Fresnel Zones – What are they and why are they so important?

When an installation of a wireless point to point bridge is designed and installed, it is generally thought that line of sight is required. But there is also a requirement for clearance of what is known as the Fresnel Zone (Fresnel pronounced with a silent s (Fre:nell), named after French physicist Monsieur Fresnel).

Phase Cancellation

Having direct line of sight will not always give you a perfect connection. While radio waves will aim straight from transmitter tot he receiver, the problem arises with radio waves that emanates from the transmitter that spread out at an angle from the transmitter. If these signals don’t encounter any obstacles, then they will just keep going until they dissipate. However, more often the case is that they will hit a solid surface, which can deflect the signal and reach the receiving end, which can be "out of Phase" which can reduce the power of the arriving signal, also called "Phase Cancelling".

Multiple Frensel Zones

Fresnel created a calculation on how out of phase the wave would be between the transmitter and the receiver. The Fresnel zone is a 3-D cylindrical ellipse shape (like a cigar or sausage) and is made up of multiple zones, Zone 1 being the strongest area for signal strength, Zone 2 being the weaker, Zone 3 being weaker still and so on. There are many fresnel zones, but only the first 3 have the any major effects on signal strength. Phase cancelling effect in even numbered zones have the maximum effect, while in zones with odd numbers can have a positive effect to signal strength.

So in order to maximise the signal strength at the receiver, you want to minimise the any out of phase signals from reaching the receiving end by making sure the strongest signals do not bump into any obstacles. The general rule of thumb is that the 1st Fresnel zone must be 60% clear of obstruction from the centre line of sight to the outer boundary of the 1st Fresnel zone to maintain a good connection.

Curvature of the Earth is an obstruction

For longer distance links, lets say 7 miles and above, the curvature of the earth comes into play and may become an obstruction into the Fresnel zone and cause signal loss. This is because the longer the distance between the Transmitter and receiver, the great the radius of the Fresnel Zones. There fore the height of the transmitter and the receiver become an important consideration for long distance links to ensure clearance of the gound level to maximise signal strength.