Gary Schafer wrote:
On Tue, 23 May 2006 23:44:50 +1000, 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?

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

Does sea water make a good enough ground without radials?

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

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

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.

All ideas and comments appreciated.

Will


Hello Gary,

Although I'm not advocating "radials" or any other marine HF grounding
approach, I am trying to understand the reasoning used by those who do
advocate. I've tried to set up instrumentation that would allow
comparative measurements of alternative "ground" properties for yachts,
but the problems involved have been overwhelming. I think I understand
why the world is not awash in empirical data in this area, especially
compared to what is available for land-based verticals.

First, radials on a boat are not usually better than a good ground to
seawater.

I think the word "radials" in this thread ought to be in quotes, since
we're really not talking about conventional symmetrical radials from
which radiation is substantially canceled. As you point out, the
"radials" being discussed in this thread are simply a horizontal part of
the radiating antenna and not really radials at all.

Having said that, I would welcome learning your basis for the
conclusion, and any info you can offer on how much better and in what
way. Do you have a measurement of the HF "ground resistance" provided by
seawater using a Dynaplate? Would your conclusion change for a vessel on
the Chesapeake where salinity is quite low?

If you do use radials they need to be resonant which means they need
to be ¼ wavelength long at each frequency of operation or they need to
be tuned with a loading coil to make them resonant. The reason is that
if they are not resonant you will get little current into them. Your
antenna system will be unbalanced and being that the radials will
usually be closer to other wires etc. on the boat they will couple
into them before they couple to the sea. That will make the tuner coax
and control cables radiators as well, because of the higher impedance
of the radials.

We don't seem to require that the backstay (or whatever vertical
radiator we are using) be a physical 1/4 wavelength when we use a tuner.
Why would we impose that requirement on the horizontal part of the
radiating system? Isn't the famous 100 square feet of copper approach
analytically equivalent to a nonresonant "radial"?

It is true that proximity of the horizontal radiator to other wiring can
cause problems, and this may be an unequivocal disadvantage to the
approach. Of course, an entire sailboat is in the reactive near-field
region of the vertical radiating element regardless of the "ground" used
and so the coupling issue is a matter of degree.


If the radials are mounted in the bottom of the hull right near the
water they have a chance of coupling to the sea. But if you go to that
trouble it is much easier to couple directly to the seawater with some
metal under the boat. (dynaplate, through hulls etc.)

Radials that do not couple the energy to the sea act as part of the
antenna. That wouldn't be bad if that radiation went where you wanted
it to go but a large part of it will get into all sorts of things on
the boat that you don't want it to.

There are two issues he interference (already discussed above) and
efficiency. Assuming no communication value is given to near-vertical
radiation angles (a mistake, in my opinion, for the lower frequencies),
the efficiency of an L-shaped, center-fed radiator is probably no worse
than 3 dB below that of the same vertical element worked against a
perfect ground plane. That is based on simply assuming that 100% of the
power that would have been radiated by the horizontal element is
dissipated in the "ground". In practice, the typical Dynaplate-based
ground plane will be less than perfect, even over salt water, and the
horizontal element will most definitely radiate some power. Hence, my
suggestion than 3 dB is worst case and I would not be shocked to see the
Dynaplate-based system less efficient. Data, data, data.


Radials on a boat are different than when laid over earth. When laid
over earth there is tight coupling to the earth and length is not
important.

If we're now talking about real radials, I wonder if the statement is
true. Real radials elevated three feet above seawater might couple very
tightly. It would be interesting to see some analysis or measurements of
this. But we're talking about "radials" in this thread and the statement
is not relevant to them.

When they are elevated above earth they need to be resonant
in order to work. Short radials do little good in this situation.
A boat installation is similar to an elevated installation on land.

As mentioned above, I believe elevated radials (real radials) need to be
resonant and symmetrical in order to cancel radiation from them. The
problem is semantic, of course. A radiating structure with horizontal
and vertical components can be 100% efficient with the horizontal
components nowhere near resonant lengths. There will be effects on
horizontal and vertical patterns but that is impossible to generalize.


