Larry makes some good points, although they are couched a little too
heavily in absolutes. I don't agree that the losses with a two-foot
separation would necessarily be unacceptable.

Leaving aside whether the near-field losses will arise from electric
(displacement currents as Larry suggests) or magnetic fields, almost all
metal structures above the deck will alter the near-field properties of
a deck-mounted antenna.

Regarding the sufficiency of a two-foot separation, I would point out
that most HF mobile antennas are mounted well within two feet of the
vehicle's vertical metal components for something like two feet of
vertical rise. There is ample evidence that these installations "work",
or if they do not, it is usually not because of proximity to the
vehicle's vertical surfaces. HF antennas on aircraft have been run
entirely along the metal fuselage with separations on the order of two
feet. These "work" also.

So before rejecting the customary deck-mounting of a 23-foot whip with a
horizontal support, it would be useful to provide the OP with some
concrete indication of how well the antenna is likely to perform
compared to the next best alternative. While I can't do that
quantitatively, (I did suggest an experiment, which a competent tech
could do fairly quickly) it is my experience that such an installation
could be satisfactory.

With only the top 16 feet as a radiator, near-field losses become
negligible but tuner losses predominate. At higher frequencies (above 8
MHz, for example) 16 feet in the clear will probably outperform 23 feet
mounted next to the pilothouse. At lower frequencies it is more
difficult to say which would be better.

Lacking further information from the OP, that is probably the best we
can offer. The antenna, tuner, and SSB manufacturers should be contacted
to mine their experience.

For the strong of heart, center- and/or top-loading could even be
employed in some circumstances to shape the current distribution on the
whip in such a way as to reduce losses near the pilothouse.

Out of curiosity, did the boat's designer/naval architect not address
this design detail? Surely it is not an out-of-the-ordinary design question.

Chuck

Larry wrote:
bradleyj wrote in
:


I understand that it is important to keep the whip from coming
too close to any metal. This is an obvious problem, since this boat is
all aluminum. Any suggestions or references would be appreciated.




You're very lucky, as HF operators go. The metal boat will make an
excellent "ground plane", the counterpoise plastic boat owners dream of.

What kills HF is any PARALLEL metal which the RF waves intercept on their
way out of the boat. This actually creates a capacitor, its two plates
being the mast at boat ground and the whip, with the air as its
insulator, thus shunting the RF energy off to ground. Two feet is not
enough, infinity is. The further you can get the whip away from any
metal rising over it, the better. The best place is atop the mast with
the whip sticking way up above it, but this isn't very practical as
you'll be going under overhead obstructions. So, let's mount the whip as
far away from the mast as we can get, including its guy wires and other
metal/wire/conductive things going aloft.

Let's put the base for the whip on TOP of the nice metal pilot house
ground plane (which ends up perpendicular to the whip). This makes USE
of the excellent ground plane effect of the horizontal pilot house roof,
while keeping the E-field from intersecting it.

PLEASE DO NOT MOUNT THE WHIP ON THE SIDE OF ANY METAL HOUSE if you can at
all help it. The shunt capacitance of the whip near the wall of the
pilot house exterior mounted for convenience on the side somewhere is
just awful...sucking your signal off to that metal along side the whip.
Signals suck like that.

You didn't mention a flybridge atop the pilot house so I'll assume you
don't have one. So, let's mount the whip dead centerline of the pilot
house roof, about a foot from the forward edge of it, with the
weatherproof tuner right next to it. The tuner's output wire to the whip
should be as short as you can possibly make it as that becomes part of
the antenna (and part of the shunt signal off to metal problem).

The ideal mount would be a hole in the pilot house roof with an insulator
mounted THROUGH the roof with the tuner mounted safely INSIDE the pilot
house right next to where the feed-thru mount's "hot" bolt protrudes
inside. Of course, though I'd be proud as hell of it, myself, the
yachties would scowl, seeing the tuner overhead of the pilothouse...(c;

Be sure, wherever you mount the whip/tuner to put a heavy metal stainless
strap from the tuner's ground bolt to the BARE METAL under the closest
leg of the tuner where it's bolted to the pilot house roof, a fantastic
ground.

I'm green with envy. Lionheart has a long strap ground to the engine
block...the only metal mass of any consequence in the plastic ketch....
(sigh).

Larry W4CSC
Lionheart WDB-6254
Icom M802, Icom AT-130, 55' long insulated backstay....poorly
grounded...dammit.