Which statement characterizes the far field?

Which statement characterizes the far field?

• A. The electric and magnetic fields are purely non-propagating far from the source.
• B. The near-field region begins at several wavelengths from the source.
• C. Electric and magnetic fields are at a distance of a few wavelengths from the emitter and are the best place to sample.
• D. The fields only exist inside the emitter.

Answer: (C)

In the far field, electromagnetic energy travels away from the source as a propagating wave. The electric and magnetic fields are perpendicular to each other and to the direction of travel, they are in phase, and their strengths decrease with distance roughly as 1/r. This region is where measurements reflect the actual radiation pattern of the source rather than stored or reactive energy near the source. Being a distance of a few wavelengths from the emitter puts you in that propagating, radiating regime. It’s far enough that reactive near-field effects have diminished, so the fields you sample represent the true radiated field rather than energy stored close to the source. The exact boundary depends on the source size and frequency, but the idea is that the far field is where the wave is essentially transverse and radiation-dominated, making it the best place to sample for characterizing emission. The other statements mischaracterize the far field. The far field is not purely non-propagating; that describes reactive near-field behavior. The near-field itself is generally closer in, within roughly a wavelength or so (depending on details like antenna size). And the fields do exist outside the emitter; they are not confined to inside the source.

In the far field, electromagnetic energy travels away from the source as a propagating wave. The electric and magnetic fields are perpendicular to each other and to the direction of travel, they are in phase, and their strengths decrease with distance roughly as 1/r. This region is where measurements reflect the actual radiation pattern of the source rather than stored or reactive energy near the source.

Being a distance of a few wavelengths from the emitter puts you in that propagating, radiating regime. It’s far enough that reactive near-field effects have diminished, so the fields you sample represent the true radiated field rather than energy stored close to the source. The exact boundary depends on the source size and frequency, but the idea is that the far field is where the wave is essentially transverse and radiation-dominated, making it the best place to sample for characterizing emission.

The other statements mischaracterize the far field. The far field is not purely non-propagating; that describes reactive near-field behavior. The near-field itself is generally closer in, within roughly a wavelength or so (depending on details like antenna size). And the fields do exist outside the emitter; they are not confined to inside the source.