I didn't guess, as well as I don't take "as is" a claimed number.
I prudentially estimated a reasonable breakdown voltage taking into
account cable dimensions, dielectric thickness and the dielectric
constant of the used insulating material.
I don't know where does the 300V RMS figure cames from, but if the
sizes I estimated for an RG8X cable are comparable with the real ones,
that breakdown voltage of 300V is by far not a realistic figure.
Considering the distance between the concentrical conductors, even with
an air gap (dielectric constant 1), the breakdown voltage is higher
I went to that http://www.thewireman.com/coaxdata.pdf but I don't think
that is a page from the Bible.
For example an RG142 (a teflon insulated coax cable) is reported with
the same 1400V voltage of an RG58 that uses a polyethylene dielectric,
another strange data is the mini RG174 with 1100V, at least if compared
with the RG58 data shown.
> ---------- Initial message -----------
> From : "Gary J. Ferdinand W2CS" <email@example.com>
> To : <firstname.lastname@example.org>, <email@example.com>, <firstname.lastname@example.org>,
> Cc :
> Date : Fri, 2 Mar 2001 07:26:23 -0500
> Subject : RE: [TowerTalk] Coax Rating
> 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
> | 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
> same "sleeve" adapter to PL259 coax connectors as RG59.
> | The voltage rating of an RG59 with a solid polyethylene is 2.3 KV
> | But the inner diameter of an RG8X must be a bit bigger since the
> | 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.
> not size per se, is the dominant factor. My data shows RG59 at more
> 1.7KV rms.
> | The lower dielectric constant is 1.5 for foam polyethylene and 2.26
> | 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
> | and with foam poyethylene dielectric is minimum the half of what it
> | with a compact poyethylene.
> Why guess? Check out information available on the web. The best
> been able to get states that RG8X has a rating of 300Vrms. This is
> http://www.thewireman.com/coaxdata.pdf . It is interesting to note
> 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
> | 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
> 50 ohms, as is the source. So, VSWR is 1:1. In that case we can use
> P=E*E/R -> E=SQRT(PR). At 1500W output E=SQRT(1500*50)=SQRT(75000)
> volts (rms).
> So, unless I've slipped a decimal point it would appear that in the
> case of an ideal match, no sharp bends in the coax, undamaged,
> 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%
> than the minimum. Remember, this thread started with the assumption
> feeding a 3/2-wave antenna. Do not assume this is a perfect 50-ohm
> (More like 70+, or a 1.5:1 VSWR.)
> Now throw in typical ham practicalities... The bend where the coax
> into the house, a wrap of the coax around the center insulator, the
> whipping in a strong wind. Coax on the ground mistakenly having been
> on. All these things potentially, though in small ways to be sure,
> the breakdown rating by altering the physical characteristics of the
> Finally, throw in the 1 db or so loss typically found in such coax at
> over a 100 ft run a year or so after the coax has been in the
> you have the recipe for a transmission line failure. Other options
> I'd divide power by two and even then treat this coax very carefully!
> Gary W2CS
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