We're not discussing theory, we're discussing implementation. VHF and HF
differ significantly due to near-field considerations as well as physical
dimensions. A ground plane for a VHF whip can live on the same pole. For
HF, you'd need a MUCH bigger boat.
We once asked the lab to do a finite element analysis of a hunk of beef
being roasted. They came back later and said there wasn't enough computer
power in the world to do the job due to the various elements (water, fat,
protein, bone, etc.) By the same token, HF antenna theory is useless
because so many things on the boat are part of the "near field." One can't
ignore the rails, the other antennae, the people walking about, ...
In general, I tend to agree with "me" that the sea is the significant ground
plane if we're talking small boats (under 65').
400 sq.ft. of copper is 20' x 20' or so. The wavelength of 4215 KHz is 75
meters = 277 feet.
The magic comes in when the various elements interact. Even small dish
antenna are tested for their radiation pattern before going into production.
Theory, hah. Google antenna "near field" and you'll get sufficient theory,
mostly applicable to small dishes, to convince you.
As Arthur C. Clarke stated, "Any technology which is sufficiently advanced
(or complex) is indistinguishable from magic."
Roger
http://home.earthlink.net/~derbyrm
"chuck" wrote in message
ink.net...
Thanks for the comments, Roger.
While I don't disagree with what you have said (except that antennae are
magic), I don't believe your remarks touch on the issues here.
The typical marine VHF antenna, for example, does not depend upon the sea
for its operation. I am assuming the usual quarter-wave ground plane
vertical atop a mast. Do you believe that because VHF and HF antennas
involve different frequencies the underlying theory is different?
A backstay "vertical" antenna may or may not depend on the sea for its
return path. Me has unequivocally asserted that it always depends on the
sea and thus its ground must be coupled to the sea. (400 sq. ft. of copper
is no better than nothing at all he believes)
Conventional antenna theory suggests that a backstay "vertical" could
utilize a counterpoise or other return path element (like a horizontal
dipole half) with no coupling to the sea at all.
Me asserts this is false: that such an antenna will not work.
I attempted to present a "reductio ad absurdum" argument showing that if
Me is correct, his reasoning leads to absurd results. There is no magic
here.
I would be pleased to reconsider any of my comments if they appear
incorrect or incomprehensible.
Regards,
Chuck
Roger Derby wrote:
When you talk of capacitive coupling, frequency does matter. (Xc =
1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8
MHz and VHF at 180 MHz.
"Ground" is one of those elusive concepts that get more magic/conundrum
(aka BS) than it deserves. A full dipole needs no ground. The whip or
backstay needs a ground plane so that its "virtual image" creates a full
dipole.
Note that aircraft use HF communications with a half dipole antenna
(trailing wire) with no ground plane. Of course they do have an
excellent antenna height. (Don't hold the end in your fingers to test on
the surface. When your boss hits the transmit key, it hurts, for weeks.)
Antennae are magic.
Roger
http://home.earthlink.net/~derbyrm
"chuck" wrote in message
ink.net...
You seem to be saying that sea water (which is one heck of a lot less
conductive than copper -- I mean orders of magnitude less conductive) is
the only rf ground (return path) that works? And that the only way to
utilize it in a fiberglass hull is with capacitive coupling?
By your reasoning, radio communication from a vessel on fresh water is
impossible.
Or, as a corollary, radio communication from a vessel on the hard is
impossible because the capacitive coupling to the sea is over a distance
of more than 12".
And of course, for VHF, we all use the equivalent of copper that is not
capacitively coupled to the sea. Since we all agree that VHF works fine
that way, can you tell us at what frequency the laws of electromagnetic
radiation "jump ship" and no longer work the same as at VHF? A reference
would be most welcome. The issue here is radiation, not propagation.
Can you also provide a reference to the published and repeatable testing
of 400 sq. ft. of copper ground that didn't work any better than having
nothing at all? Radio stations thousands of miles from the sea would be
amazed to learn that their ground systems don't work better than nothing
at all.
Why not post your theory on rec.radio.amateur.antenna and see how it is
received there?
Suggest you reef those sails a tad, Me.
Chuck
Me wrote:
In article .com,
"Skip Gundlach" wrote:
As further background, we have full rails, with the gates combined
electrically with brass straps belowdecks, attached to the arch, the
pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail,
unless you count the inner rails, plus the arch. In addition we have
the standard 4" copper strapping leading to a sintered bronze Guest
plane below the boat, and also connected to a 3x5' plate under the
workbench top. I think we have a reasonably good ground.
You will never know if you have a "reasonably good ground", unless
you get yourself an Impedance Bridge, and check it at the frequencies
that you commonly work. Anything that is more than 12" away from the
water, isn't going to add "diddley-squat" toward building a Low
Impedance
Wideband RF Ground System, and anyone who tells you otherwise, is just
as
uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be.
I have seen all kinds of Systems that looked very impresive, untill they
were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in
the Cabin Overhead was proffered, as a really good RF Ground, by a well
known Boat Builder, 20 years ago. It didn't work any better than
having nothing at all, when tested, in a real radio enviorment. If
you got a Plastic Hull, you are NEVER going to get a Real RF Ground,
UNLESS the hull builder was smart, (they never are) and put 200+ Sq
Ft of screen under the gellcoat down by the keel. Cellulose hulls
are just as bad, and harder to retrofit that Plastic ones. Like I said
in my first reply, Autotuners were invented to allow any "Dufus" to
think he install an MF/HF Marine Radio System, and save himself all that
money he would have paid a Compitant Radioman.
SGC Autotuners are some of the worst of the lot, even if they did steal
the design from the real inventers. SGC couldn't even copy the design
correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper
Bag".
Me