Editing Fresnel zone (section)

==Significance==
In any wave-propagated transmission between a transmitter and receiver, some amount of the radiated wave propagates off-axis (not on the line-of-sight path between transmitter and receiver). This can then [[Reflection (physics)|deflect]] off objects and then radiate to the receiver. However, the direct-path wave and the deflected-path wave may arrive out of [[Phase (waves)|phase]], leading to [[destructive interference]] when the phase difference is half an odd integer (<math>{(2z+1)/2, z \in \mathbb Z}</math>) multiple of the [[frequency|period]]. The n-th Fresnel zone is defined as the locus of points in 3D space such that a 2-segment path from the transmitter to the receiver that deflects off a point on that surface will be between n-1 and n half-wavelengths out of phase with the straight-line path. The boundaries of these zones will be ellipsoids with foci at the transmitter and receiver. In order to ensure limited interference, such transmission paths are designed with a certain clearance distance determined by a Fresnel-zone analysis. 

The dependence on the interference on clearance 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 (electronics)|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.

Fresnel zones are seen in [[optics]], [[radio communications]], [[electrodynamics]], [[Banana Doughnut theory|seismology]], [[acoustics]], [[gravitational radiation]], and other situations involving the radiation of waves and [[multipath propagation]]. Fresnel zone computations are used to anticipate obstacle clearances required when designing highly directive systems such as [[Microwave transmission|microwave]] [[parabolic antenna]] systems. Although intuitively, clear line-of-sight between transmitter and receiver may seem to be all that is required for a strong antenna system, but because of the complex nature of radio waves, obstructions within the first Fresnel zone can cause significant weakness, even if those obstructions are not blocking the apparent line-of-sight signal path. For this reason, it is valuable to do a calculation of the size of the 1st, or primary, Fresnel zone for a given antenna system. Doing this will enable the antenna installer to decide if an obstacle, such as a tree, is going to make a significant impact on signal strength. The rule of thumb is that the primary Fresnel zone would ideally be 80% clear of obstacles, but must be at least 60% clear.