Peter, to follow up on a few of your RF questions here:
The idea behind the Fresnel zones is that objects (larger than one wavelength, a few centimeters at the frequencies we're dealing with here) within the zones will reflect the incoming radio wave from the transmitting radio. As seen by the receiver, there will be two signals- one coming directly from the transmitting antenna, and the reflection coming from the object in the Fresnel zone. The reflected signal, having a longer overall path length, will be slightly out of phase compared to the direct wave, and will destructively interfere with the direct wave, lowering the overall received power level seen by the receiving radio. This is called multipath interference. Therefore, unless you're using very high gain antennas (large parabolic dishes) with high directivity, you won't gain anything by pointing them at the sky or away from the object in the Fresnel zone. You'll lose more signal by mis-aiming the antennas than you will lose from the multipath interference.
Regarding antenna polarization, your flat panel antennas are certainly polarized and must be oriented in the same polarization at each end.
Finally, if you have a way to check the received power level at your existing radios, you will want to adjust the transmitter output power of each end so that the received power is within a reasonable range. Generally speaking, for a link of that distance, you should aim for something in the -60 dBm range. Anything hotter than a -50 and you start to get into front-end overload territory, and anything weaker than a -70 and you're beginning to run on thin fade margins.
Also, I disagree that shielded Ethernet cable is unnecessary. For the very low additional cost of shielded outdoor cat-5, it's well worth your effort if you're running new cable. Of course every installation is different, but why risk ethernet errors due to some large air conditioner or something on the roof spewing EMI?
Patrick Shoemaker
Vector Data Systems LLC
shoemakerp@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com