As to 3dB being 'best' for a sailboat, the reason is that the vertical azimuth gain pattern of any single whip antenna looks like a donut centered on the wip. A flattish donut (think Frisbee), like a 9dB antenna, is strongest in a horizontal plane with the antenna vertical. Stray above and below the horizontal, and the gain drops off fast. But it's very strong in or near that horizontal plane (9dB). Drop down to a lower gain, and the donut pattern gets thicker, and a 3dB sailboat mast antenna has a fat donut (think Dunkin' Donuts brown sugar), with lower gain in the horizontal plane, but the gain doesn't drop off as fast as you move above or below horizontal - or tilt the antenna from vertical, as sailboats do when heeled.BOAT wrote:With all that jazz I'm getting here about my radio signals falling out of the wire because it's bent too sharp (so don't bend it real tight) or wrong wire (see my links above) and "base loading" (that's just the antenna construction - the lump at the bottom so the whip can be short - see links above) and fabricating antenna mounts (I can't even weld!) (I put up a link to a ratchet mount, but just a common 90 degree fixed antenna mount is fine) it's scaring me (no reason it should) so I'm thinking of just replacing my existing radio and giving up on the idea of having two radios. ( )
So a reduced gain, but an increase in elevation plus less reduction in transmitted signal strength due to heeling is why they're what's normally used on sailboat masts. Power boats don't heel under normal conditions, and they don't have much elevation, so a flatter pattern and higher transmitted signal strength makes more sense.
And a 6 ft or more white fiberglass antenna on top of a mast would look funny.
The first graphic (that I found by sniffing around) shows the relative signal strength at various angles (though actual azimuth angles aren't shown). You can see that where the envelope of the red 6dB meets the yellow 3dB, an increase in heel angle would render the 6dB just about useless, but the gain of the 3dB is still high, and doesn't drop off much with greater heel angle, up to a point. The blue 9dB is out of the running long before that angle.
Like this one - heel the boat a bit, and where a horizontal line intersects the 'envelope' would be in the negative gain region, depending on heel. That's why you don't use a high-gain antenna on a mast - tilt it even a little, and it's mostly going into the water or up to an airliner. Like a directional TV antenna, when aimed away from the transmitters - it can't grab much signal, and can even reject it with prejudice.
So you want your antenna 'aimed' at the other guy's antenna, and unless you have a way to manually tilt the antenna on top of your mast to compensate for heel, it's best just to have one with a 'wider' up/down patten, at the expense of lower power in any direction overall. Because you never get something for nothing.
Like a floodlight vs a spotlight, both with the same wattage. Spotlight has brighter light at it's best angle, and floodlight covers a broader angle at a lower intensity.