Handleiding Icom IC-M802 (pagina 72 van 78) (Engels)

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ANTENNA AND GROUNDING CONSIDERATIONS

2001 NEW 2001 NEW

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

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-

IC-M802_USA.qxd 02.7.9 10:59 Page 64

Icom IC-M802 Handleiding

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