----- Original Message ----- 
From: "Alan B. Pearce" <A.B.Pearce@rl.ac.uk>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, January 26, 2006 10:38 AM
Subject: Re: Re[2]: [EE] 2.4 GHz over water???


> OK, I have thought about it, and remember the problems that TV link
> technicians had many years ago with a link that went over water - using an
> RF frequency in this general range. They had problems with the link fading
> and dropping out, in a cyclic manner. It turned out that the mid point of
> the link was a tidal inlet, and the reflection off the water had a path
> length that changed with the height of the tide, and produced cancellation
> with the direct line path. The solution was to ensure that one end of the
> link could not "see" the water at any time.
>

This example is for fixed stations - and this is absolutely true. If
something is in the first fresnel zone you will get additional attenuation
change which can lead to fading. But it is extremely unlikely that 2 boats
on the water can hold their positions so exactly that the fading will be
constant.

I have calculated the link for the standard WLANs power out and colinear
antennas on both boats with an antenna gain of 6 dB over an isotropic
antenna. The additional fresnel zone attenuation is 8 dB, the free space
path loss is around 100 dB. So the dynamic range at the receiver at 11
MBit/s is around 6 dB, which is not really much but it will work under
normal circumstances.
If you want to have the calculation please let me know.

Enrico
OE1EQW


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