*Question E9A08:
Which frequency band has the smallest first Fresnel zone?*  

*Correct Answer is  A: 5.8 GHz*  

### AI Response: (may contain errors)  

To determine which frequency band has the smallest first Fresnel zone, we need to understand how the Fresnel zone is affected by frequency.

The radius of the first Fresnel zone ($F_1$) can be calculated using the formula:

$$
F_1 = \sqrt{\frac{\lambda D}{2}}
$$

where:
- $F_1$ = radius of the first Fresnel zone
- $\lambda$ = wavelength of the signal
- $D$ = distance between the transmitter and receiver

The wavelength ($\lambda$) is inversely proportional to frequency ($f$):

$$
\lambda = \frac{c}{f}
$$

where:
- $c$ = speed of light (approximately $3 \times 10^8$ m/s)
- $f$ = frequency in Hz

### Explanation of the Correct Answer:
1. **Correct Answer: 5.8 GHz**
   - At 5.8 GHz, the wavelength is shorter than at lower frequencies. This results in a smaller radius for the first Fresnel zone. Since the Fresnel zone is smaller, it means that the path between the transmitter and receiver is less obstructed, which is beneficial for line-of-sight communication.

### Explanation of Incorrect Answers:
2. **Other Choices:**
   - **3.4 GHz:**
     - The wavelength is longer than at 5.8 GHz, leading to a larger first Fresnel zone compared to 5.8 GHz.
   - **2.4 GHz:**
     - Similar to 3.4 GHz, the wavelength is even longer, resulting in an even larger first Fresnel zone.
   - **900 MHz:**
     - This frequency has the longest wavelength of the options given, which results in the largest first Fresnel zone. 

### Summary:
- The first Fresnel zone decreases in size as frequency increases. Therefore, the 5.8 GHz band has the smallest first Fresnel zone among the options provided. 
- Remember, higher frequencies lead to shorter wavelengths, which in turn lead to smaller Fresnel zones, making them more suitable for line-of-sight communication.