the radiation zone

Around any radiating source, space divides into regions. Close in, the fields are complicated and cling to the source; far out, they simplify into clean outgoing waves. The radiation zone (also far zone or wave zone) is that faraway region where the fields have organized themselves into genuine radiation that escapes to infinity.

Quantitatively, the radiation zone is the region r >> lambda, many wavelengths from a source itself much smaller than lambda. There the electric and magnetic fields fall off as 1/r, are mutually perpendicular and both transverse to the outward radial direction n, and locally satisfy E = cB like a plane wave. Crucially, the Poynting vector (energy flux) then falls as 1/r^2 — exactly the rate at which the area of a sphere grows — so the total power crossing any large sphere is independent of its radius: energy really is being carried away, not merely stored. Nearer in, the near zone (r << lambda) is dominated by quasi-static fields that fall faster (like 1/r^2 or 1/r^3) and carry no net power to infinity.

The 1/r behavior is the whole point: any term falling faster than 1/r contributes zero net power at infinity, because the energy flux (about field-squared, so 1/r^2 or steeper) dies off faster than the sphere grows. This is why, of all the terms in the Liénard-Wiechert fields, only the acceleration (1/r) part is called the radiation field. Almost all of radiation theory — antenna patterns, Larmor's formula, scattering cross-sections — is computed in the radiation zone.