Topband: RG-6 questions

[Tom W8JI]

>
> Since it is used the receiving it is only rated at 300v. I'd be concerned 
> about transmitting with high power or a high SWR.
>

That is NOT the voltage breakdown of the coax from center to shield. That is 
some a wiring class voltage, similar to the jacket punch through to a bare 
external conductor.

If you take regular foam dielectric "RG6" (which is almost never a real RG6 
style)  cable and strip back the end, and high pot the cable, the center to 
shield dielectric breakdown of cable with a flaw is over 12 kV.

The weakest points, by far, will be the ends. That is where it is just an 
air gap with no dielectric. The breakdown there will depend on the connector 
style, but is typically a few kV for a properly installed F fitting.

If you obtain cable with internal manufacturing flaws, like a center 
conductor that has seriously migrated off to the side, an air gap in the 
insulation, or a powdered area where the foam didn't form, the voltage will 
be considerably lower.

I test all of my cables with high voltage. A properly installed UHF 
connector, on ANY type of RG8X through LMR400 or RG8 cables, will high pot 
over 4-5kV. That will be the air gap voltage in the connector. If you get 
just one little sharp shield strand near the center, that voltage can drop 
to 1000 volts or less.

A typical RG8X cable, even foam dielectric, will high pot to 10 kV or more. 
8X is typically less than "F6" (RG6) type cables, because the internal 
spacing is less, but still more than the connector.

The worse connectors are BNC, F, and type N. Type HN and others, including 
UHF, are far more.

Anyone looking at cable advertised voltage ratings for power is making a 
mistake. With a reasonable SWR, other than connector voltage failures, the 
issue will always be heating.

In intermittent duty, I certainly wouldn't worry about RG6 type cables at 
1500 watts below ~15 MHz.

73 Tom