In which region is it best to sample electromagnetic fields emitted by a source?

In which region is it best to sample electromagnetic fields emitted by a source?

• A. Near Field.
• B. Mid Field.
• C. Far Field.
• D. Source Field.

Answer: (C)

The best region to sample electromagnetic fields emitted by a source is the far field. In this region the radiated field dominates and behaves like a traveling plane wave. Electric and magnetic components are perpendicular and in phase, and the ratio between them matches the intrinsic impedance of free space, making it straightforward to relate measurements to actual radiated power and the antenna’s pattern. The field strength falls off predictably with distance (roughly 1/r), and the wavefronts are effectively planar over the measurement area, so measurements are consistent and interpretable. Close to the source, in the near field, reactive (non-radiating) fields dominate; energy is stored rather than radiated, and the field distribution depends strongly on the source geometry and distance, making measurements complex and not representative of what’s actually emitted. The mid-field (Fresnel) region is a transition zone with both reactive and radiative components, which also complicates interpretation.

The best region to sample electromagnetic fields emitted by a source is the far field. In this region the radiated field dominates and behaves like a traveling plane wave. Electric and magnetic components are perpendicular and in phase, and the ratio between them matches the intrinsic impedance of free space, making it straightforward to relate measurements to actual radiated power and the antenna’s pattern. The field strength falls off predictably with distance (roughly 1/r), and the wavefronts are effectively planar over the measurement area, so measurements are consistent and interpretable.

Close to the source, in the near field, reactive (non-radiating) fields dominate; energy is stored rather than radiated, and the field distribution depends strongly on the source geometry and distance, making measurements complex and not representative of what’s actually emitted. The mid-field (Fresnel) region is a transition zone with both reactive and radiative components, which also complicates interpretation.