*Question T3C05:
Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?*  

*Correct Answer is  A: Knife-edge diffraction*  

### AI Response: (may contain errors)  

To understand why "Knife-edge diffraction" is the correct answer for the question about how radio signals can travel beyond obstructions, let's break down each option:

### Correct Answer: Knife-edge diffraction
- **Explanation**: Knife-edge diffraction occurs when a radio wave encounters an obstacle (like a building or hill) and bends around the edge of that obstacle. This bending allows the signal to reach the receiver even if there is a direct line-of-sight obstruction. The phenomenon is particularly significant at higher frequencies where the wavelength is comparable to the size of the obstacle.

### Why the Other Choices are Incorrect:
- **B: Faraday rotation**  
  - **Explanation**: Faraday rotation refers to the rotation of the plane of polarization of a radio wave as it passes through a magnetic field. While it affects the polarization of signals, it does not help signals travel around obstructions.

- **C: Quantum tunneling**  
  - **Explanation**: Quantum tunneling is a quantum mechanical phenomenon where particles pass through a barrier that they classically shouldn't be able to. This concept does not apply to radio waves in the context of propagation and obstructions.

- **D: Doppler shift**  
  - **Explanation**: Doppler shift is the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. It affects the perceived frequency of signals but does not enable signals to bypass physical obstructions.

### Summary
- **Knife-edge diffraction** allows radio signals to bend around obstacles, making it the correct choice for this question.
- The other options relate to different physical phenomena that do not facilitate the propagation of radio waves around obstructions.

This understanding is crucial for the Extra Class exam, as it highlights the practical aspects of radio wave behavior in real-world scenarios.