*Question T7C05:
Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?*  

*Correct Answer is  A: To protect the output amplifier transistors*  

### AI Response: (may contain errors)  

The correct answer to the question is:

**A: To protect the output amplifier transistors.**

### Explanation of the Correct Answer:
- **Solid-state transmitters** are designed with output amplifier transistors that can be damaged by excessive heat or current. 
- When the **Standing Wave Ratio (SWR)** increases, it indicates that there is a mismatch between the transmitter and the antenna, leading to reflected power.
- This reflected power can cause the transistors to operate outside their safe limits, potentially resulting in overheating or failure.
- To prevent damage, most solid-state transmitters automatically reduce their output power when the SWR exceeds a certain threshold.

### Explanation of the Incorrect Choices:
- **B: To comply with FCC rules on spectral purity**
  - While compliance with FCC rules is important, the primary reason for reducing power at high SWR is to protect the hardware, not directly for spectral purity.
  
- **C: Because power supplies cannot supply enough current at high SWR**
  - This is not accurate. Power supplies are typically designed to handle the required current. The issue at high SWR is more about the reflected power damaging the transmitter rather than the power supply's capability.

- **D: To lower the SWR on the transmission line**
  - Reducing output power does not lower the SWR. SWR is a function of the impedance mismatch between the transmitter and the load (antenna). The transmitter reduces power to protect itself, not to correct the SWR.

### Useful Concepts:
- **SWR (Standing Wave Ratio)** is a measure of how well the load (antenna) is matched to the transmission line. An SWR of 1:1 is ideal, indicating perfect matching.
- **Reflected Power** can be calculated using the formula:
  
  $$ P_{reflected} = P_{incident} \times \left( \frac{SWR - 1}{SWR + 1} \right)^2 $$

  where:
  - $P_{incident}$ is the power sent from the transmitter.
  - $SWR$ is the standing wave ratio.

Understanding these concepts will help you grasp why solid-state transmitters behave the way they do under high SWR conditions.