*Question E4B03:
Which S parameter is equivalent to forward gain?*  

*Correct Answer is  C: S21*  

### AI Response: (may contain errors)  

In the context of S parameters, which are used to describe the electrical behavior of linear RF networks, each parameter has a specific meaning related to how signals are transmitted and reflected in a network.

### Explanation:

- **S21 (Correct Answer):**  
  - **Forward Gain:** S21 represents the forward transmission coefficient. It measures how much of the input signal at port 1 is transmitted to port 2. This is equivalent to the forward gain of the network.
  - **Formula:**  
    $$S_{21} = \frac{\text{Output voltage at port 2}}{\text{Input voltage at port 1}}$$

- **S11:**  
  - **Input Reflection Coefficient:** S11 measures the amount of the input signal that is reflected back from port 1. It is not related to gain but rather to how much signal is reflected at the input.
  - **Formula:**  
    $$S_{11} = \frac{\text{Reflected voltage at port 1}}{\text{Incident voltage at port 1}}$$

- **S12:**  
  - **Reverse Gain:** S12 represents the reverse transmission coefficient. It measures how much of the input signal at port 2 is transmitted to port 1. This is not the forward gain but rather the reverse gain.
  - **Formula:**  
    $$S_{12} = \frac{\text{Output voltage at port 1}}{\text{Input voltage at port 2}}$$

- **S22:**  
  - **Output Reflection Coefficient:** S22 measures the amount of the output signal that is reflected back from port 2. Like S11, it is related to reflection, not gain.
  - **Formula:**  
    $$S_{22} = \frac{\text{Reflected voltage at port 2}}{\text{Incident voltage at port 2}}$$

### Summary:

- **S21** is the correct choice because it directly measures the forward gain from port 1 to port 2.
- **S11** and **S22** are related to reflections, not gain.
- **S12** measures reverse gain, not forward gain.