bob wrote:
Roy Lewallen wrote:
I put this aside until I could do a little modeling. A lot of postings
have been made in the interim, but I don't see too much in the way of
answers. I'll try to answer some of your questions.

I think we all would be interested in how a small piece of metal buried
in sea water can provide an efficient ground versus one or 2 elevated
radials. I also dont see how its efficient concentrating all your
current in such a small area. Since the salinity of salt water is not
constant using one or 2 radials on yacht would be more efficient.

Imagine for a moment that instead of salt water that the ocean was
covered by a thick metal plate. How would you effectively use that as a
ground? Salt water isn't as good a conductor as metal, but it acts more
like that than dirt.

I don't know how much the salinity or conductivity of sea water varies,
but suspect that even at its worst it's quite a good conductor.

How can salt water which would have some resistance even though
its
conductivity is high compared to earth behave better than copper wire
when returning antenna currents to the feedpoint.

Cross sectional area. Replacing all the sea water with copper would
improve it, but scattering a bunch of copper radials out and replacing
only tiny parts of it wouldn't make much difference. And the loss is so
low to begin with that even replacing it with copper wouldn't make any
difference.

The small amount of
current flowing in a 100 watt signal i would not want to waste sending
it into salt water.

It won't go in very far. It'll stay very close to the top. And the waste
is negligble.

Salt makes good resistors, why would you introduce
a loss into the equation which radials seem to eliminate even though we
dont have ground loss over sea water? We also have the issue of the
skin depth of sea water to consider.

Solid salt is actually a decent dielectric, I believe. Again, the trick
is cross sectional area. The current is spread over a large area of
water, so the overall loss is negligible.

The analysis I did took skin effect into consideration. The skin depth
is even less in metal, yet metal has low RF loss.


Yes. A foot-long wire "ground rod" below the antenna provides a nearly
lossless ground connection at HF.

This is not how most yachts connect their ground connections. They
connect to the sea cocks well below the top of the water anywhere for
3ft to 5 ft down. Some even use slim flat ground shoes again well below
the water line. Its impractical for a any sail vessel to maintain a
connection to sea water close to the surface because loading and the
yacht heeling when sailing.

I'm sorry to hear that, because any connection below a few inches is
ineffective at HF.


How can a piece of copper metal about 1 ft square equal several
radials laying on the boats deck?

Radial wires are used for land based systems because of the poor
conductivity of soil. Radial wires reduce the resistance of the path
current takes going to and from the antenna base. Salt water is a good
conductor and doesn't need -- and won't benefit from -- radial wires.

Indeed radials do perform this way. I would still want to use radials
wires even 1 or radials wires even on a yacht since the length of the
radials will have a greater capacity to sea water ground.
It also would
be more efficient in providing a current return.

More efficient than a deep plate, for sure. Not any more efficient than
a foot long uninsulated wire extending downward from the surface. But by
all means use whatever makes you feel well grounded.


Why do i have to use copper foil when most other people suggest
using ordinary copper wire?

You don't. And won't copper corrode rapidly in salt water?

Over seawater what would be the best number of radials to use
considering that maximum length i can run is 40 ft. I am planning to
use a backstay antenna with a SGC 230 Tuner.

None. A simple wire down into the water is adequate. Or use a small
plate very near the surface if you prefer.

Again yachts bury their ground connection well below the skin depth.
Some even run the ground wire from the tuner down to the keel which is
well submersed in salt water.

If the wire is uninsulated, the first few inches of the wire will
provide the ground connection. If it's insulated, they'll have no HF
ground connection at all except what's provided by capacitive coupling
through the first few inches of insulation.

All they are using is one short piece of
foil that is behaving like a small radial. We will see what the models say.

By all means, do some modeling. The only program I know of which will
allow modeling submerged conductors is NEC-4 and derivatives.

2 elevated radials over seawater versus a 1 ft square piece of metal
buried below the skin depth. My money would be on the radials.

Certainly elevated radials would be better than metal more than a skin
depth or two deep. Better yet is a wire extending from the surface to a
few skin depths. Why isn't that possible?

Incidentally, I'm not proposing replacing the standard grounding system,
which I'm sure is important for other uses including, probably,
lightning protection. It will just need to be supplemented if you want
an effective HF ground.

Roy Lewallen, W7EL