Question E4B11:
Which of the following can be measured with a vector network analyzer?  

Choices:  
A: Input impedance  
B: Output impedance  
C: Reflection coefficient  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

A vector network analyzer (VNA) measures complex S-parameters, from which you can directly obtain impedance and reflection-related quantities. Therefore, it can measure all items listed.

Why D is correct (All these choices are correct):
- VNAs measure the complex reflection coefficient at each port: $S_{11}$ (input) and $S_{22}$ (output).
- From $S_{11}$ or $S_{22}$, you can compute the corresponding impedance seen at that port.
- The reflection coefficient itself is a direct VNA measurement.

Why A, B, and C alone are not the best answers:
- A: Input impedance can be derived from $S_{11}$, but the VNA can measure more than just this.
- B: Output impedance can be derived from $S_{22}$, but again the VNA does more.
- C: Reflection coefficient is directly measured, but the VNA also provides impedance and other parameters.
- Each is correct individually, but incomplete; only D captures the full capability.

Useful formulas:
- Reflection coefficient:
  $$\Gamma = \frac{Z - Z_0}{Z + Z_0}$$
- Impedance from reflection coefficient:
  $$Z = Z_0 \frac{1 + \Gamma}{1 - \Gamma}$$
- Return loss (dB):
  $$\mathrm{RL} = -20 \log_{10} |\Gamma|$$
- VSWR:
  $$\mathrm{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$

Notes:
- Input impedance is typically found from $S_{11}$ with the other port terminated in $Z_0$.
- Output impedance is typically found from $S_{22}$ with the input port terminated in $Z_0$.