Each electromagnetic field can be divided into four zones: the near zone, the intermediate zone, the far zone, and the plane-wave zone. The near zone is the portion of the field close to the source. It is defined as the region where stored energy is much greater than any radiating energy. The far zone is the region where:

1) the stored field energy is much less than the radiating energy,

2) the wave impedance is approximately ho = 120p, and 3) the electric and magnetic fields are perpendicular to one another.

The intermediate zone is the region between the near and far zones. The plane-wave zone is the region in the far zone where the radiation can be approximated as plane waves.

This last zone is different from the others because its definition depends on the size of the receiving antenna.

Just as a basketball’s surface appears curved to a human but relatively flat to a tiny microorganism on the surface, and the Earth’s surface appears flat to a walking human, the apparent flatness of a curved wavefront depends on the relative size of the observer.

In optics, the plane-wave zone is called the Fraunhofer zone, and the combination of the three other zones is called the Fresnel zone.

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