first Fresnel zone

first Fresnel zone [¦fərst frə′nel ‚zōn]

(electromagnetism)

Circular portion of a wavefront transverse to the line between an emitter and a more distant point, where the resultant disturbance is being observed, whose center is the intersection of the front with the direct ray, and whose radius is such that the shortest path from the emitter through the periphery to the receiving point is one-half wavelength longer than the direct ray.