Re: webpage

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

My 1954 copy of the ARRL Antenna Book states that RG-8U will
> take 4kV rms.  If we allow a factor of five for safety under
> damped wave conditions, then 800V into 50 ohms would still
> be 12,8kW :-) but this assumes a VSWR of 1:1 i.e. no
> standing waves on the line.  Even 400Vrms would be 3,2kW, so
> it's going to be OK for quite reasonable power levels.  More
> than this and you may be looking at Andrews Heliax (TM)
> which I have seen at UK ham rallies but which is undoubtedly
> going to be more common in the USA since you have many more
> high power ham stations.  I think that holds good for at
> least 12kVrms which ought to be enough for more power than I
> am ever likely to run!  Then there's the hardline used by
> the broadcast industry . . . if the pros can run hundreds of
> kW then there is plenty of mileage in 50 ohm coiling.

Typical voltage rating for the 1/2" type coax (RG-8, RG-213, RG-11, etc.) is
5000 Vrms, which as Duncan points out, gives you plenty of margin for a high
VSWR (which I suspect any sort of TC is going to have.....)

Hardline/Heliax, generically air dielectric coax, in the same size, has a
lower rating (the breakdown voltage for polyethylene is much higher than
that for air).  They use air dielectric because it is VERY low loss,
particularly at higher frequencies (i.e. 900 MHz Cell sites, long cable TV
distribution runs, etc.), where PE insulated coax has losses in the tens of
dB per 100 meters range.

You CAN build transmission lines with high breakdown voltages, and air
dielectric, but they have to be big.  They do this all the time for TV
transmitters, and the like.  You can also use an insulating gas (like SF6 or
Freon) and/or pressurize it.) Open wire 2 conductor or 4 conductor lines are
also popular for high powers, although a pain to construct and route.

For ultimate power handling, albeit at high frequencies, there is always
waveguide.. The Deep Space Network sends something like 400 kW (continuous,
not pulsed) at S band (2-3 GHz) through a fairly large waveguide up to the
feed to communicate with spacecraft a long, long distance away. (The fact
that they also have a huge antenna helps)

It's also important to remember that 50 ohms is a compromise impedance
between most efficient  power transmission and lowest loss.  There is
nothing sacred about 50 ohms.

In the TC area, you might want to choose a low impedance to keep the
voltages low. (E = sqrt (P*R))