WiFi (2.4Ghz) Link Calculation

While searching around on how to calculate the maximum distance of a WiFi connection, I came across this very useful link budget calculator.

So far we’re still searching for a way to calculate the area of a fresnel zone which falls below ground. Based on a 1km link the zone starts at the antenna (8m high), extends to the user (1m high), and the fresnel zone has a radius at its centre point of 5.6m. This zone will intersect the ground at 371.181m and 990.905m from the antenna – thanks to Rory McCann for calculating this.

We’re still trying to work out the area of the Freshnel which travels underground. After doing this, we only then need to calculate the loss in dB. Any ideas?