*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 the relationship between frequency and the size of the Fresnel zone.

### Explanation:

1. **Fresnel Zone Basics:**
   - The Fresnel zone is an elliptical area around the line of sight path between a transmitter and receiver. It is important for understanding signal propagation and potential obstructions.
   - The size of the Fresnel zone is inversely proportional to the square root of the frequency. This means that as frequency increases, the Fresnel zone becomes smaller.

2. **Formula for the First Fresnel Zone Radius:**
   - The radius of the first Fresnel zone at its widest point can be calculated using:
     $$ F_1 = \sqrt{\frac{\lambda \cdot d_1 \cdot d_2}{d_1 + d_2}} $$
   - Where:
     - $F_1$ is the radius of the first Fresnel zone.
     - $\lambda$ is the wavelength of the signal.
     - $d_1$ and $d_2$ are the distances from the transmitter and receiver to the point of interest, respectively.

3. **Frequency and Wavelength Relationship:**
   - Wavelength ($\lambda$) is inversely related to frequency ($f$):
     $$ \lambda = \frac{c}{f} $$
   - Where $c$ is the speed of light ($3 \times 10^8$ m/s).

### Analysis of Choices:

- **A: 5.8 GHz** (Correct Answer)
  - Higher frequency means a smaller wavelength, leading to a smaller Fresnel zone.

- **B: 3.4 GHz**
  - Lower frequency than 5.8 GHz, resulting in a larger Fresnel zone.

- **C: 2.4 GHz**
  - Even lower frequency, thus an even larger Fresnel zone compared to 5.8 GHz.

- **D: 900 MHz**
  - The lowest frequency among the choices, resulting in the largest Fresnel zone.

### Conclusion:

- The correct answer is **A: 5.8 GHz** because it has the highest frequency, which results in the smallest first Fresnel zone.
- The other choices are incorrect because they have lower frequencies, leading to larger Fresnel zones.