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

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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.

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.

"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

Reg, G4FGQ wrote:
"At radio fgrequencies 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."

In general, Reg is correct.

I`ve installed a marine radio on a yacht, engine powered not the sailing
kind. It worked well as expected and the owner paid promptly. I`ve also
installed radios of various types in many vessels including large ones
used on the high seas and small work boats serving the near offshore.
But, I never installed an antenna using radials on a boat or ship.
Radials can be hazardous to eyes and body parts at sea. For VHF, a
1/2-wave coaxial antenna has the same gain as a ground plane but needs
no radials. For HF, it`s easy and effective to use a loaded vertical
antenna against the sea as a return path.

Best regards, Richard Harrison, K5WZI