*Question G4A06:
What is the purpose of an antenna tuner?*  

*Correct Answer is  C: Increase power transfer from the transmitter to the feed line*  

### AI Response: (may contain errors)  

An antenna tuner is a device used to match the impedance of the transmitter to the impedance of the antenna system, which includes the feed line and the antenna itself. This is crucial for efficient power transfer.

1. **Correct Answer: C - Increase power transfer from the transmitter to the feed line**

   - The primary purpose of an antenna tuner is to ensure that the impedance seen by the transmitter is the same as its output impedance, typically 50 ohms. This impedance matching maximizes power transfer according to the maximum power transfer theorem, which states that maximum power is transferred when the load impedance equals the source impedance.

2. **Why the other choices are incorrect:**

   - **A: Reduce the SWR in the feed line to the antenna**  
     An antenna tuner does not change the SWR (Standing Wave Ratio) on the feed line itself. It only makes the transmitter see a 1:1 SWR, which is ideal for power transfer. The SWR on the feed line remains the same unless the antenna system is adjusted.

   - **B: Reduce the power dissipation in the feedline to the antenna**  
     While an antenna tuner can help ensure that the transmitter operates efficiently, it does not directly reduce power dissipation in the feed line. Power dissipation is primarily a function of the feed line's characteristics and the SWR along it.

   - **D: All these choices are correct**  
     Since choices A and B are incorrect, D cannot be correct.

3. **Useful Formula:**

   - **Maximum Power Transfer Theorem:**  
     For maximum power transfer, the load impedance ($Z_L$) should match the source impedance ($Z_S$). In the context of radio systems, this means matching the transmitter's output impedance to the antenna system's impedance using an antenna tuner. 

By using an antenna tuner, you ensure that the transmitter operates efficiently by seeing the correct load impedance, thus maximizing power transfer to the antenna system.