On Mon, 10 Sep 2007 20:01:16 -0500, "amdx" wrote:

How many wavelengths above ground (roof) would be considered Free Space?

Hi Mike,

I'm not sure that is very important in the scope of practicality. More
about what can go wrong below.

Regarding transmission power: it's not microwatts but it is 18 milliwatts.

Sufficient for a lot of applications. Given you are on a boat, you
have your limitations. I don't suppose the horizon is more than 10
miles away if it is a sail boat with a tall mast. To another,
similarly equipped boat, you can roughly double your range. Standing
on the deck of a small boat, you probably can only see out to 3 or 4
miles.

Fresnel Zone; I didn't really understand the why, but jist of it is, an
object
can be outside the line of site, but be in what is called the Fresnel Zone
and this object will attenuate the signal.

No.

Fresnel also goes by the name "picket fencing." This means that
reflections from nearby objects, and the direct signal cancel each
other. There are dead zones for reception. Shift things a half wave
(not very far for the band we are talking about) and they add
constructively. There are also intermediate zones which for digital
transmission can be problematic. For mobile operations like VHF in a
car, the signal from a fixed source will alternately fade and build as
though you are listening through a picket fence rushing past the
window. A choppy water can really create a witch's brew of wave
mixing (no pun intended).

Here, the solution is a gain antenna that is focused (aimed) at the
source giving it a stovepipe view, so to speak. This cuts down on all
reflections contributions and you do this not for the gain, but for
the directivity (which is euphemistically called gain). However, a
highly directive antenna, up high on a boat, in the same chop, will
bob and weave and runs the risk of that stovepipe view looking
somewhere other than at the source. Mixed bag.

73's
Richard Clark, KB7QHC