Hi Gary,
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
than 300V.
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.
73,
Mauri I4JMY
>  Initial message 
>
> From : "Gary J. Ferdinand W2CS" <w2cs@ipass.net>
> To : <i4jmy@iol.it>, <aa4lr@arrl.net>, <w2cs@ipass.net>,
<towertalk@contesting.com>
> Cc :
> Date : Fri, 2 Mar 2001 07:26:23 0500
> Subject : RE: [TowerTalk] Coax Rating
>
> 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 3/2wave antenna. Do not assume this is a perfect 50ohm
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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