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

*Correct Answer is  D: 450 ohms*  

### AI Response: (may contain errors)  

Window line, also known as ladder line, is a type of parallel-conductor transmission line commonly used in amateur radio. It is characterized by its open, ladder-like structure, which reduces losses compared to coaxial cables.

1. **Correct Answer: 450 ohms**  
   - The nominal characteristic impedance of window line is typically 450 ohms. This high impedance is due to the wide spacing between the conductors and the low dielectric constant of the air between them. The design minimizes losses and allows for efficient power transfer over long distances.

2. **Why Other Choices Are Incorrect:**
   - **50 ohms (Choice A):** This is the characteristic impedance of most coaxial cables used in amateur radio, such as RG-58 or RG-213, not window line.
   - **75 ohms (Choice B):** This impedance is common for television coaxial cables, like RG-59 or RG-6, but not for window line.
   - **100 ohms (Choice C):** This is not a standard impedance for common transmission lines used in amateur radio.

3. **Useful Formulas or Rules of Thumb:**
   - The characteristic impedance \( Z_0 \) of a parallel-conductor line can be approximated by:
     $$ Z_0 = \frac{276 \log_{10} \left( \frac{D}{d} \right)}{\sqrt{\varepsilon_r}} $$
     where:
     - \( D \) is the center-to-center distance between the conductors.
     - \( d \) is the diameter of the conductors.
     - \( $$ {\varepsilon_r} $$ \) is the relative permittivity of the material between the conductors (approximately 1 for air).

Understanding these concepts helps in recognizing why window line has a nominal impedance of 450 ohms, making it suitable for specific applications in amateur radio.