*Question G9A03:
What is the nominal characteristic impedance of “window line” transmission line?*  

*Correct Answer is  D: 450 ohms*  

### AI Response: (may contain errors)  

The correct answer to the question about the nominal characteristic impedance of "window line" transmission line is **450 ohms**. 

### Explanation of the Correct Answer:
- **Window line** is a type of open-wire transmission line that is often used in amateur radio applications. Its characteristic impedance is typically around **450 ohms**. This high impedance is beneficial for certain antenna systems, particularly for balanced antennas.

### Why the Other Choices are Incorrect:
- **50 ohms**: This is the standard impedance for many coaxial cables used in RF applications, particularly for low-loss transmission lines and antennas. It is not the characteristic impedance of window line.
  
- **75 ohms**: This impedance is commonly used for video and broadcast applications, such as in cable television systems. It is not applicable to window line transmission lines.
  
- **100 ohms**: This impedance is often associated with twisted pair cables used in data communications (like Ethernet), but it does not represent window line.

### Useful Formulas or Rules of Thumb:
- The characteristic impedance ($Z_0$) of a transmission line can be calculated using the formula:

$$
Z_0 = \sqrt{\frac{L}{C}}
$$

where:
- $L$ = inductance per unit length (in henries per meter)
- $C$ = capacitance per unit length (in farads per meter)

- For open-wire lines like window line, the impedance is influenced by the spacing between the conductors and their diameter. The typical range for window line is around **300 to 600 ohms**, with **450 ohms** being a common nominal value.

This information should help you understand why **450 ohms** is the correct answer and clarify the characteristics of different types of transmission lines.