Mauri,
Comments interspersed with yours.
| This stuff with RG8X is getting interesting.
| Well, after some researches it seems to me that what is sold in US as
| an RG8X has a size that's not so different from an RG59, formerly
| 0.242".
That is true. The physical, outer diameter is extremely close. It uses the
same "sleeve" adapter to PL259 coax connectors as RG59.
| The voltage rating of an RG59 with a solid polyethylene is 2.3 KV RMS.
| But the inner diameter of an RG8X must be a bit bigger since the cable
| is 50 OHms instead of 75, and the dielectric constant of foam PE is
| less than with a compact type.
Solid poly dielectric has a much higher voltage rating than foam. Material,
not size per se, is the dominant factor. My data shows RG59 at more like
1.7KV rms.
| The lower dielectric constant is 1.5 for foam polyethylene and 2.26 for
| solid polyethylene and all this leads to a consequential decrease in
| the voltage rating of the cable.
| Considering a safe margin, the voltage rating for a cable of that size
| and with foam poyethylene dielectric is minimum the half of what it is
| with a compact poyethylene.
Why guess? Check out information available on the web. The best info I've
been able to get states that RG8X has a rating of 300Vrms. This is from
http://www.thewireman.com/coaxdata.pdf . It is interesting to note that
RG174 is 1100V, RG58 is 1400V, RG59 is 1700V and RG8 is 4000V.
| Neglecting ohmic and expecially dielectric losses (low because foam
| type and because frequency is not high), and further halving the max
| applicable voltage (1.150/2=575V) to be "really" reliable (halving the
| voltage makes the power decreased to 1/4), with an SWR of 1:1 a safe
| max power for that cable could be (575x575)/50=6612 Watt.
Interesting approach. Now let's try it will real numbers:
Let's assume what you did that the load is purely resistive and is exactly
50 ohms, as is the source. So, VSWR is 1:1. In that case we can use DC
calculations:
P=E*E/R -> E=SQRT(PR). At 1500W output E=SQRT(1500*50)=SQRT(75000)=274
volts (rms).
So, unless I've slipped a decimal point it would appear that in the best
case of an ideal match, no sharp bends in the coax, undamaged, lossless
cable, and with 1500W output, we come out 26 volts short of the rated
breakdown voltage of the cable, or about 9% short of breakdown.
VSWR by definition is the ratio of maximum to minimum voltage on the
transmission line. With a VSWR of 1.3:1 the maximum value is 30% higher
than the minimum. Remember, this thread started with the assumption he was
feeding a 3/2-wave antenna. Do not assume this is a perfect 50-ohm load.
(More like 70+, or a 1.5:1 VSWR.)
Now throw in typical ham practicalities... The bend where the coax goes
into the house, a wrap of the coax around the center insulator, the coax
whipping in a strong wind. Coax on the ground mistakenly having been walked
on. All these things potentially, though in small ways to be sure, change
the breakdown rating by altering the physical characteristics of the foam
dielectric.
Finally, throw in the 1 db or so loss typically found in such coax at 7MHz
over a 100 ft run a year or so after the coax has been in the elements, and
you have the recipe for a transmission line failure. Other options are more
prudent.
I'd divide power by two and even then treat this coax very carefully!
73,
Gary W2CS
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