In article t,
chuck wrote:

\
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.

What you need to understand is just what kind of antenna are you trying
to describe, here as a MF/HF Marine Antenna? Are you thinking a Marconi,
Vertical Dipole, Offcenter Feed Marconi, or just what? The standard
MF/HF Marine Antenna, usually is considered a Marconi, and that is what
MOST both commercial and non-commercial MF/HF Marine antennas end up
being. So lets discuss Marine Marconi Antennas, and what makes good
ones and bad ones. Marconi Antennas are charactorized by 1/4 Lambda
Vertical Radiating Element, sitting in close proximity to a LOW
Impedance RF Ground System Perpendicular to the 1/4wave Vertical.
In the MF/HF Marine enviorment we add a Tuner, Auto or Manual so as
to be able to tune the Vertical to a 1/4 Wave Electrical Length and
Resonance on each frequency that the vessel maybe required to use in the
MF/HF Marine Frequency Bands, which cover 1.6Mhz to 25 Mhz in frequency.

Ok now lets look at what the vertical is. A Backstay, a Whip, a Loaded
Whip, a Loaded Whip with wire under it. It really doesn't matter, as
they all will be tuned to resonance, in either 1/4Wave, or 3/4Wave by
the tuner, against the impedance of whatever RF Ground is connected to
the tuner.

Hence the "Old RadioMans Addage", "If you have a Good RF Ground,
anything will radiate a good signal, even a wet noodle, but even the
best antenna will radiate poorly if it is working against a poor RF
Ground."

It is the RF Ground, that determines how WELL Marconi Antennas work.
Always has, and always will.

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?

Ok, lets look at what RF Ground really means in MF/HF Marine Antenna
Systems. Put on your "Bruce's Special RF Glasses" and look at you
vessel, and lets see what the Marine enviorment really looks like to
RF. Lets take a look at the wood or plastic hulled vessel over there,
what DO WE SEE? Well, we see the vertical radiator, we see the salt
water, we see the metal on the boat that is grounded electrically
together, (bonded) we see the engine if it has one, we see the piping
and wiring that is all connected electrically, or bonded to the engine,
and nothing else. Now remember that for a Marconi Antenna, IT IS THE
RF GROUND that determines the efficency of the system. What makes a
good low impedance RF Gound? Large Flat area, perpendicular to the
Radiator, very electrically conductive. Sounds just like Salt Water,
doesn't it. Hmmmm, wonder why most really good systems use the WATER
as the RF Ground? Dahhhh. Again, remember that we are looking for a LOW
Impendance RF Ground that doesn't have much of a reactive component to
it, so that it will be good just about anywhere in the 1.6- 25Mhz range.

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"?

Ok, now lets look at what we have to use to build a good LOW Impedance RF
Ground with what we see using "Bruce's Special RF Glasses". One thing to
think about, Do we really want to make a direct connection to the WATER,
for our RF Ground, and if so do we want to make that Direst Connection
a DC CONNECTION? This question is where "Elctrolysis" comes into play,
and is beyound the scope of this lecture, so for convience lets assume
we want NO DC Path to the WATER. So how do we then couple the RF to the
water effectivly? Well, we use a Capacator, and a capacitor is made up
of "Two Plates seperated by an insulator. Our capacitor has one plate
as the Salt Water, and the insulator is the Wood or Plastic Hull, and
the other plate we need to build out of what we have aboard that we can
see with our "Special Glasses". Now what are the factors that increase
the capacative coupling in a capacator? Plate surface area, and plate
speration, so we want as much surface area as we can get, as close to
the Salt Water, as we can get. The higher the coupling, the lower the
impedance of the RF Gound System, and the BETTER the antenna system
wiull function.

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.


Not a real big issue here. Near field is basically unimportant in Marconi
Antenna Systems, except for Near Field Grounded Verticals within a few
feet of the Vertical Antenna.


Chuck
NT3G

End of Lecture Part 1 MF/HF Marine Radio Antenna
System Design / Simplified

It's the Ground, dummy, the RF Ground........

Bruce in alaska an Old MF/HF Marine RadioMan from way back.....
--
add a 2 before @