Question E9B08:
What is the far field of an antenna?  

Choices:  
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 is  D: The region where the shape of the radiation pattern no longer varies with distance  

Correct answer: D — The region where the shape of the radiation pattern no longer varies with distance

1) Why D is correct
- The far field (Fraunhofer region) is defined as the region far enough from the antenna that the angular distribution of the radiated fields (the pattern) is fixed. Only the magnitude changes with distance, not the pattern shape.
- In this region:
  - Fields are predominantly transverse, with $E$ and $H$ in phase and orthogonal.
  - Field strengths fall off as $1/r$.
  - The wave impedance is approximately that of free space: $E/H \approx \eta_0 \approx 377\ \Omega$.

2) Why the other choices are incorrect
- A: The ionosphere is unrelated to defining near/far field regions of an antenna. Refraction in the ionosphere is a propagation phenomenon, not an antenna field-region definition.
- B: “Dissipates over a specified time period” describes a time process, not a spatial region around an antenna.
- C: Field strengths are not constant in the far field; they decrease approximately as $1/r$. What is constant is the pattern shape (the relative angular distribution), not the absolute field level.

3) Useful formulas and rules of thumb
- Far-field boundary (for an antenna of maximum dimension $D$ and wavelength $\lambda$):
  $$R_{FF} \approx \frac{2D^{2}}{\lambda}$$
- In the far field:
  $$|E|, |H| \propto \frac{1}{r},\quad \frac{|E|}{|H|} \approx \eta_0 \approx 377\ \Omega$$
- Reactive near field (for electrically large antennas) is roughly within:
  $$r \lesssim 0.62\,\sqrt{\frac{D^{3}}{\lambda}}$$
- Practical rule: Beyond several wavelengths (and certainly beyond $2D^{2}/\lambda$), measurements reflect the true radiation pattern.