*Question E4B05:
What three test loads are used to calibrate an RF vector network analyzer?*  

*Correct Answer is  B: Short circuit, open circuit, and 50 ohms*  

### AI Response: (may contain errors)  

To calibrate an RF vector network analyzer (VNA), three specific test loads are used: a short circuit, an open circuit, and a 50-ohm load. These are essential for ensuring accurate measurements of impedance and reflection coefficients.

### Explanation of the Correct Answer (B):
- **Short Circuit**: This provides a known reflection coefficient of $-1$. It reflects all the incident power back to the source.
- **Open Circuit**: This also reflects all the incident power back to the source, but with a reflection coefficient of $+1$.
- **50 Ohms**: This is the standard reference impedance for most RF systems, providing a reflection coefficient of $0$ when perfectly matched.

These three conditions allow the VNA to establish a baseline for measuring unknown impedances by comparing them to these known standards.

### Why the Other Choices are Incorrect:
- **Choice A (50 ohms, 75 ohms, and 90 ohms)**: These are different impedance values, but they do not provide the necessary reflection extremes (short and open) needed for calibration.
- **Choice C (Short circuit, open circuit, and resonant circuit)**: While the short and open are correct, a resonant circuit is not a standard calibration load because its impedance can vary with frequency.
- **Choice D (50 ohms through 1/8 wavelength, 1/4 wavelength, and 1/2 wavelength of coaxial cable)**: These are transmission line lengths, not standard calibration loads. They do not provide the necessary reflection coefficients for calibration.

### Useful Concepts:
- **Reflection Coefficient ($\Gamma$)**: This is a measure of how much of the signal is reflected by an impedance mismatch. It is defined as:
  $$ \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0} $$
  where $Z_L$ is the load impedance and $Z_0$ is the characteristic impedance (usually 50 ohms).

Understanding these concepts and the role of each test load is crucial for effectively using a VNA in RF measurements.