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.
73,
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
by
> 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
can
> 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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