Join Date: Aug 2006
Location: Arlington, VA
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It's a deep and complex subject (no pun intended).
And, as you say, empirical data are hard to come by. But, evidence-based experience isn't. There's tons of that around. And, the science is pretty good re: attenuation of RF signals in various mediums, including salt water.
Here are a few "facts", followed by some experience-based opinion.
1. Attenuation of RF signals in seawater is acute...on the order of 70-80db per foot at the HF frequencies used for marine SSB.
2. Therefore, you cannot communicate from one station to another by sending RF signals through seawater.
3. RF energy pumped into seawater at depths greater than a foot or so is almost entirely consumed by heating the surrounding water.
4. There's little if any basis in fact to support the notion of "100 square feet of copper" to form a good RF ground, however much it's been touted by "authoritative" sources including engineers, radio consultants, authors, and manufacturers. Even Gordon West now says you don't need that; he recommends just running a wide strap to the nearest bronze thru-hull and forget it. And Stan Honey says much the same thing about RF grounds in his piece for West Marine.
5. You don't necessarily need to couple an antenna system on a boat to the seawater in order to have an efficient radiator of RF energy. If you don't believe this, consider the lowly dipole...especially the vertical dipole. You will look long and hard to find a more effective antenna, and it works well with NO separate RF ground, NO connection to seawater, NO connection to the hull or tanks or engine or keel, etc.
6. There are several ways to create an effective RF ground on a boat. Among these are the traditional "coupling to seawater" route...with it's many variants...and, just as effective, the "pseudo-ground", e.g., radials...tuned or not.
1. It may well be that the apparent efficiency of dynaplates, engine connections, tanks, keel, etc. comes not from the contact with water or even necessarily from the proximity and capacitative coupling to water, but rather from the copper strips and heavy wires used in such connections actually acting as radials.
2. The "seawater as the perfect ground" statement is often misinterpreted and/or misrepresented. In fact, the reflective quality of the surface of seawater is what's most important, to serve as a springboard for the RF emissions as they take off for the ionosphere and "hop" to their destination.
3. Mobile installations aboard aircraft, in vehicles, and fixed installations ashore can be very efficient without bonding to seawater. There are lots of ways to construct a suitable counterpoise.
4. The best antenna system is one which works well on your boat, and may take a variety of forms depending on the boat, the rigging and equipment, and other factors.
Last edited by btrayfors; 03-17-2008 at 10:06 PM.