*Question E4A08:
Which of the following is used to measure SWR?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

To measure Standing Wave Ratio (SWR), all the provided options can be used effectively. Here's a breakdown:

1. **Correct Answer: All these choices are correct**
   - **Directional wattmeter**: This device measures the forward and reflected power in a transmission line, allowing you to calculate SWR using the formula:
     $$SWR = \frac{P_{forward}}{P_{reflected}}$$
   - **Vector network analyzer (VNA)**: A VNA can measure the impedance of the antenna system and calculate SWR based on the reflection coefficient. It provides a detailed analysis of how the antenna interacts with the transmission line.
   - **Antenna analyzer**: This tool is specifically designed to measure SWR directly, along with other parameters like impedance and resonant frequency. It simplifies the process for operators.

2. **Why the other choices are incorrect**:
   - There are no incorrect choices in this case, as all options can measure SWR. Each tool has its own method and level of detail, but they all serve the purpose of determining SWR.

3. **Useful formulas**:
   - The basic formula for SWR is:
     $$SWR = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$
     where $|\Gamma|$ is the reflection coefficient, which can be derived from the voltage standing wave pattern on the transmission line.
   - The relationship between reflected power and SWR can also be expressed as:
     $$P_{reflected} = P_{forward} \times \left( \frac{SWR - 1}{SWR + 1} \right)$$

In summary, all the choices provided can measure SWR, making "All these choices are correct" the right answer. Understanding how each tool works will help you in practical applications and exam scenarios.