Icom 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 usually use a stainless steel hose clamp to grab each underwater metal source. Now let's take a look at grounding from a slightly more technical 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 technical 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 techniques 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 frequency applications will utilize both the seawater as well as radiator for its entire operation. Like two kids on a teeter-totter, 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 antenna system- one-half wavelength long on the frequency band of operation with voltage and current loops equally distributed 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 counterpoise to push off the signal from. In technical terms, the less RF ground, the higher the radiation resistance of the antenna system. This radiation resistance 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 actually 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 behavior 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 UNWANTED OCCURRENCES. SURFACE AREA For marine SSB radios a good radio frequency ground system 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 hydraulic 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 surface 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 together cement hulls can be used quite nicely as a ground counterpoise system. Copper foil and grounding screen is available from most marine electronic distributors as well as marine electronic dealers. 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 difficult 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 frequency energy passes through fibreglass, so an incapsulated ground system works just as well as one that is actually exposed to the seawater. Incapsulated lead keels with a halfinch of resin also work well. Any RF ground system that is capacitively coupled to the seawater is enhanced by the seawater itself. Developing the ground system for radio frequency applications (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 trapcircuits at certain radio frequencies, and will appear invisible as an effective way of coupling your set to your ground system. That's right, copper foil must be run from below the wa-