Roy Lewallen wrote:
chuck wrote:
. . .
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. . .

Oops. The DC resistance would be 33 / (1 * 1) / 1.524 ~ 22 ohms.

Roy Lewallen, W7EL

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.

Will wrote:
I want to set up a hf antenna for my sailboat.

I have read various guides from Icom etc.

They suggest running copper foil to a Dynaplate and use sea water as
the ground. How can this work when the Dynaplate is below sea water?

I don't know anything about Dynaplates, but if it's on the hull, it's
very near the surface of the water. Any current it conducts will flow
along the top of the water displaced by the hull. If, on the other hand,
it's really under any depth of water at all, it'll be invisible to RF
and might as well not be there.

Is sea water equal to copper wire radials as a RF ground system?

Yes.

Does sea water make a good enough ground without radials?

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

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.

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.

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

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.

Bob

Roy Lewallen, W7EL

Roy Lewallen wrote:
Roy Lewallen wrote:
chuck wrote:
. . .
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. . .

Oops. The DC resistance would be 33 / (1 * 1) / 1.524 ~ 22 ohms.

Roy Lewallen, W7EL

Thank you for the detailed response, Roy.

A couple of issues still trouble me however.

If the skin depth at 14 MHz is about 2.4 inches, can we roughly assume
that the RF resistance of that path is no less than 52.8 ohms (2.4*22
ohms)? This assumes most of the RF current would occur in the top one
inch (attenuation at one inch would be about 15 dB), and that the
resistance at 14 MHz is equal to the DC resistance.

A path one inch deep by 16 feet long (1/4 wavelength at 14 MHz) would
then have no less than 26.4 ohms resistance at 14 MHz.

Now imagine a system of multiple one foot wide by 16 feet long copper
radials on the ground with 26.4 ohm resistance distributed uniformly in
each radial. Obviously such a system will be lossy, with an average
radial resistance of 13.2 ohms.

While the analogy is a stretch, it illustrates the difficulty I am
having in understanding how seawater can be considered more efficient
than even a single slightly elevated radial, which is reported to be
less than 1 dB worse than 120 quarter wavelength buried radials
(ignoring slight pattern distortion). So even if seawater does
constitute a less lossy ground plane than a single radial (yeah, apples
and oranges, but we can weigh their juices I think) it would be better
by less than 1 dB. .

Then there is the issue of the one foot long "grounding rod" immersed in
the sea. If the above back-of-the-envelope analysis is valid, it would
seem that a even one inch long rod would be more than sufficient. If we
were dealing with a pool of liquid mercury or silver, this would have
considerable intuitive appeal for me. But the seawater model is
troubling. I imagine seawater to be a lot like earth, except more
homogeneous and with orders of magnitude higher conductivity. And I
imagine a perfect ground plane to have conductivity orders of magnitude
higher than seawater. I imagine even a modest system of copper radials
to appear more like liquid mercury than seawater does.

Where am I going astray?

73,

Chuck
NT3G

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

chuck wrote:

If the skin depth at 14 MHz is about 2.4 inches, can we roughly assume
that the RF resistance of that path is no less than 52.8 ohms (2.4*22
ohms)? This assumes most of the RF current would occur in the top one
inch (attenuation at one inch would be about 15 dB), and that the
resistance at 14 MHz is equal to the DC resistance.

What you'd need to do is look at the I^2 * R loss for every little pie
slice of water the current flows through. It's greatest near the antenna
(assuming a vertical) where the current density is greatest. In that
region, the current density is greatest and R is also greatest, so
that's where the majority of loss occurs. (Which is why a radial wire
field is useful for land installations -- its resistance is least near
the antenna.) So you can't just calculate a single value of R or I based
on the current and cross section at some point -- the entire area over
which the current is flowing must be taken into account. The modeling
program does just that.

Don't get too worried about the skin depth. Shallower skin depth is an
indication of a better conductor. The skin depth in metal is extremely
thin, yet it's a better conductor yet.

A path one inch deep by 16 feet long (1/4 wavelength at 14 MHz) would
then have no less than 26.4 ohms resistance at 14 MHz.

True but irrelevant. The current at the far end is much less than the
current at the near end.

Now imagine a system of multiple one foot wide by 16 feet long copper
radials on the ground with 26.4 ohm resistance distributed uniformly in
each radial. Obviously such a system will be lossy, with an average
radial resistance of 13.2 ohms.

