Understanding the Concept:
The space surrounding a transmitting antenna is divided into three distinct regions based on how the electromagnetic fields behave as you move further away from the antenna structure:
• Reactive Near-Field Region: The zone immediately next to the antenna structure where inductive and capacitive fields dominate.
• Radiating Near-Field (Fresnel Zone) Region: The intermediate zone where the radiation fields begin to dominate, but the angular distribution of the fields still depends on the distance from the antenna.
• Far-Field (Fraunhofer Zone) Region: The outer region where the wave behaves as a plane wave, and its spatial power distribution pattern becomes independent of distance.
Step 1: Identifying the Regions and their Names
In antenna theory, the primary regions are named after the scientists who discovered the corresponding optical diffraction behaviors:
• The Fraunhofer region refers explicitly to the Far-Field zone, where waves are parallel and highly predictable.
• The Fresnel region refers explicitly to the radiating Near-Field zone, located between the reactive near-field and the far-field boundary.
Step 2: Defining the Boundary Limits
For an antenna with a maximum dimension $D$ operating at a wavelength $\lambda$, the boundary separating the Fresnel radiating near-field from the Fraunhofer far-field is defined by the standard equation:
\[
R = \frac{2D^2}{\lambda}
\]
Any distance closer than this boundary value $R$ falls within the near-field classification. Therefore, the Fresnel zone is directly associated with the near field characteristics of electromagnetic wave propagation.
Step 3: Disproving alternative choices
• Far Field: Known exclusively as the Fraunhofer zone, which is the opposite of the Fresnel region.
• Electrostatic Field Reactive Field: These terms describe static electric charges or the immediate storage zone within a fraction of a wavelength from the antenna, rather than the broader radiating Fresnel region.