On Wed, 05 Nov 2003 23:32:57 GMT, "Gordon Wedman"
wrote:

The other day I was wandering around one of our marinas trying to steal
ideas from other boats and I came across an aluminum pilot-house sloop that
may have come over from Europe. I noticed that the SSB coax was held away
from the backstay turnbuckle and wire by ~1 inch plastic spacers. I've
never seen this before and the previous owner didn't do it on my boat. I've
been thinking of upgrading the ancient SSB system on my boat and was
wondering if these standoffs were something recommended.
Anyone know about these? Thanks
Gord


Any time the output antenna "hot" of an HF tuner is near anything
metal, a capacitor is formed between the antenna wire and the metal
objects. This capacitor is in parallel with the output of the
transmitter and must be minimized. If it becomes a substantial
capacitor (wire close to object), the output tuning capacitor in the
tuner will run out of range (low as it goes) and the antenna won't
tune properly, especially on the upper frequencies. So, we isolate
the wire as far as practical from all metal objects, especially large,
grounded metal objects. It is also very important that the antenna
lead on the antenna side of the tuner be STABLE, and not flopping
around, which causes this natural capacitance to anything to CHANGE
during transmissions. If the wire is moving around near metal
objects, the shunt capacitance constantly changes, ruining the tune of
the tuner. So, this boat had a proper installation.....isolated on
long insulators with many of them that would hold the wire stable as
it traversed the sheet metal.

The effects of shunt capacitance in any HF antenna situation with long
wires is on the OTHER end of the antenna from the feed point.....at
the insulator at the top of the backstay. This point in the antenna
is the highest impedance (nearly infinity we hope) point of the
system. The voltage at the upper insulator of a
shorter-than-quarter-wave wire is always very high. Any capacitance
to a metal object causes a lot of the signal to be shunted off to that
object, and mostly lost, wasted. So, it is very important to make
sure the upper insulator is NOT installed too near the masthead, but
back down the backstay a ways and the upper end of the insulated
backstay is never near metal objects, like boom lifting devices made
of stainless cable, etc. The tuning of the backstay will go all crazy
every time one of these metal cables moves near the upper end of the
backstay.

We Geoffrey acquired Lionheart, an Amel Sharki 41 ketch, the owner
reported the insulated backstay antenna didn't work very well and he
never found out why. After I took it over I noted how close the boom
lift was to the backstay when the boom was on centerline, where it
made my signal on HF just SUCK! So, we changed out the metal cable
for insulated line. HF signals are now very acceptable no matter
where the mainsail ends up. Getting the metal away from the antenna's
upper end high impedance point solved the problem.