I seem to recall the Fresnel radius calculation formula from the CWNA Study Guide...
r = 43.3 X (sqr/rt of (dist divided by (4 X freq))
Would this give you a reasonably accurate diameter/radius using any/all types of antenna.
Unless I'm missing something, it seems to me that a pair of Cisco 1400's using the dish antenna (5 Gig)over 2 miles would have a pretty narrow beam width compared to a couple of Cisco 1300's using Yagi's (2.4 Gig)over the same distance.
Also, the above is related to what I suspect is a row of trees along a riverbank that is directly within the LOS... A GPS will give me height above sea level for my two end-points, but how in the heck would someone go about finding the height of the potential tree obstruction about mid-way across the link?
As Brianh already pointed out, the size of the Fresnel Zone has nothing to do with the gain or beamwidth of the antennas. It is based ONLY on distance between the antennas and frequency/wavelength of the signal. Note the variables in the equation: antenna gain isn't in there.
I point this out so emphatically because it's a common mistake to see that the Fresnel Zone is obstructed and think that a higher gain antenna will fix it. The higher gain antenna probably WILL have the effect of increasing RSSI on the other end of the link, simply because more energy is getting transmtited in the direction of the other antenna, but it WON'T change the degree to which your Fresnel Zone is obstructed, and it WON'T change the fact that lots of your signal's energy is getting absorbed by that obstruction.
Thanks for the detailed explanation, much appreciated.