Short Wave Sportfishing wrote in
:

Properly installed, there is no honest advantage to a 8' antenna over
a 3' antenna for small boats.

For larger boats, that's a whole different ball of wax. :)



It's not on larger boats, either. At 55' atop the mainmast of an Amel
Sharki 40 ketch, the Metz easily outperforms the 8' monsters, which must
be mounted lower.

There is a problem with these "High Gain" antennas that's not
addressed....BOATS ROLL AND PITCH.

The horizontal radiation pattern of a 1/2 wave whip, like the end-fed
Metz, looks just like a donut that has no hole with the whip sticking up
where the hole should be. Its "gain" over the isotropic source is 3 db
because of this donut. The isotropic source's radiation pattern is a
sphere. If you took that sphere and pushed the north and south poles of
it to the center, the sides bulge out twice as wide....the 3db measured
at the equator, for you cartographers in the audience. The equator is
fatter.

Now, if we took that Metz radiation balloon, the donut, and put it
between two plates in a vice, when we squeezed the vice, the balloon
between the plates would be squeezed even more and its equator would
bulge out even more as we flattened it so the plates were only an inch
apart. (Our balloon can take any pressure.) This is what the radiation
pattern looks like for the 8-9 db fiberglass colinear "whip", which is
actually a phased array of 1/2 wave dipoles inside a fiberglass tube to
hold them in place. 8' antennas have two dipoles spaced 1/2 wavelength
apart to create this donut that looks like you stepped on it and
flattened it.

Now, this gain IS real gain AT THE EQUATOR when either whip is held
pointing at zenith, straight up and down. Too bad your boat NEVER points
it there, only in passing through it. The pattern is ALWAYS
perpendicular to the plane of the whip passing through the middle of it.

So,



\ Tip of whip
\
\
\
X Boat heeled this way
\
\
\
\
Boat ^-pattern edge heeled to right

God that's awful but its the best I can think of. AS you can see, the
donut has tipped with the whip as the boat rolls. The pattern rolls,
too! All the stations off to starboard or port DON'T see the signal
caused by the EDGE of the donut. They see something MUCH less. How much
less is determined by the tilt of the donut and how squished out it is.
If it's squished hard, like the 8' whip (compared to the Metz' fat
donut), the part of the pattern pointing at the receiving antenna afar is
much less thick than the fat Metz donut.

The effect of rolling and pitching at the receiver you're trying to
impress is that the Metz antenna's signal at that far-off receiver runs a
course from our reference signal, for this explanation, down to 60% of
reference signal. The big whip's signal, on the other hand, being
flatter produces about 20% more signal when the antenna is vertical, but
the dip in signal as the boat rolls is down 80% or more because its donut
is so squished. Very little signal points to the receiver when the donut
is pitched over like this.

That's what the REAL difference between them is, at sea, in the real
world. If your VHF has an S-meter on it, watch it in rolling seas and
you can see the other guy's rolling and pitching making it roll up and
down as his donut's peak and minimum pass by you. The Metz, or any half-
wave with the fat donut, has lots less pitch/roll fading and better comms
when it really counts....precisely why the CG uses it on THEIR pitching
and rolling craft in heavy weather.

Besides, I've submarined the Metz on the jetboat a few times when the bow
became a shovel.....and it survived what would rip the fiberglass whip
right off, probably in splinters....(c;


Larry
--
Search youtube for "Depleted Uranium"
The ultimate dirty bomb......