There is an effect called the Fresnel ellipsoid which in simple terms encapsulates all the energy paths from the transmitter antenna to the receiver antenna. Any obstruction that is in the path of the ellipsoid (shaped a bit like a cigar between transmitter and receiver antenna) interferes and saps energy from the signal. On this page you will find a Fresnel radius calculator which helps when planning point to point wifi links to avoid obstructing the signal.

When we plan a wifi link we consider link budget to see the gains and losses involved. But in general these calculations are taken separately and do not include the Fresnel effect. It is always wise to consider this effect and use a fresnel radius calculator as there are many link situations that require you to compensate in some way for this effect. To follow link budget calculations see our page

## What is the Fresnel Radius Take a look at the diagram. What we see is the Fresnel envelope within which all our radio signal paths are contained. This model shows that the shape is an ellipsoid from one antenna to the other. Additionally the cross section is more or less circular. As you move one from one antenna to the other the radius of the cross section increases to a maximum radius midway between both antennas and the decreases in a corresponding way as you approach the other antenna.

This middle point defines the maximum Fresnel Radius FR (fresnel radius) and marks the most prominent point to avoid obstacles in the signal path. The Fresnel radius calculator will measure the FR at other points which will obviously be smaller than this maximum radius but needs to be calculated as obstacles could be anywhere on the signal path.

We choose a point along the signal path on the ground where we measure the FR because there is an obstacle at that point. This can then be used to measure obstacle clearance and the clearance must be at least the FR to avoid interfering with the signal paths. Notice that the FR is centred on the direct line between antennas.

## A Practical Example Fresnel Radius Calculation

First lets assume we are proposing to mount two antennas 10 km apart. Lets also say both antennas are provisionally to be erected 520 meters above some chart datum like sea level. Then assume you calculated a hill of altitude 510 meters at 4 km distance from the left hand Antenna. Using these numbers we compute from the fresnel radius calculator that the FR is 10.2 meters. In this case we are just about scrapping the envelope. The clearance here is in fact only 10 meters. Had we made an error in the height of the hill and it should be 512 hight then we would be eating into our signal envelope.
There is no tolerance here so it might be a good idea to build some in to account for errors in measurement. The obvious thing to do here is to elevate the antennas to compensate. 