RF101 - Fresnel Zones
For RF communications, a Fresnel zone, named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoids in the radiation pattern of a circular aperture. Fresnel zones result from diffraction by the circular aperture. The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first.
If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver (Direct Line of Sight - LOS). However, if there are reflective surfaces along the path, such as bodies of water or smooth terrain, the radio waves reflecting off those surfaces may arrive either out of phase or in phase with the signals that travel directly to the receiver. Waves that reflect off of surfaces within an even Fresnel zone are out of phase with the direct-path wave and reduce the power of the received signal. Waves that reflect off of surfaces within an odd Fresnel zone are in phase with the direct-path wave and can enhance the power of the received signal. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the signal-to-noise ratio. Fresnel provided a means to calculate where the zones are--where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone they will be 180 to 360 degrees out of phase, and so on. Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power.
To maximize receiver strength, one needs to minimize the effect of obstruction loss by removing obstacles from the radio frequency LOS. The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone. Obstacles that appear in the first Fresnel zone have a typical loss impact to the received signal even with direct LOS available and should be avoided.
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