chuck wrote:

A few additional questions along these lines for the group (with some
paraphrasing):

1. What is the skin depth in salt water at 14 MHz?

About 2.4 inches.

How would this affect
a ground plate at four feet below the surface?

A ground plate at that depth would be invisible to RF. It might as well
not be there. This is, of course, assuming it actually has 4 feet of
water above it and not a boat's hull and air.

2. What would the ohmic losses be over a one square foot by 33 foot path
through salt water?

Let's see, salt water conductivity is about 5 S/m, which is 1.524 S/ft.
So the *DC* resistance of that piece of sea water would be 1.524 * 33 /
(1 * 1) ~ 50 ohms. But the RF resistivity would be much greater because
only the outer few inches would carry any current.

3. How well would the ground plate work on fresh water bodies, such as
much of the Chesapeake, the Great Lakes, and various rivers and
tributaries often used by cruisers? How would it compare with radials
over fresh water?

Fresh water is quite a different story. The skin depth in *pure* fresh
water at 14 MHz is 156 feet. But "fresh water" is far from pure.
Unfortunately I don't have any ready data on "typical" "fresh water". So
the skin depth is somewhere between 2.4 inches and 156 feet. Not much
help. If the water is pretty pure, radials near the surface would be an
improvement over a ground plate.

4. Can anyone cite a published and reproducible study in which the RF
losses through salt water were measured and compared with losses through
one or more copper wire "radials" on or below deck of a typical cruising
vessel? Or is there a published theoretical analysis of this comparison?
Looking for more than the casual, anecdotal stuff.

No. An NEC-4 model shows a one-foot wire to provide a nearly perfect
ground in salt water. But that falls far short of your requirement.

5. Will a four foot length of wire dropped into sal****er provide a
"good" RF "ground" and on what is the answer based?

Yes, but one foot does just as well -- any current on the wire will drop
to essentially zero within the first foot, so the remainder might as
well not be there. This is from an NEC-4 model.

I need enlightenment!

So do we all.

Roy Lewallen, W7EL