【FAQ】What is the Fresnel zone in a radio system?

A Fresnel zone is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. Transmitted radio waves can follow slightly different paths before reaching a receiver, especially if there are obstructions or reflecting objects between the two. The waves can arrive at slightly different times and will be slightly out of phase due to the different path lengths. Depending on the magnitude of the phase shift, the waves can interfere constructively and destructively. The size of the calculated Fresnel zone at any particular distance from the transmitter and receiver can help to predict whether obstructions or discontinuities along the path will cause significant interference.

Fresnel Zone radius formula is  (λ is the wave length, D is the distance between A and B ).