I see some guys on the Maritime mis-information forum spouting about
using radials on a boat.

You are much better off getting a connection to the sea for your
ground. It is one of the best ground planes available.

I'm curious about how these ground plates work. The attenuation in
seawater of RF at 10 MHz is on the order of 30 dB/foot. If a ground
plate is four feet below the surface, how does it work? Copper's
conductivity is orders of magnitude better than seawater. Do we know the
break even point for a copper wire ground plane vs. seawater? Reflection
properties of seawater OTOH are well-known and documented.

http://ecjones.org/physics.html
Ionospheric Physics of Radio Wave Propagation

The usual instructions for installing ground plates recommend bonding
the RF ground connection to metal tanks, the engine, lifelines, the mast
and associated steel rigging (other than the insulated backstay) etc.
Sometimes I wonder if all that metal is not forming the "other half of
the dipole", pretty much independently of the ground plate's existence.
Since a good many installations disregard your recommendation to mount
the tuner close to the ground plate, rather than close to the antenna,
that 10 feet or so may contribute to the "size" of the counterpoise when
a ground plate is used.

I suppose this is heresy, but disproving it would require testing a
ground plate with NO connections to the tuner other than transmitter,
antenna and ground plate. A manual tuner would make sense for the test.
Of course the transmitter would have to be run from an isolated battery
so as not to bond the tuner to the boat's DC ground wiring. To make
sense of the feedpoint impedance, the antenna should be a quarter
wavelength to provide a known radiation resistance, but much of it would
be buried in the hull, etc. It quickly becomes complex.

Another factor is that at the higher frequencies, a typical backstay
approaches or exceeds a half-wavelength, in which case even a "short"
length of wire, with or without a ground plate, will work well. If the
ground plate is providing one of the best ground planes available, its
impedance should actually decrease with decreases in frequency, given
the attenuation of seawater as a function of frequency. Thus, the ground
plate should be spectacular at 160 meters. Alas, I have not seen even
anecdotal information on that.

To do so you
need a very short ground lead from the tuner to the sea connection.
The best way to accomplish that is to mount the tuner down in the hull
right next to the sea ground connection within a foot or so. Then run
your antenna lead from the tuner to the antenna as much in the clear
as you can.

Remember that no matter where the tuner is mounted the antenna starts
at the ground connection. If you have a 10 foot lead from the tuners
ground connection to the sea water connection that 10 feet will
radiate like the antenna. Problem is so will the coax and tuner
control lines radiate and couple into all sorts of places you don't
want it to.


Without some kind of isolation, that is true. The flip side, though, is
that (as you pointed out) the antenna starts 10 feet "lower" and a good
deal of radiation from that 10 foot length is now inside the hull. If
your objection to the use of "radials" is unwanted coupling, then this
must surely fall into the same potential (no pun) category.

By placing the tuner at the ground connection you have control over
what radiates and what does not.

Copper foil for the ground lead to the tuner makes a lower impedance
path than doe's wire. You want the lowest impedance path you can get
for the ground lead.

73
Gary K4FMX


Be interested in your thoughts on these issues, Gary. Sorry for the length.

73,

Chuck
NT3G

In article t,
chuck wrote:

\
Hello Gary,

Although I'm not advocating "radials" or any other marine HF grounding
approach, I am trying to understand the reasoning used by those who do
advocate. I've tried to set up instrumentation that would allow
comparative measurements of alternative "ground" properties for yachts,
but the problems involved have been overwhelming. I think I understand
why the world is not awash in empirical data in this area, especially
compared to what is available for land-based verticals.

First, radials on a boat are not usually better than a good ground to
seawater.

I think the word "radials" in this thread ought to be in quotes, since
we're really not talking about conventional symmetrical radials from
which radiation is substantially canceled. As you point out, the
"radials" being discussed in this thread are simply a horizontal part of
the radiating antenna and not really radials at all.

