In addition to Richard's words.

One thing to also keep in mind is that once you reach the maximum range
of a "lower" gain system, changing to a higher (antenna) gain system
doesn't realize a huge increase in coverage. (Ignoring the gain effects
of height that is)

The theory of course says that if you have 6dB more gain you will cover
twice the distance, simple inverse square law stuff. What happens though
is once you are at maximum distance with the lower gain system, the path
losses past that point are much higher than what you get from simple
inverse square law. I forget the actual numbers now but whilst still in
line of sight of the antenna (and a little more) you'll get roughly the
6dB loss for every distance doubling. Past LOS and out to some distance
(500km?) you lose a huge amount more, maybe 30dB per distance doubling.
After 500km the loss curve is steeper still. I'll admit I cant remember
the numbers nor the distances well but you can see that once past LOS,
where you will be at maximum low gain coverage, an extra few dB of
antenna gain wont make a lot of difference. (You can model this if it is
important to know)

The best analogy I ever heard of for describing antenna gain was those
foam rubber stress balls. In its spherical state it represented a point
source radiator. As you squash down on the ball, it becomes more like a
pattern for a normal dipole . ie the diameter in the horizontal gets
larger. Higher and higher collinear gain is represented by pushing down
harder on the ball still. You could eventually make it into a very thin
pancake with a large diameter (and be very stress relieved!). You can
also see from this analogy that in its spherical state a lot of
radiation also goes up and down, where it isn't much use normally. As it
flattens you get less and less up or down angle radiation. There is a
downside to that for example when you are skating up and down waves,
your pattern is skying one side and burying in the water in the other.
It sometimes pays to not have a too high gain collinear on a boat for
that reason.

Hope you find this useful.

BOB W5/VK2YQA

Richard Clark wrote:

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.