Bob Nielsen wrote:
> If you need high breakdown voltage (not usually an issue with 50 ohm
> systems), sulphur hexafluoride is sometimes used.
>
SF6 would probably only be useful if the runs were straight, and the
coax large. Otherwise, the small variations in geometry would dominate
over the increased dielectric strength of the SF6 (which is about 3
times that of air).
http://home.earthlink.net/~jimlux/hv/insulgas.htm
SF6 has enough other issues (i.e. price at $8/lb) that I doubt anyone
would use it an amateur installation. It would be cheaper to just buy
bigger diameter coax. OTOH, if you're running megawatts through
waveguide (where you can't just go bigger, because of moding) or
hundreds of kV through an underground switchyard, SF6 is wonderful.
It's also a fine substitute for oil. Good dielectric strength
(90-100kV/cm or so, compared to oil at 150kV/cm or air at 30kV/cm), but
a LOT less messy. A pound of SF6 at atmospheric pressure is about 2.5
cubic feet, so if you can get small quantities at a reasonable price,
it's also cheaper than oil ($8/gallon, $65/cubic foot vs $4/cu ft for SF6)
Another alternative for those looking for exotic gases is to use
refrigerants of various kinds. They have almost as good insulating
properties as SF6, and might be more readily available in small
quantities (auto parts store). R22 is about $9/lb, R134a a bit cheaper.
Jim, W6RMK
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