On Sat, 30 Jul 2005 19:58:46 GMT, Me wrote:

In article ,
Brian Whatcott wrote:

Hehe...why don't you tell us what you really think, anonymous poster?

If it takes 200 sq ft of screen under a gel coat to make a good RF
ground, then folks who attempt a similar feat through a hull would
need about 0.5 / 0.05 X 200 sq ft of material - That's 2000 sq ft of
foil or metal mesh (for a 0.05 in gelcoat, and a mere 0.5 inch hull
thickness)

Now THAT would be quite a trick - a square about 100 ft by 20 ft.
Better not tell the folks who use an antenna coupled through a 1/4
inch glass shield - a coupler that can measure 1.5 inch square.
Admitted, this is often for FM radio (say 90 MHz) as opposed to
3MHz (?) on hf. Using these numbers for comparison,
90/3 X 0.5/0.25 X 2.25 sq in = 135 sq in of ground plane, hmmmm that's
1 sq foot in round numbers. Now that *does* look small to me.

I expect the truth lies somewhere between 1 sq ft and 2000 sq ft.
through a half inch hull section. There! How mealy-mouthed is THAT!
:-)

OK, let's get serious: how about locating a bronze through hull,
and connecting a copper foil externally in contact with it.
a square foot THERE, connected internally with good Litz wire might
make a serviceable ground....

Brian Whatcott Altus
p.s A hint for you: talking about "compitant radiomen" makes
prospective customers nervous! :-)

Nice thought Brian.... However what you seem to forget is that
capacative couping at RF Frequencies, is determined by three things.
1. Area of the Inside the hull Plate.
2. Distance between the two Plates.
3. Conductivity of the Sea Water Plate.

A 200 Sq Ft Screen in the cabin overhead isn't near (Orders of magnitude)
big enough to be an RF Ground for a MF/HF Antenna System, by itself.
Being located far (relative in capactive terms) from the other plate
(Salt water) means that the Rf couping into the water is what, can you
guess? Less than a few Picofarads. Now calculate the RF Impedance for
such a system at ANY, and All MF/HF Marine Frequencies that you like to
use, if you have enough computer power in your supercomputer. then come
back and explain it all to the rest of the world.


Me

Oh my! Anonymous poster, it was *YOUR* suggestion that an RF ground
of 200 sq ft of mesh under the (external hull) gel-coat was required
for a satisfactory RF ground at HF.

It was the original poster's suggestion of an elevated mesh that
caught your interest, not mine.

As you asked about antenna testing, I should mention that
a supercomputer is not really necessary: there is a handy dandy
gadget ( from MFJ ) which combines several RF test functions like
antenna bridge, SW ratio etc. It ran about $200 as I recall. I
satisfy myself with an LC meter these days - which gets one into the
ball park at $100.
eBay has an MFJ noise bridge at $25 currently.


Hmmm...the capacitance to ground of a few objects in my vicinity runs
about 45 pF per sq ft. Like me, standing on carpet It is just
possible a hi level ground screen of 200 sq ft might get you 200 X 45
pF = 9000 pF

At 3 MHz that would put the capacitive reactance at
1/2pi.f.C ohms = 6 ohms. Not that great. The actual value might
well be quite a bit higher than that.
But that's just me measuring with an instrument, rather than you
guessing how poor it is.
Another thing: the conductivity of sea water does not vary all that
much - it doesn't have much impact on capacitance. But there I go
again, actually measuring things!

Regards

Brian Whatcott
p.s. I have an FCC GROL+rdr. You?