The trick is determining exactly where your two end points are. For most U.S. cities, this is easy:
- Go to Maporama.com and enter your address in the "MAPS" box in the upper left, then click on the little orange arrow on the bottom right of the box
- Maporama may present a list of possible sites. If so, find yours and click on it.
- The lat/long for your site will be in the "INFORMATIONS" box under the map. Write that down.
- Repeat the above step for your other end point. Put your lats and longs into a lat/long distance calculator to get the distance
- Divide your distance in half and use a Fresnel zone calculator like the one at RadioLAN. (Hint: the Fresnel zone is largest at the mid-point.)(Don't forget to use 2400 MHz!)
- Divide the results by 2 to get the minimum height of your antennas.
Keep in mind that this assumes no obstructions between the two antennas and that both antennas are the same height. If the obstruction is nearer one of the antennas and/or the antennas are different height, the math is a bit more complicated.
The above does make for a good off-the-cuff W.A.G. though.