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Old 16-05-2006, 10:43   #31
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Join Date: Jan 2005
Location: Seattle area (Bremerton)
Boat: C&C Landfall 39 center cockpit "Anahita"
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Reiterating the basics

I'd just like to make a technical point that foils and meshes added below a tuner on cruising sailboats do not constitute a ground plane or a counterpoise because the dimensions required to do that for SSB frequencies are on the order of 40 meters in all horizontal directions outward from the ground point of the tuner for frequencies 4MHz and above and I don't think anyone on this forum has a vessel of those dimensions to successfully make that happen.

Such foil and mesh installations are attempting to accomplish is to form a capacitor where one plate is the foil or mesh and the other "plate" is the surface of the sea somewhat parallel with and below it. The amount of capacitance is reduced by the square of the distance between the two "plates" and, therefore, becomes critical for any particular installation as is the area of the mesh or foil.

Because of those somewhat indeterminite geometries I prefer to use direct seawater connections using low inductance wires to achieve good ground. Sure, if you already have the mesh or foil in place it can only help. Sure, if you use discrete capacitors that will also work. You can encapsulate the capacitors in epoxy or polyester if you very carefully do something to guarantee the integrity of the leads from breakage or salt water intrusion.

Grounding the engine block and/or the transceiver chassis has nothing to do with rf grounding to improve communictaions. It has only to do with personnell safety and bonding safety.

Backstays are used in an attempt to achieve an useful approximation to a vertical quarter-wave effective length of radiator at a particular frequency of operation. Any metal in close proximity (within a half-wavelength, which includes the whole boat probably) will affect the effective radiated output power in various directions from the radiator. The closer the metal is to the "drive point" of the antenna the more energy gets absorbed because the drive energy is derived from the current into the radiator and the maximum current of a quarter wave radiator is always maximum at the drive point and zero at the upper end. The voltage is zero at the drive point and maximum at the upper end. The "drive point" is physically at the end of the tuner terminal, NOT at the lower insulator. This is why I recommend not using a lower insulator and, instead, to drive the backstay from inside a lazarrette via the chainplate through bolts (disconnecting the chainplate from a bonding system via a knife-switch conveniently closed during lightning storms). At resonance, points reachable on deck down to the tuner don't generate much voltage to require kilo-volt insulation from adjacent contacts unless the entire set-up is too far from resonance, in which case you will not achieve worthy effective radiated output power anyway.

Of course you will not WANT to touch the backstay, yet if you do you will not get shocked. If one makes a definite "grab" onto the stay and the transmitter generating significant power one may feel nothing. It is when one brushes lightly against the stay that an rf burn happens to the outer layer of skin. If one leaves the skin there it gradually getts hotter and hurts. That is when one reacts and yells and maybe trips over junk standing the deck and falls over cracking their bones on the junk and deck. This is not good. However, the likelyhood of this happening is low and preventable by discipline like the discipline of not peeing overboard without wearing a safety harness.

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