*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 SWR (Standing Wave Ratio), you can use several different tools, each with its own advantages:

1. **Directional Wattmeter**:  
   - A directional wattmeter can measure forward and reflected power in a transmission line.  
   - SWR can be calculated using the formula:  
     $$ \text{SWR} = \frac{1 + \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}}}{1 - \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}}} $$  
   - Here, $P_{\text{fwd}}$ is the forward power, and $P_{\text{ref}}$ is the reflected power.

2. **Vector Network Analyzer (VNA)**:  
   - A VNA can measure complex impedance and reflection coefficients, which can be used to determine SWR.  
   - It provides a detailed analysis of the antenna system, including SWR across a range of frequencies.

3. **Antenna Analyzer**:  
   - An antenna analyzer is specifically designed to measure SWR and other parameters of an antenna system.  
   - It is user-friendly and provides a direct readout of SWR, making it a popular choice for amateur radio operators.

**Correct Answer: D: All these choices are correct**  
- Each of these devices can measure SWR, making option D the correct answer.

**Why the other choices are incorrect individually**:  
- Choices A, B, and C are not incorrect; they are all valid methods for measuring SWR. The question is asking which of the options can be used, and since all can be used, the correct answer is D.