bob wrote:
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

Hello Bob,

Sorry, but I'm not able to answer your question as I'm still struggling
to find an appropriate mental construct. For the moment, I'm
suspending disbelief, as they say.

Roy, W7EL, has reported model results showing that a wire (probably a
few millimeters in diameter) only one foot long will produce
near-perfect (my words) results.

The greater the cross-sectional area, the better, of course, but it
would seem not to be a critical factor based on what both Roy and Reg
reported, A one inch diameter copper pipe would probably give you some
margin based on those reports. Make it a couple of feet long and slip it
through six inches or so of one of those foam "noodles" the kids use
when swimming. That will keep it afloat, ensure it is visible, and
protect the hull from damage when it collides. Remember that you will
have to figure out how to attach this pipe to your tuner. In a lot of
installations, that will mean six feet or more of wire (from tuner to
pipe) hanging over the gunwale. That wire is effectively part of your
antenna, and it will radiate. For convenience, it would make sense to
let the pipe float away from the hull by six feet or so, but that makes
the connecting wire even longer. If you hang something like that over
the side you'll doubtless want to secure it with some kind of UV
resistant line to take the strain off the wire, especially when under
way (ugh!).

On some boats, using the stainless rudder shaft could be a better
solution if you can attach to it. It is often near the surface of the
water. On other boats, the rudder shaft exits the hull well below sea
level and that probably wouldn't work.

Experiment by all means, and if you go the copper pipe route, just
remember that your zinc will be protecting whatever copper you immerse.
If you use a lot of copper (like a 1 foot diameter by 1 foot long
cylinder made of copper flashing with lots of holes drilled in it),
expect accelerated depletion of zinc.

Good luck.

Chuck

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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

To illustrate the order of magnitude of the effects :

An ideal shape of ground electrode is a hemisphere, of diameter D
metres, pressed into the soil, flush with the soil surface..

Its resistance to the soil is easily proved and calculated :

R = S / Pi / D ohms,

where S is the soil's resistivity in ohm-metres and D is the diameter
in metres. Pi = 3.14. The metric system is by far the most simple.

The resistivity of salt sea water is 0.22 ohm-metres, constant
wherever you may sample and test. Unaffected by the melting of the
glaciers.

So with a diameter of 0.22 metres = 9 inches, the electrode resistance
= 1 ohm. Low enough?

If the ground electrode is a ball with diameter = 9 inches, immersed
in sea water at a sensible depth, then the electrode resistance will
be halved.

At radio frequencies the impedance of the connection to ground will be
that of the connecting wire only, even before the resistance of the
connecting wire to the water is taken into account. The high
permittivity of water will also tend to decrease impedance at RF.

Another illustration, following Lord Kelvin :

The resistance of a ground rod to soil is given by :

R = S / 2 / Pi / L * ArcSinh( 2 * L / D ) ohms,

where S = soil resistivity, L = rod length in metres and D is rod
diameter. ArcSinh is the inverse hyperbolic Sine function you will
find on your pocket scientific calculators.

So in sea water, at low frequencies, a rod 12 inches in length and a
diameter of 1 inch will have a resistance of 1.2 ohms. At HF, because
of the very low propagation velocity in water, propagation effects
predominate and the rod must be considered as a very lossy
transmission line. But its impedance to ground is still very low
because Zo is very small.

So the hull of a metal boat makes an excellent ground. Just connect
to it with an alligator clip at the end of a length of wire and stop
worrying about it.

By the way, the practical units of resistivity in ohm-metres should be
much preferred to the academic units of milli-Siemens. When dealing
with milli-Siemens I find I have to stand on my head and look
backwards.

1 milli-Siemens = 1000 ohm-metres.

The clock tells me it's 7.30 in the morning in Birmingham, the idle,
depressed ex-industrial city of the Midlands, where there used to be
10,000 factories, now superceded by the hardworking Chinese, and I'm
already half way down a bottle of Spanish Campaneo red. Hic!
----
Reg, G4FGQ.

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

An arithmetical correction. I forgot to divide by Pi.

The resistance of a hemispherical electrode, 9 inches diameter, in
salt sea water, is even smaller. It is only 0.32 ohms.

Incidentally. the resistance of a flat circular disk of diameter D
metres, in contact with the soil surface, is given by :

R = S / 2 / D ohms,

where S = soil resistivity in ohm-metres.

In sea water, a disk of 12 inches diameter has a resistance of 0.37
ohms. Which is negligible in comparison with the radiation resistance
of a 1/4-wave vertical antenna of 36 ohms.

Careful readers should make a note of these hints and tips, free to
USA citizens, in their notebooks.

My own notebooks extend from volumes A to letter S. I'm wondering who
to leave them to in my Will & Last Testament.
----
Reg, G4FGQ.

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

"Reg Edwards" wrote:
The clock tells me it's 7.30 in the morning in Birmingham, the idle,
depressed ex-industrial city of the Midlands, where there used to be
10,000 factories, now superceded by the hardworking Chinese, and I'm
already half way down a bottle of Spanish Campaneo red. Hic!

Reg, FYI: "NAPA, Calif. - French and California winemakers marked the 30th
anniversary of the storied Paris tasting with another sip-and-spit showdown.
California won - and by more than a nose. Native wines took the top five of
10 spots, with a 1971 Ridge Monte Bello cabernet sauvignon from the Santa
Cruz mountains coming out on top Wednesday."
--
73, Cecil, W5DXP

Cecil Moore wrote:
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't find the hole.

Didn't they notice that the water lever dropped?

Not usually. They fill the hole in the water with so much money they
never notice anything else.
:-)

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.

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.


Roy, after re-reading the above comment this morning, I realized you
probably meant that RF currents flowing to the plate could simply travel
from the plate along the hull/water boundary to the water line, and from
there, change direction and travel over the water's surface. Much as
currents would travel up and down the surface of a large ocean wave
(except for hull/water vs. air/water boundary differences).

If so, then your model results show the Dynaplate's location is not
critical as long as it is attached to the hull and it need not be
mounted at the waterline?

Is my reading of your comment correct?

Thanks.

73,
Chuck


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

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