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[TowerTalk] Coax Rating

To: <>
Subject: [TowerTalk] Coax Rating
From: (
Date: Fri, 2 Mar 2001 15:23:42 +0100
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 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. 

Mauri I4JMY

> ---------- Initial message -----------
> From    : "Gary J. Ferdinand  W2CS" <>
> To      : <>, <>, <>,        
> 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 
> | 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 
> | 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 
info I've
> been  able to get states that RG8X has a rating of 300Vrms.  This is 
> . 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 
> calculations:
> 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 
he was
> 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 
> dielectric.
> 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 
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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