dear
it is the ellipsoidal area which the re is clear view or in other the ellipsoidal area the you have line of sight between receiver end and transmitter end .
and you can calculate the area of the ellipsoidal by this equation .
R=17.32(D/4f)^1/2
is it easy to calculate the hieght of the antenna of any microwave hop,if the area u are planning for mountainee area where the terrain factor is maximum i.e 42