While the analogy is a stretch, it illustrates the difficulty I am
having in understanding how seawater can be considered more efficient
than even a single slightly elevated radial, which is reported to be
less than 1 dB worse than 120 quarter wavelength buried radials
(ignoring slight pattern distortion). So even if seawater does
constitute a less lossy ground plane than a single radial (yeah, apples
and oranges, but we can weigh their juices I think) it would be better
by less than 1 dB. .

The problem is that the analogy is too much of a stretch. Too many
incorrect assumptions were made, resulting in an invalid conclusion.

Then there is the issue of the one foot long "grounding rod" immersed in
the sea. If the above back-of-the-envelope analysis is valid, it would
seem that a even one inch long rod would be more than sufficient.

As it turns out, a one inch rod is nearly as good, even though it
doesn't extend to the entire depth where significant current is flowing.
Half the total current is below about 1.7 inches deep. To connect
directly with essentially all the current requires at least several skin
depths. Here's the relative current on a foot long wire directly below a
quarter wave vertical at 14 MHz:

Depth (in.) I
0.5 0.81
1.5 0.53
2.5 0.35
3.5 0.23
4.5 0.15
5.5 0.10
6.5 0.07
.. . .
10.5 0.01
11.5 0.006

If we
were dealing with a pool of liquid mercury or silver, this would have
considerable intuitive appeal for me. But the seawater model is
troubling. I imagine seawater to be a lot like earth, except more
homogeneous and with orders of magnitude higher conductivity. And I
imagine a perfect ground plane to have conductivity orders of magnitude
higher than seawater. I imagine even a modest system of copper radials
to appear more like liquid mercury than seawater does.

At RF, taking skin depth into account, there's about 5 orders of
magnitude difference between the conductivities of copper and average
soil. Sea water is 30 times more conductive (at RF) than average soil,
so it's still far short of copper. But Suppose we had a conductor which
was 10 orders of magnitude more conductive than copper -- would it make
any difference if our ground plane was made out of that or out of
copper? How about 3 orders of magnitude less conductive? The fact is
that in this application, 30 times better than soil is adequate for the
water to behave a lot more like copper than like soil.

A modest system of radials in soil looks like very, very small cross
sections of copper (remember the skin depth in copper!) separated by
very large regions of soil.

Out of curiosity, I altered the conductivity of the water in my computer
model. Dropping it by a factor of 10 at DC (about 3 at RF) results in a
reduction of about one dB in field strength, or about 25% in efficiency
when using a single ground wire. So salt water has just about the
minimum conductivity you can get by with if you want really good
efficiency with a single ground wire.

Where am I going astray?

In oversimplifying the problem and using analogies which aren't quite right.

Roy Lewallen, W7EL

The permittivity, K, of water is about 80.

The relative velocity of propagation along a wire immersed in water is
about VF = 1/Sqrt( K ) = 0.11

At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water
is only 1.1 metres = 43 inches long.

Furthermore, in salt sea water, considering a wire as a transmission
line, dielectric loss is so high there is little or no current flowing
at the end of a quarterwave radial wire. Longer wires can be
disregarded because they carry no current.

So, at 7.5 MHz, there is no point in considering a system which has
more than a radius of 1.1 metres. At higher frequencies the radius is
even less.

A copper coin, 1" in diameter, immersed in a large volume of salt
water, has an impedance low enough to be used as an efficient ground
for a 1/4-wave HF vertical antenna. It is limited by its power
handling capacity.

I have made measurements years ago but have no records as I didn't
attach any importance to them at the time. And still don't.

Unpolluted, clean, fresh pond water, is a different kettle of fish.
Permittivity is still about 80 but the resistivity is very much
greater. About 1000 ohm-metres is a reasonable value.
----
Reg.

Reg Edwards wrote:
The permittivity, K, of water is about 80.

The relative velocity of propagation along a wire immersed in water is
about VF = 1/Sqrt( K ) = 0.11

At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water
is only 1.1 metres = 43 inches long.

Furthermore, in salt sea water, considering a wire as a transmission
line, dielectric loss is so high there is little or no current flowing
at the end of a quarterwave radial wire. Longer wires can be
disregarded because they carry no current.

So, at 7.5 MHz, there is no point in considering a system which has
more than a radius of 1.1 metres. At higher frequencies the radius is
even less.

A copper coin, 1" in diameter, immersed in a large volume of salt
water, has an impedance low enough to be used as an efficient ground
for a 1/4-wave HF vertical antenna. It is limited by its power
handling capacity.

