*Question G9A01:
Which of the following factors determine the characteristic impedance of a parallel conductor feed line?*  

*Correct Answer is  A: The distance between the centers of the conductors and the radius of the conductors*  

### AI Response: (may contain errors)  

The characteristic impedance of a parallel conductor feed line is primarily determined by the physical dimensions of the conductors and their arrangement. 

### Correct Answer: A
**Why is this correct?**  
The characteristic impedance ($Z_0$) of a parallel conductor feed line can be calculated using the formula:

$$
Z_0 = \frac{138 \log_{10} \left( \frac{D}{r} \right)}{\sqrt{\epsilon_r}}
$$

where:
- $D$ = distance between the centers of the conductors
- $r$ = radius of the conductors
- $\epsilon_r$ = relative permittivity of the dielectric material between the conductors (often assumed to be 1 for air)

This formula shows that both the distance between the conductors and their radius directly influence the characteristic impedance.

### Why the Other Choices are Incorrect:
- **B: The distance between the centers of the conductors and the length of the line**  
  The length of the line does not affect the characteristic impedance. It affects the total impedance at the load but not the inherent characteristic impedance of the line itself.

- **C: The radius of the conductors and the frequency of the signal**  
  While the radius of the conductors is relevant, the frequency does not directly determine the characteristic impedance. Instead, it affects the propagation characteristics and losses in the line.

- **D: The frequency of the signal and the length of the line**  
  Similar to choice B, the frequency and length do not determine the characteristic impedance. They may influence other factors like attenuation and phase velocity but not the impedance itself.

### Summary
- The characteristic impedance is determined by the physical dimensions of the conductors (distance and radius).
- Length and frequency are important for other aspects of transmission lines but not for calculating characteristic impedance. 

This understanding is crucial for designing and analyzing RF systems effectively.