Gary Schafer wrote:
On Tue, 23 May 2006 23:44:50 +1000, Will
wrote:

I want to set up a hf antenna for my sailboat.

I have read various guides from Icom etc.

They suggest running copper foil to a Dynaplate and use sea water as
the ground. How can this work when the Dynaplate is below sea water?

Is sea water equal to copper wire radials as a RF ground system?

Does sea water make a good enough ground without radials?

How can a piece of copper metal about 1 ft square equal several
radials laying on the boats deck?

Why do i have to use copper foil when most other people suggest using
ordinary copper wire?

Over seawater what would be the best number of radials to use
considering that maximum length i can run is 40 ft. I am planning to use
a backstay antenna with a SGC 230 Tuner.

All ideas and comments appreciated.

Will


Hello Gary,

Although I'm not advocating "radials" or any other marine HF grounding
approach, I am trying to understand the reasoning used by those who do
advocate. I've tried to set up instrumentation that would allow
comparative measurements of alternative "ground" properties for yachts,
but the problems involved have been overwhelming. I think I understand
why the world is not awash in empirical data in this area, especially
compared to what is available for land-based verticals.

First, radials on a boat are not usually better than a good ground to
seawater.

I think the word "radials" in this thread ought to be in quotes, since
we're really not talking about conventional symmetrical radials from
which radiation is substantially canceled. As you point out, the
"radials" being discussed in this thread are simply a horizontal part of
the radiating antenna and not really radials at all.

Having said that, I would welcome learning your basis for the
conclusion, and any info you can offer on how much better and in what
way. Do you have a measurement of the HF "ground resistance" provided by
seawater using a Dynaplate? Would your conclusion change for a vessel on
the Chesapeake where salinity is quite low?

If you do use radials they need to be resonant which means they need
to be ¼ wavelength long at each frequency of operation or they need to
be tuned with a loading coil to make them resonant. The reason is that
if they are not resonant you will get little current into them. Your
antenna system will be unbalanced and being that the radials will
usually be closer to other wires etc. on the boat they will couple
into them before they couple to the sea. That will make the tuner coax
and control cables radiators as well, because of the higher impedance
of the radials.

We don't seem to require that the backstay (or whatever vertical
radiator we are using) be a physical 1/4 wavelength when we use a tuner.
Why would we impose that requirement on the horizontal part of the
radiating system? Isn't the famous 100 square feet of copper approach
analytically equivalent to a nonresonant "radial"?

It is true that proximity of the horizontal radiator to other wiring can
cause problems, and this may be an unequivocal disadvantage to the
approach. Of course, an entire sailboat is in the reactive near-field
region of the vertical radiating element regardless of the "ground" used
and so the coupling issue is a matter of degree.


If the radials are mounted in the bottom of the hull right near the
water they have a chance of coupling to the sea. But if you go to that
trouble it is much easier to couple directly to the seawater with some
metal under the boat. (dynaplate, through hulls etc.)

Radials that do not couple the energy to the sea act as part of the
antenna. That wouldn't be bad if that radiation went where you wanted
it to go but a large part of it will get into all sorts of things on
the boat that you don't want it to.

There are two issues he interference (already discussed above) and
efficiency. Assuming no communication value is given to near-vertical
radiation angles (a mistake, in my opinion, for the lower frequencies),
the efficiency of an L-shaped, center-fed radiator is probably no worse
than 3 dB below that of the same vertical element worked against a
perfect ground plane. That is based on simply assuming that 100% of the
power that would have been radiated by the horizontal element is
dissipated in the "ground". In practice, the typical Dynaplate-based
ground plane will be less than perfect, even over salt water, and the
horizontal element will most definitely radiate some power. Hence, my
suggestion than 3 dB is worst case and I would not be shocked to see the
Dynaplate-based system less efficient. Data, data, data.


Radials on a boat are different than when laid over earth. When laid
over earth there is tight coupling to the earth and length is not
important.

