On Wed, 8 Aug 2007 12:07:27 -0400, "Robert11"
wrote:

So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db),

Hi Bob,

This is usually achieved through co-linear elements. That is, one
vertical element stacked upon (above) the other, inline. Many such
"gain" antennas' details of construction are obscured by a fiberglass
shell.

or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.

Quite so. Unfortunately there is more to "gain" (or effective
sensitivity) than co-linear elements at the frequencies you are
interested in. More important is height which can make a substantial
difference in perceived "gain." If you invest any of your interest in
raising an antenna, it would reward you to also hoist a co-linear
design instead of a rubber duckie.

Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?

As stated, the co-linears. They are common.

Now, as to the need for gain. The marine band is principally limited
to line-of-sight transmissions - hence the advice for height. The
higher you are, the further you can see. The earth's horizon can be
expressed as being the square root (twice the height in feet) miles
away. So, if you are using a rubber duckie, that horizon would be
roughly 3.5 miles away (barring obstructions). If you hoisted it 30
feet in the air, the horizon would be about 8 miles away, and less
likely to be obstructed (except at the far end). If your 30 foot
antenna is listening to a ship's antenna at 30 feet, your greatest
range would be roughly 16 miles. An airplane at 30,000 feet would
stay within your range out to 245 miles.

Without obstructions, you could probably hear them (airplanes or
ships) quite clearly even if they transmitted only 100mW of power.
Gain would be unnecessary.

However, we can anticipate variables to this such as those neglected
obstructions, and what would be called propagation. The signal
becomes weaker by degrees, or by huge plunges. Some of this can be
made up for by more power by the transmitter, or more gain in
either/both antennas.

73's
Richard Clark, KB7QHC