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[TowerTalk] Coax Rating

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Subject: [TowerTalk] Coax Rating
From: (
Date: Fri, 2 Mar 2001 11:59:56 +0100
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 
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.
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.
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.
Now, it's true that an high SWR may somewhere produce an higher voltage 
along the line if the impedance gets transfomed, but it's also true 
that a terrible impedance transformations may appear when the load is 
like a short or an open circuit.
In practice even a mismatched antenna doesn't resembles such kind of 
extreme loads and the voltage swings can't be so big when the SWR is 
different than 1:1 but still reasonable.
Also with an SWR of 1:2 or 1:3, the voltage rating of an RG8X cable 
shouldn't be a matter of failures even at 1.5 Kw at low HF bands.
If the cable fails, the reasons must be found elesewere than in the 
foam, obviously unless the cable has been mechanically damaged. An 
event that is finally not so rare with foam cables without a semirigid 

Mauri I4JMY

> The issue is probably more of having a proper match to the feedline. 
> you are feed a purely resistive 50 ohm load, RG-58 or RG-8X should be 
> more than capable of handling the full amatuer legal limit (and then 
> some).
> However, if you are feeding an antenna with a complex impedance, 
> incur higher losses, and there's a chance that the transformed 
> could cause a high-voltage point within the coax -- potentially 
causing a 
> breakdown.
> Bill Coleman, AA4LR, PP-ASEL        Mail:
> Quote: "Not within a thousand years will man ever fly!"
>             -- Wilbur Wright, 1901

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