If we're now talking about real radials, I wonder if the statement is
true. Real radials elevated three feet above seawater might couple very
tightly. It would be interesting to see some analysis or measurements of
this. But we're talking about "radials" in this thread and the statement
is not relevant to them.

When they are elevated above earth they need to be resonant
in order to work. Short radials do little good in this situation.
A boat installation is similar to an elevated installation on land.

As mentioned above, I believe elevated radials (real radials) need to be
resonant and symmetrical in order to cancel radiation from them. The
problem is semantic, of course. A radiating structure with horizontal
and vertical components can be 100% efficient with the horizontal
components nowhere near resonant lengths. There will be effects on
horizontal and vertical patterns but that is impossible to generalize.


I see some guys on the Maritime mis-information forum spouting about
using radials on a boat.

You are much better off getting a connection to the sea for your
ground. It is one of the best ground planes available.

I'm curious about how these ground plates work. The attenuation in
seawater of RF at 10 MHz is on the order of 30 dB/foot. If a ground
plate is four feet below the surface, how does it work? Copper's
conductivity is orders of magnitude better than seawater. Do we know the
break even point for a copper wire ground plane vs. seawater? Reflection
properties of seawater OTOH are well-known and documented.

http://ecjones.org/physics.html
Ionospheric Physics of Radio Wave Propagation

The usual instructions for installing ground plates recommend bonding
the RF ground connection to metal tanks, the engine, lifelines, the mast
and associated steel rigging (other than the insulated backstay) etc.
Sometimes I wonder if all that metal is not forming the "other half of
the dipole", pretty much independently of the ground plate's existence.
Since a good many installations disregard your recommendation to mount
the tuner close to the ground plate, rather than close to the antenna,
that 10 feet or so may contribute to the "size" of the counterpoise when
a ground plate is used.

I suppose this is heresy, but disproving it would require testing a
ground plate with NO connections to the tuner other than transmitter,
antenna and ground plate. A manual tuner would make sense for the test.
Of course the transmitter would have to be run from an isolated battery
so as not to bond the tuner to the boat's DC ground wiring. To make
sense of the feedpoint impedance, the antenna should be a quarter
wavelength to provide a known radiation resistance, but much of it would
be buried in the hull, etc. It quickly becomes complex.

Another factor is that at the higher frequencies, a typical backstay
approaches or exceeds a half-wavelength, in which case even a "short"
length of wire, with or without a ground plate, will work well. If the
ground plate is providing one of the best ground planes available, its
impedance should actually decrease with decreases in frequency, given
the attenuation of seawater as a function of frequency. Thus, the ground
plate should be spectacular at 160 meters. Alas, I have not seen even
anecdotal information on that.

To do so you
need a very short ground lead from the tuner to the sea connection.
The best way to accomplish that is to mount the tuner down in the hull
right next to the sea ground connection within a foot or so. Then run
your antenna lead from the tuner to the antenna as much in the clear
as you can.

Remember that no matter where the tuner is mounted the antenna starts
at the ground connection. If you have a 10 foot lead from the tuners
ground connection to the sea water connection that 10 feet will
radiate like the antenna. Problem is so will the coax and tuner
control lines radiate and couple into all sorts of places you don't
want it to.


Without some kind of isolation, that is true. The flip side, though, is
that (as you pointed out) the antenna starts 10 feet "lower" and a good
deal of radiation from that 10 foot length is now inside the hull. If
your objection to the use of "radials" is unwanted coupling, then this
must surely fall into the same potential (no pun) category.

By placing the tuner at the ground connection you have control over
what radiates and what does not.

Copper foil for the ground lead to the tuner makes a lower impedance
path than doe's wire. You want the lowest impedance path you can get
for the ground lead.

73
Gary K4FMX


Be interested in your thoughts on these issues, Gary. Sorry for the length.

73,

Chuck
NT3G