S11 in S-parameters corresponds to which quantity?

• A. Transmission from port 1 to port 2
• B. Reflection coefficient at port 1
• C. Reflection at port 2
• D. Insertion loss

Answer: (B)

In RF networks, S-parameters describe how signals propagate between ports when the other ports are terminated in matched loads. S11 is the reflection coefficient at the input port, indicating what fraction of the incident signal at port 1 is reflected back due to impedance mismatch. It is defined as S11 = b1 / a1 with port 2 matched (a2 = b2 = 0), so it directly quantifies the input reflection. This is why it corresponds to the reflection at port 1. The other quantities mapped to the choices are different: transmission from port 1 to port 2 is S21, reflection at port 2 is S22, and insertion loss relates to how much signal is lost in transmission (often tied to |S21| under matched conditions), not S11.