*Question E4B04:
Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?*  

*Correct Answer is  A: S11*  

### AI Response: (may contain errors)  

The correct answer to the question about which S parameter represents input port return loss or reflection coefficient is **S11**.

### Explanation of the Correct Answer (S11):
- **S11** is defined as the reflection coefficient at the input port of a two-port network. It quantifies how much of the input signal is reflected back due to impedance mismatches.
- The return loss can be calculated from the reflection coefficient using the formula:
  
  $$ 
  \text{Return Loss (dB)} = -20 \log_{10} |S11| 
  $$

- A higher return loss indicates better matching and less signal reflection.

### Why the Other Choices are Incorrect:
- **S12**: This parameter represents the reverse transmission coefficient, indicating how much signal is transmitted from port 2 to port 1. It does not relate to input port reflection.
  
- **S21**: This parameter represents the forward transmission coefficient, showing how much signal is transmitted from port 1 to port 2. Like S12, it does not provide information about reflection at the input port.

- **S22**: This parameter represents the reflection coefficient at the output port (port 2). It measures how much of the signal is reflected back from the output, not the input.

### Summary:
- **S11** is crucial for understanding how well the input port is matched to the system, which is essential for minimizing signal loss and maximizing efficiency in RF systems. 

Remember, in RF design, minimizing reflections (high return loss) is key to ensuring effective signal transmission.