Icom IC-M802 Instruction Manual page 72

14
ANTENNA AND GROUNDING CONSIDERATIONS
everything of ground potential. If you can get at your keel
bolt, or tap a screw into the keel, your grounding is done.
Lead incapsulated keels are the ultimate in grounds, and you
may need nothing further.
In powerboats, since there's no keel, you're going to need to
come up with at least 100 square feet of RF ground surface
below the water line. This means you must pick up as many
ground potentials below the water line as possible. We usu-
ally use a stainless steel hose clamp to grab each underwa-
ter metal source.
Now let's take a look at grounding from a slightly more tech-
nical point of view. Don't worry, we won't get over-technical,
and it's easy to read and understand. If you're letting a tech-
nical friend of technician put in your equipment, make sure
they read over this section several times. The facts here are
based on hundreds of hours of doing and undoing, different
types of ground systems. In all cases, following these tech-
niques will give you the results that you want and that's long
range and the loudest signal on the band.
ANTENNA GROUND PRINCIPLES,
TECHNICALLY SPEAKING
The marine antenna system for low, medium, and high fre-
quency applications will utilize both the seawater as well as
radiator for its entire operation. Like two kids on a teeter-tot-
ter, the system works well if there is a balance between the
antenna radiator and the seawater ground. This balanced
antenna system may electrically be compared to a dipole an-
tenna system— one-half wavelength long on the frequency
band of operation with voltage and current loops equally dis-
tributed throughout the half-wave length system. In marine
applications where a vertical antenna is used, this system is
more precisely referred to as a Hertz antenna set-up. The
white fiber glass whip is tuned to an electrical one-quarter
wavelength and the ground system will make up the other
one-quarter wavelength. We technically call the RF ground
system a "counterpoise," and the antenna the "radiator."
If either a one-quarter wavelength antenna or ground system
is missing or inadequate, radio reception and transmission
range will be severely reduced. How good was your car radio
reception when someone broke off your whip antenna? The
same degradation of range also takes place when there is
little or no RF ground system for antenna to work against in a
marine installation.
Imagine a swimmer making a flip turn, but not having the
side of the pool to push off from. The same thing happens
with radio wave transmissions on single sideband. The most
powerful antenna will not radiate a signal if it has not coun-
terpoise to push off the signal from.
In technical terms, the less RF ground, the higher the radia-
tion resistance of the antenna system. This radiation resis-
tance will lead to substantial power loss and single sideband
equipment will not only perform poorly, but also get quite hot
in the transmitter section. An inadequate single sideband RF
ground will also lead to "hot mikes" where the operator actu-
ally receives a radio frequency burn each time the mike is
held next to his mouth. Poor RF grounding will also lead to
erratic movement of analogue dial instruments, bizarre be-
havior of automatic pilots while transmitting on the sideband
64
set, and may even cause burn-outs of tiny integrated circuits
in companion marine electronic gear aboard. ISOLATING
YOUR RF GROUNDING SYSTEM FROM YOUR DC
GROUNDING SYSTEM WILL PREVENT THESE UN-
WANTED OCCURRENCES.
SURFACE AREA
For marine SSB radios a good radio frequency ground sys-
tem will consist of a minimum of 100 square feet of metal
below the waterline. Now we know you're going to jump out
of your chairs when you read this, but don't be overwhelmed.
There are plenty of underwater metals that we might attach
to in order to obtain this amount of counterpoise below the
waterline. Lead keels incapsulated within fiber glass will
make excellent surface area grounds in sailboat applications.
The tough part will be getting to the lead keel or the exposed
keel bolt.
In other marine installations, stainless steel tanks, copper hy-
draulic lines, and through-hulls, will help make up the RF
ground counterpoise system.
Boat manufacturers have the capabilities of adding a terrific
RF ground system when the hull is being laid up. Lightweight
copper screen is one of the best ways to provide a good sur-
face area ground. Copper screen could be laminated inside
the fiber glass layers as the hull is under construction. Thin
sheets of copper foil could also be used in the manufacturing
process of the hull. Even the conducting mesh that holds to-
gether cement hulls can be used quite nicely as a ground
counterpoise system.
Copper foil and grounding screen is available from most ma-
rine electronic distributors as well as marine electronic deal-
ers. Thickness of the screen and foil is not important, the
most common foil is generally one to four mills thick and
comes in three-inch wide rolls of just about any length you
want. Copper window screen (if you can find it) is usable,
thickness is not important. Radio frequency energy travels
on the outside of this conducting surface called skin effect,
eliminating the need for thick grounding materials.
Since grounding foil and screen is relatively expensive, most
boat builders will simply leave out this grounding process
and expect the customer to provide their own RF ground
once the boat is finished. This is a shame— it's so easy to
build in when the hull is under construction, and far more dif-
ficult to add after the vessel is fully completed.
The copper foil and screen does not actually need to contact
seawater in order to create the ground system. Radio fre-
quency energy passes through fibreglass, so an incapsu-
lated ground system works just as well as one that is actually
exposed to the seawater. Incapsulated lead keels with a half-
inch of resin also work well. Any RF ground system that is
capacitively coupled to the seawater is enhanced by the sea-
water itself.
Developing the ground system for radio frequency applica-
tions (as opposed to DC grounding systems) requires that all
ground connections be interlaced using copper foil. Round
ground wires are out! Wires actually look like inductive trap-
circuits at certain radio frequencies, and will appear invisible
as an effective way of coupling your set to your ground sys-
tem. That's right, copper foil must be run from below the wa-