I have made measurements years ago but have no records as I didn't
attach any importance to them at the time. And still don't.

Unpolluted, clean, fresh pond water, is a different kettle of fish.
Permittivity is still about 80 but the resistivity is very much
greater. About 1000 ohm-metres is a reasonable value.
----
Reg.



Interesting info, Reg.

I also made some kitchen table-top sal****er measurements about a year
ago, but at much lower frequencies than you discuss. My measurements are
not handy at the moment, but they don't comport with yours. I utilized a
variety of electrode geometries: concentric, 4 pole, parallel plate,
etc. Measurements of electric field strength, conductivity, path
conductance, etc. are not difficult but interpretation of the data
stumped me.

As you remember, the conductance of a sal****er path is a direct
function of the path's cross-sectional area. A penny doesn't produce
much of a cross-sectional area at its end of the path. Maybe your
pennies are better than ours, Certainly worth more.

73.

Chuck

Reg Edwards wrote:
The permittivity, K, of water is about 80.

The relative velocity of propagation along a wire immersed in water is
about VF = 1/Sqrt( K ) = 0.11
. . .

When the material is conductive, like salt water, you also have to
consider the conductivity in determining velocity factor. The velocity
factor in salt water is 0.0128 at 7.5 MHz, 0.0175 at 14 MHz (based on
conductivity of 5 S/m and dielectric constant of 81).

Incidentally, you can get this information directly from EZNEC,
including the demo program. Select a real ground type, then find the
velocity factor, skin depth, and other information in Utilities/Ground Info.

Roy Lewallen, W7EL

chuck wrote:
Reg Edwards wrote:
The permittivity, K, of water is about 80.

The relative velocity of propagation along a wire immersed in water is
about VF = 1/Sqrt( K ) = 0.11

At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water
is only 1.1 metres = 43 inches long.

Furthermore, in salt sea water, considering a wire as a transmission
line, dielectric loss is so high there is little or no current flowing
at the end of a quarterwave radial wire. Longer wires can be
disregarded because they carry no current.

So, at 7.5 MHz, there is no point in considering a system which has
more than a radius of 1.1 metres. At higher frequencies the radius is
even less.

A copper coin, 1" in diameter, immersed in a large volume of salt
water, has an impedance low enough to be used as an efficient ground
for a 1/4-wave HF vertical antenna. It is limited by its power
handling capacity.

I have made measurements years ago but have no records as I didn't
attach any importance to them at the time. And still don't.

Unpolluted, clean, fresh pond water, is a different kettle of fish.
Permittivity is still about 80 but the resistivity is very much
greater. About 1000 ohm-metres is a reasonable value.
----
Reg.



Interesting info, Reg.

I also made some kitchen table-top sal****er measurements about a year
ago, but at much lower frequencies than you discuss. My measurements are
not handy at the moment, but they don't comport with yours. I utilized a
variety of electrode geometries: concentric, 4 pole, parallel plate,
etc. Measurements of electric field strength, conductivity, path
conductance, etc. are not difficult but interpretation of the data
stumped me.

As you remember, the conductance of a sal****er path is a direct
function of the path's cross-sectional area. A penny doesn't produce
much of a cross-sectional area at its end of the path. Maybe your
pennies are better than ours, Certainly worth more.

73.

Chuck
Hi Chuck

So what would be the best size cross sectional area to achieve a close
to perfect RF ground from 1 to 30 mhz over sea water? Considering things
like corrosion, fowling, growth on the plate over time and any other
factors that would deteriorate the effectiveness of this connection. You
would want adequate safety margin when using this kind of simple direct
contact.

Bob

Roy Lewallen wrote:
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.

I dont have NEC4 is it too much to ask you to run the model. Radials
over sea water versus a direct connection?



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?

Theres no easy way of making sure that the wires will submerge
precisely or close to the ideal skin depth. The loading and heeling of
the yacht would affect this depending on the sailing position wind
speed and other factors. The motion of the waves and swell conditions
will also be another variable. It would work great when you anchored.


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

Well if you read the many sailing web pages and the Icom marine
guides they all advocate installing your RF ground system well below
the skin depth of salt water. They also advocate bonding all your on
board metals to submerged objects like the keel and copper ground
shoes, which is clearly wrong.

A yacht with elevated radials installed below the deck would radiate a
better signal in my view. However what constitutes an effective radial
system over seawater for frequencies between 1 and 30 mhz using a
random wire backstay antenna versus a direct connection to sea water i
cant answer without the modeling software.

Bob