Question
What is the far field of an antenna?
Answer Options
- A) The region of the ionosphere where radiated power is not refracted
- B) The region where radiated power dissipates over a specified time period
- C) The region where radiated field strengths are constant
- D) The region where the shape of the radiation pattern no longer varies with distance
Correct Answer: D
Explanation
The electromagnetic field generated by an antenna is divided into two regions: the near field and the far field. The near field (or Fresnel region) is complex, with energy being stored and returned to the antenna, and the pattern changing significantly with distance. The far field (or Fraunhofer region) is where the field has stabilized.
Antenna performance characteristics, such as gain and directivity, are measured only in the far field. The far field is the region where the shape of the radiation pattern no longer varies with distance. In this region, the wave front is essentially flat, and the electric and magnetic fields are perpendicular, functioning as a true plane wave propagating into space.
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