What you need to understand is just what kind of antenna are you trying
to describe, here as a MF/HF Marine Antenna? Are you thinking a Marconi,
Vertical Dipole, Offcenter Feed Marconi, or just what? The standard
MF/HF Marine Antenna, usually is considered a Marconi, and that is what
MOST both commercial and non-commercial MF/HF Marine antennas end up
being. So lets discuss Marine Marconi Antennas, and what makes good
ones and bad ones. Marconi Antennas are charactorized by 1/4 Lambda
Vertical Radiating Element, sitting in close proximity to a LOW
Impedance RF Ground System Perpendicular to the 1/4wave Vertical.
In the MF/HF Marine enviorment we add a Tuner, Auto or Manual so as
to be able to tune the Vertical to a 1/4 Wave Electrical Length and
Resonance on each frequency that the vessel maybe required to use in the
MF/HF Marine Frequency Bands, which cover 1.6Mhz to 25 Mhz in frequency.

Ok now lets look at what the vertical is. A Backstay, a Whip, a Loaded
Whip, a Loaded Whip with wire under it. It really doesn't matter, as
they all will be tuned to resonance, in either 1/4Wave, or 3/4Wave by
the tuner, against the impedance of whatever RF Ground is connected to
the tuner.

Hence the "Old RadioMans Addage", "If you have a Good RF Ground,
anything will radiate a good signal, even a wet noodle, but even the
best antenna will radiate poorly if it is working against a poor RF
Ground."

It is the RF Ground, that determines how WELL Marconi Antennas work.
Always has, and always will.

Having said that, I would welcome learning your basis for the
conclusion, and any info you can offer on how much better and in what
way. Do you have a measurement of the HF "ground resistance" provided by
seawater using a Dynaplate? Would your conclusion change for a vessel on
the Chesapeake where salinity is quite low?

Ok, lets look at what RF Ground really means in MF/HF Marine Antenna
Systems. Put on your "Bruce's Special RF Glasses" and look at you
vessel, and lets see what the Marine enviorment really looks like to
RF. Lets take a look at the wood or plastic hulled vessel over there,
what DO WE SEE? Well, we see the vertical radiator, we see the salt
water, we see the metal on the boat that is grounded electrically
together, (bonded) we see the engine if it has one, we see the piping
and wiring that is all connected electrically, or bonded to the engine,
and nothing else. Now remember that for a Marconi Antenna, IT IS THE
RF GROUND that determines the efficency of the system. What makes a
good low impedance RF Gound? Large Flat area, perpendicular to the
Radiator, very electrically conductive. Sounds just like Salt Water,
doesn't it. Hmmmm, wonder why most really good systems use the WATER
as the RF Ground? Dahhhh. Again, remember that we are looking for a LOW
Impendance RF Ground that doesn't have much of a reactive component to
it, so that it will be good just about anywhere in the 1.6- 25Mhz range.

If you do use radials they need to be resonant which means they need
to be ¼ wavelength long at each frequency of operation or they need to
be tuned with a loading coil to make them resonant. The reason is that
if they are not resonant you will get little current into them. Your
antenna system will be unbalanced and being that the radials will
usually be closer to other wires etc. on the boat they will couple
into them before they couple to the sea. That will make the tuner coax
and control cables radiators as well, because of the higher impedance
of the radials.

We don't seem to require that the backstay (or whatever vertical
radiator we are using) be a physical 1/4 wavelength when we use a tuner.
Why would we impose that requirement on the horizontal part of the
radiating system? Isn't the famous 100 square feet of copper approach
analytically equivalent to a nonresonant "radial"?

Ok, now lets look at what we have to use to build a good LOW Impedance RF
Ground with what we see using "Bruce's Special RF Glasses". One thing to
think about, Do we really want to make a direct connection to the WATER,
for our RF Ground, and if so do we want to make that Direst Connection
a DC CONNECTION? This question is where "Elctrolysis" comes into play,
and is beyound the scope of this lecture, so for convience lets assume
we want NO DC Path to the WATER. So how do we then couple the RF to the
water effectivly? Well, we use a Capacator, and a capacitor is made up
of "Two Plates seperated by an insulator. Our capacitor has one plate
as the Salt Water, and the insulator is the Wood or Plastic Hull, and
the other plate we need to build out of what we have aboard that we can
see with our "Special Glasses". Now what are the factors that increase
the capacative coupling in a capacator? Plate surface area, and plate
speration, so we want as much surface area as we can get, as close to
the Salt Water, as we can get. The higher the coupling, the lower the
impedance of the RF Gound System, and the BETTER the antenna system
wiull function.

