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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