What does S21 represent in S-parameters?

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

Answer: (A)

S-parameters describe how signals move between ports when all other ports are properly terminated. Each S-parameter tells you the response at one port due to excitation at another.

S21 specifically is the forward transmission from port 1 to port 2. It measures how much of the signal injected into port 1 appears at port 2, including both magnitude and phase, with the network matched at all ports. It is not about reflections at a port—that would be S11 (reflection at port 1) or S22 (reflection at port 2). It’s also not the loss itself, though the loss you experience between input and output is often discussed in terms of the magnitude of S21 (insertion loss is related to |S21|^2). So the best description is: the transmission from port 1 to port 2.