*Question E0A05:
What hazard is created by operating at microwave frequencies?*  

*Correct Answer is  B: The high gain antennas commonly used can result in high exposure levels*  

### AI Response: (may contain errors)  

The correct answer to the question about hazards created by operating at microwave frequencies is:

**B: The high gain antennas commonly used can result in high exposure levels.**

### Explanation of the Correct Answer:
- **High Gain Antennas**: At microwave frequencies, antennas often have high gain to focus the energy in a specific direction. This can lead to increased exposure levels for anyone near the antenna, as the concentrated microwave energy can cause thermal effects in biological tissues. 

### Why the Other Choices are Incorrect:
- **A: Microwaves are ionizing radiation**:  
  This is incorrect because microwaves are non-ionizing radiation. Ionizing radiation, such as X-rays and gamma rays, has enough energy to remove tightly bound electrons from atoms, which can cause damage to biological tissues. Microwaves do not have this capability.

- **C: Microwaves are in the frequency range where wave velocity is higher**:  
  This statement is misleading. The speed of electromagnetic waves, including microwaves, is constant in a vacuum (approximately $3 \times 10^8$ m/s). The frequency does not affect the speed of the wave in a vacuum; it only affects the wavelength. Therefore, this choice does not accurately describe a hazard.

- **D: The extremely high frequency energy can damage the joints of antenna structures**:  
  While high-frequency energy can cause heating effects, it does not specifically damage the joints of antenna structures. The concern with microwave frequencies is primarily related to human exposure and thermal effects, not structural integrity.

### Useful Rules of Thumb:
- **Safety Guidelines**: Always follow safety guidelines for exposure to RF energy, especially when using high gain antennas. The Federal Communications Commission (FCC) and other regulatory bodies provide exposure limits to ensure safety.
- **Distance and Power**: The power density of microwave radiation decreases with distance from the antenna. A common rule of thumb is to maintain a safe distance from high gain antennas to minimize exposure.

By understanding these concepts, you can better prepare for questions related to microwave hazards on the Extra Class exam.