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[TowerTalk] coax rating

To: <>
Subject: [TowerTalk] coax rating
From: (
Date: Fri, 2 Mar 2001 17:00:59 +0100
Hi Dan,

generally it's like that, at least at low HF where ohmic losses are 
dominant over the dielectric losses.
The stand-off voltage of air corresponds more or less to 50 volts per 
mil.(mil=.001") and any isolating material with an higher dielectric 
constant makes the breakdown voltage a minor problem.
May be not everyone knows that optimum diameter ratio for voltage 
breakdown in coax is 2.7 corresponding to a 60-ohms impedance (in the 
past a standard impedance in some European countries).
Another hint not very much known is that since the dielectric constant 
of polyethylene is 2.3 the impedance of a 77-ohm coax air line is just 
reduced to 51 ohms if only filled with polyethylene.

Mauri I4JMY

> Seems to me that there was an article in QST 10 or 15 yrs ago by some
> guy who worked for a major cable mfr. about coax cable ratings. I
> believe he stated that the h.f. power rating was primarily determined 
> temperature rise in the center conductor not voltage breakdown in the
> dielectric. Skin effect caused greater loss at higher frequencies.
> Teflon dielectric cable had higher power ratings because the teflon 
> withstand much higher temperatures than polyethelene. Does anyone else
> recall this or is my imagination playing tricks on me?
> 73, Dan, N5AR

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