One thing to keep in mind is that the noise floor is not static. Also environmental changes will impact your links.
I try to design links to have more fade margin than needed. I can alsways turn down the power to what is needed but I sleep better knowing that I have extra margins.
Jerry makes a great point.
Links that are out in the boonies, I usually just try and calculate enough margin to deal with the environment.
Links in the city or over cities, I calculate for the environment, and then go extra to deal with issues like interference.
I’ve had much better performance from the PacWireless Grid’s- I won’t use the circular grids at all anymore.
http://www.pacwireless.com/products/GDxx-MT.shtml
GD-29 MT is what I use on all my 10/20 Backhaul links now.
They are a bit steep on the shipping but anything that size is going to be.
As long as spectrum is clean (ie no noise or interference), and just run 10 mbps I would go with 27RD. Canopy says 35 miles in their specs with reflector at both ends in optimal conditions.
It will differ from person to person as how much margin one would keep in your links. Jerry in 7.3.6 how do you lower power in 5.7 Ghz. There are only 2 steps, ie low and normal. You cant raise the power by just 2-3 dB.