It is true that proximity of the horizontal radiator to other wiring can
cause problems, and this may be an unequivocal disadvantage to the
approach. Of course, an entire sailboat is in the reactive near-field
region of the vertical radiating element regardless of the "ground" used
and so the coupling issue is a matter of degree.


Not a real big issue here. Near field is basically unimportant in Marconi
Antenna Systems, except for Near Field Grounded Verticals within a few
feet of the Vertical Antenna.


Chuck
NT3G

End of Lecture Part 1 MF/HF Marine Radio Antenna
System Design / Simplified

It's the Ground, dummy, the RF Ground........

Bruce in alaska an Old MF/HF Marine RadioMan from way back.....
--
add a 2 before @

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.

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?

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

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.

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

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

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

On Sun, 28 May 2006 04:59:49 +1000, bob wrote:

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.

Hi Bob,

As I offered some time ago, how deep is fairly immaterial and your
dismissal of "many sailing web pages, Icom" and so on to then come to
the conclusion that they are "clearly wrong" is not quite so clear
why.

There are only two paths to that ground system well below the skin
depth of salt water:
(1) Through the water;
(2) Through a lot of air within the boat.

For (1), that already takes care of itself, but is a very odd method
to getting to that dynaplate. Besides, a wire tacked to the outside
of the hull, or worse simple thrashing in the surf, has got to add to
drag. Following this wet path automatically snubs how much current
will make it to the plate anyway. As Reggie offers, after 40 inches
at 7MHz it is immaterial and that wet path to the plate makes the
plate simply a tie-point. In short, you have to penetrate that skin
depth to get beneath it. Penetrating it solves the "problem" of going
too deep.

For (2), what do you stand to lose with a deeper connection that is
approached through the interior of the boat? This is a matter of
matching characteristics, which lead to issues of loss. You have the
same connection loss anyway you look at it.

So, what value is there in these page's recommendations? It insures a
connection.

Of course, I could be wrong. I've only had experience in electronics
on Big Gray Boats in salt water, or in Big Gray Submarines beneath
salt water. Ground was everywhere and death as far away as a power
lead (several dozen nuclear warheads were only slightly further away).

73's
Richard Clark, KB7QHC

bob wrote:

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

Sure. How many, how high, and how long? The foot-long wire produces
efficiency of virtually unmeasurably close to 100%. So radials can't be
significantly better. What sort of efficiency are you expecting from the
radials?

[I wrote:]
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.

Why can't you extend a wire or strip all around your boat, or make it
several feet long? There's no penalty for having it extend beyond
several skin depths. Fishermen somehow manage to keep their lines and
nets in the water -- surely you can work out a way to keep a wire in the
water.

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.

Yes. There's a vast amount of incorrect information on the web. Have you
ever browsed around audiophile pages dealing with speaker wire? I'm sure
there are innumerable astrology pages, too.

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.

All I can do is present the results that physical laws dictate. It's not
uncommon for that to be inadequate to change a person's beliefs.

Roy Lewallen, W7EL

Roy Lewallen wrote:
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.

That hole in the water is pretty important. It not only allows the
plate a few feet down on the hull to make a good connection, it also
makes the boat float better.

I'm sure sooner or later someone will tell you there isn't a hole
because where they put their boat, because when they take the boat out
of the water they can find the hole.

73 Tom

wrote:
I'm sure sooner or later someone will tell you there isn't a hole
because where they put their boat, because when they take the boat out
of the water they can find the hole.

Didn't they notice that the water lever dropped?
--
73, Cecil http://www.qsl.net/w5dxp