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