Fresnel zone

A Fresnel zone (/freɪˈnɛl/ fray-NEL), named for physicist Augustin-Jean Fresnel, is one of a series of concentric prolate ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system. The concept is used to understand and compute the strength of waves (such as sound or radio waves) propagating between a transmitter and a receiver.

Each Fresnel zone is defined by the phase-shift (or lack thereof) which ultimately occurs when a transmitted sine wave deflects off an object within that region and continues onward to the receiver. These regions can be important to take into account because a sine wave following one of these paths may arrive at the receiver out of sync with the line-of-sight sine wave by anywhere from 1º to 359º of phase shift (or more). If an object is within a particular Fresnel zone region, a part of the original signal which is not moving towards the receiving antenna and would otherwise have continued onward into space, may be partially deflected by that object back to the receiving antenna. The deflected signal can result in constructive interference or destructive interference at the receiver, depending on the path length and the resulting degree of phase shift in the deflected wave as it arrives at the receiver.

This is the cause of the picket-fencing effect when either the radio transmitter or receiver is moving, and the high and low signal strength zones are above and below the receiver's cut-off threshold. The extreme variations of signal strength at the receiver can cause interruptions in the communications link, or even prevent a signal from being received at all.

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