Roy Lewallen wrote:
bob wrote:
. . .
One thing i am curious about is that if you suspend a radiator or
conductor well below skin depth in air whats the radiation efficiency
like of that conductor? To me on a yacht it looks like a conductor
suspended in a U channel suspended in air with the top open and
exposed. Maybe it will operate like a waveguide with some cut off
frequency, this is a wild guess.Maybe someone who can model conductors
below ground can model this. Most yachts have at least 3ft of
freeboard above water to play with.

But who knows there seems to conflicting advice on several points.
Maybe Roy can clarify all these issues with his models.

There's no good way to model this situation with the tools I have.
Conceptually, though, you can get a fair idea by imagining the sea to be
made of metal. The boat makes a depression in the surface, and you can
connect to the metal at the bottom of the depression just as you can
anywhere else on its surface.

Imagine that the depression is perfectly round and symmetrical, with a
grounded vertical extending up from its center. The current flowing into
the "ground" connection at the bottom of the depression will flow along
the conductor's surface, up the sides of the "dish" to the flat water
surface. The total current flowing from the center of the dish upward to
the flat surface will equal the current flowing up the vertical wire
near its base. The fields from the two will nearly cancel, so there'll
be little radiation from the vertical in the region below the flat
surface of the water. The vertical above that point should radiate
normally. This simplification will of course be modified by the reality
of a non-symmetrical hull, but it helps in getting a general idea of
what will happen.

Another way to look at the situation is to view the depression as the
outer conductor of a shorted coaxial cable, with the "ground wire"
extending down to its center as its center conductor, and the shorted
end the bottom of the depression. This shows you'd get some inductive
reactance in your connection to the surface of the water. A rough
calculation would probably get you in the ballpark of the actual value.
This doesn't represent loss, however.

The main thing, though, is that connection to the water requires a
conductor either in contact with or capacitively coupled to the water's
surface or only a very short distance below it (assuming salt water),
whether the surface is below the boat's hull in a depression or on the
flat surface of the ocean.


Back around the beginning of this thread a thought occurred to me, and
I'm surprised that no one else mentioned it.

Unless we are planning on putting out ship on the great Salt lake, of
one of the few other salt water inland lakes or seas, we're going to be
putting the thing in the ocean.

Immediately, one sees that if a ground is at the surface of the water,
at many points it will be 4 or more feet under the water. Dem boats
rock! There are moments that it will be quite a distance under water,
depending on the sea state.

Depending on the load, the water line is going to be different, and
will be changing constantly as fuel and food is used.

Certainly any capacitive coupling through the hull makes for a variable
capacitor? Does this have an effect?

Will we eventually come to the conclusion that we can't put radios on
ships?? ;^)


- 73 de Mike KB3EIA -