"Short Wave Sportfishing" wrote in message
...
On Wed, 20 Apr 2005 19:27:22 -0400, "JimH" wrote:


"Dan Krueger" wrote in message
hlink.net...
The difference in four feet (assuming the same gain the the antennae)
you
will only be able to communicate 1.18 miles further.

True, or close to it depending on the db gain you used. ;-)

Calculation for Range of an Antenna:

6db Antennas: Square Root of Height (in feet) above water x 1.15= Range in
miles
9db Antennas: Square Root of Height (in feet) above water x 1.52= Range in
miles

So we are talking a range of roughly 3 miles for the 4 foot vs 4.25 miles
for the 8 foot.

Not a big difference as you said.

You have to remember that the db figure is based on a free space
formula - in short, an imaginary antenna range - it's all math and
perfect performance.

In the real world it's not any where near 6 db for a variety of
reasons including standing waves, ground conditions, radio
performance, height, ground plane, data, data, data. The reason
manufacturer's can advertise the specs is because, in theory, that's
what the antenna will produce given perfect conditions.

Also be careful about the line-of-sight deal. FM also relies on
ground wave much as AM does - it's the reason you can receive an FM
station more than line-of-sight from the FM station antenna.

By the way, there is no such thing as a 9 db antenna unless you are
talking about a Yagi.

Later,

Tom

Yeah, but the point, as Dan brought out, is there is only about a 25%
increase in range when going with an 8 foot antenna of equal ratings and
positioned at the same height (from the waterline) on the boat, which turns
out to be a little more than a mile.