Question E4B04:
Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?  

Choices:  
A: S11  
B: S12  
C: S21  
D: S22    

Correct Answer is  A: S11  

To identify the S-parameter for input port return loss/reflection (equivalent to VSWR), remember that S-parameters relate incident and reflected traveling waves at each port.

Why A (S11) is correct:
- $S_{11}$ is the input port reflection coefficient: the ratio of reflected to incident wave at port 1 with port 2 terminated in $Z_0$.
- Its magnitude equals the input reflection coefficient $|\Gamma_\text{in}|$, from which return loss and VSWR are derived.

Useful formulas:
- Reflection coefficient: $$|\Gamma| = |S_{11}|$$
- Return loss (dB): $$\mathrm{RL} = -20 \log_{10} |\Gamma|$$
- VSWR: $$\mathrm{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$

Why the others are incorrect:
- B: $S_{12}$ is reverse transmission (from port 2 to port 1), i.e., isolation or reverse gain, not a reflection parameter.
- C: $S_{21}$ is forward transmission (gain) from port 1 to port 2, not a reflection parameter.
- D: $S_{22}$ is the output port reflection coefficient (return loss at port 2), not the input.

Quick rule of thumb for a 2-port:
- $S_{11}$ = input match (reflection at port 1)
- $S_{22}$ = output match (reflection at port 2)
- $S_{21}$ = forward gain
- $S_{12}$ = reverse isolation