (1dB/100 ft or so) and figure out how many watts per 
meter you're dissipating and decide if the dielectric is going to 
melt.  or, you can ballpark the RMS current and compare it to 
standard 60Hz wiring ampacity ratings (figuring that insulated wire 
is insulated wire)
It's a PE dielectric, which has a temperature limit of 85C, if not lower.


Or, look it up in the manufacturer's data (if it exists)..
RG-8X is s0mething like Belden 9258
http://bwccat.belden.com/ecat/pdf/9258.pdf

Inner conductor is 0.058 inches in diameter  somewhere between 
AWG15-16 (kind of tricky, because it's stranded)  4.3 ohms/1000 ft 
which is about like AWG 16.5

But what you really want is on page 3 of the spec sheet.. at 10 MHz, 
1kW, at 50 MHz, 370 Watts

Somewhere on Belden's site will be how they specify power rating 
(probably cable in free air at a temperature of some value, perhaps 
with moving air)

http://www.belden.com/pdfs/Techpprs/Coaxial%20Cables%20and%20Applications.pdf

says it's based on cable in free air at 20C.

So you'll probably want to derate it a bit, unless you live somewhere 
where the sun doesn't shine and it never gets very warm.

You could do the derating using the thermal equivalent of Ohm's law 
and taking 85C as the max dielectric temp

Current = Voltage/Resistance
Heat flow = temperature difference/thermal resistance

You know that 1kW= (85-20)/theta = 65/theta.
That is, theta = 65 degrees/kW
Say you want to derate it for 40C (104F).. so the new max power would be
45 degrees / (65 degrees/kW) or about 700W.


This is for a wellmatched case.  If you have serious mismatch, the 
current will be higher in places, and the peak power dissipation will 
also be higher.  Worst case would be a perfect mismatch, where 
current will be double, so dissipation will be 4x. (In reality, some 
of the heat at the hot spot will be carried along the center 
conductor and disspated elsewhere, so this is *slightly* pessimistic.

So, if you wanted to handle the worst case at 40C, derate the cable 
to about 170W (@ 10MHz).

If you have a "tough" PA that can tolerate a substantial mismatch (or 
you have a tuner that can do the same), you can do a fun 
experiment.  Get some small coax and run a lot of power at a 
reasonably high frequency into the coax.  You'll melt spots every 1/2 
wavelength.  With radars (very high peak power) where the breakdown 
is from voltage, you can blow holes in the coax at the voltage 
maximums instead of the current maximums.

And, just to make this more fun, you could measure the velocity 
factor of your (now destroyed) coax by measuring the distance between 
maxima.  It's a bit more dramatic than a slotted line with a diode probe.

Everyone should blow up or melt some coax or connectors at least once 
in their life.  It's a valuable learning experience.  Even more so if 
you calculate what should happen ahead of time.  (There's a science 
project for some middle or high schooler who's into ham radio.)

Jim W6RMK