"thuss" wrote in news:1174842368.471514.316370
@n59g2000hsh.googlegroups.com:

I'm replacing the antenna feed line between my antenna tuner and
insulated backstay. It's currently a coax cable but then I read that
I should use insulated antenna wire and NOT use coax for the antenna
feed line. This raises some questions though, isn't the whole reason
to use coax because it's shielded which prevents the feed line from
radiating and causing RF burns in the cockpit?



The initial installation was just WRONG. Coax cable left open on the
backstay end is simply a Faraday Shield the also makes an output
capacitor because of the proximity of the ground (shield grounded) and
the center conductor hot with RF. What SHOULD have been installed was a
high voltage cable on standoff insulators or just hanging out AWAY FROM
ALL METAL OBJECTS...NOT TYWRAPPED TO THE BOTTOM METAL PARTS OF THE
BACKSTAY, PLEASE....

How long is the run from the tuner's antenna output insulator to the top
of the backstay's bottom insulator, the feedpoint of the antenna? The
tuner should be located as close to that point as is practical.
Lionheart's is less than 2' and I use a stainless wire to it.

Every piece of metal sticking up above the deck is a passive part of the
backstay antenna system and can have high voltage points, of a sort, on
them if they are of a resonant length at the frequency you are operating
on. How high depend on their proximity and parallelness to the radiating
element. This is RF, not AC power. At some frequency, all metal
rods/shrouds/masts/anything that conducts becomes resonant because of its
length. Ride by any AM radio station, whos tower is the radiating
element at all of them, and look at the insulators installed in every guy
wire around the tower to make sure each section between those insulators
is NOT a resonant length of wire (too short) which reduces secondary
radiation and absorption to insignificant values.

Now to safety. 150 watts on a marine HF isn't much RF power, relatively
speaking. Stop by Lionheart and I will let you run 150 watts whistling
into its SSB transmitter while I'm wrapped around the backstay antenna,
unharmed. Tower climbers never turn off AM radio stations to climb up
and change the lights, or even those insulators while hanging upside down
from the guy wires they are working on...which scares me to death. You
aren't burned from an high voltage wire AS LONG AS YOU ARE AT THE WIRE'S
POTENTIAL. Where you get into trouble, which is really minimal and not
really dangerous at 150 watts HF, is when you get ACROSS high voltages of
different potentials/phase...where current, which is what burns/shocks,
flows through your body. Don't touch the backstay and boom or mizzen or
that seawater-grounded metal helm wheel when the transmitter is on.

The wire coming from the tuner to the backstay, by the way, is part of
the antenna, which starts at the high voltage insulator on the tuner.


Larry
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