In the context of Huygens' principle and physical optics, it makes sense to consider a volume over which the phase of contributing waves is fairly constant, centred around the line-of-sight where the phase is stationary. Consider the situation in Figure D.l where the distance between the two antennas is d and the total distance to a scattering point is R₁ + R₂.

Inspec keywords: electromagnetic wave scattering

Other keywords: Huygens principle; line of sight; Fresnel zones; physical optics

Subjects: Electromagnetic wave propagation