Fresnel zone

Lore & Background

In the early 19th century, French scientist Augustin-Jean Fresnel created a method to calculate where the zones are — that is, whether a given obstacle will cause mostly in-phase or mostly out-of-phase deflections between the transmitter and the receiver. The primary wave travels in a relatively straight line from transmitter to receiver, but aberrant transmitted radio, sound, or light waves transmitted at the same time can follow slightly different paths, especially if there are obstructions or deflecting objects between the two. The two waves can arrive at the receiver at slightly different times, and the aberrant wave may arrive out of phase with the primary wave due to the different path lengths, leading to constructive or destructive interference depending on the magnitude of the phase difference.

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 are ellipsoids with foci at the transmitter and receiver. The first region includes the ellipsoidal space through which the direct line-of-sight signal passes; a stray component bouncing off an object within this region will have a phase shift less than a quarter-wavelength (less than 90°), potentially having a positive impact. The second region surrounds the first; a reflective object there will cause a shift greater than 90° but less than 270°, generally unfavorable. The third region surrounds the second, and deflected waves captured by the receiver will have the same effect as a wave in the first region, potentially arriving exactly in sync if shifted precisely one wavelength.

Reader's Guide

Fresnel zone clearance analysis is used to anticipate obstacle clearances required when designing highly directive systems such as microwave parabolic antenna systems. Although clear line-of-sight between transmitter and receiver may seem all that is required for a strong antenna system, obstructions within the first Fresnel zone can cause significant weakness even if they are not blocking the apparent line-of-sight signal path. The dependence on interference with clearance is the cause of the picket-fencing effect when either the radio transmitter or receiver is moving, with high and low signal strength zones above and below the receiver's cut-off threshold, potentially causing interruptions or preventing a signal from being received at all. Fresnel zones are seen in optics, radio communications, electrodynamics, seismology, acoustics, gravitational radiation, and other situations involving the radiation of waves and multipath propagation. The rule of thumb is that the primary Fresnel zone should ideally be 80% clear of obstacles, but must be at least 60% clear; some obstruction can often be tolerated, with a maximum allowable obstruction of 40% and a recommended obstruction of 20% or less.

Did You Know?

• The first Fresnel zone must be kept largely free from obstructions to avoid interfering with radio reception.
• A stray wave bouncing off an object in the first Fresnel zone will have a phase shift less than 90° and may positively impact signal strength.
• The picket-fencing effect occurs when a moving transmitter or receiver causes signal strength to vary above and below the receiver's cut-off threshold.
• Fresnel zone computations are used to anticipate obstacle clearances for microwave parabolic antenna systems.