What do S-parameters S11 and S21 represent in RF networks, and how are they typically expressed?

What do S-parameters S11 and S21 represent in RF networks, and how are they typically expressed?

• A. S11 is input return loss; S21 is forward transmission; expressed as magnitude/phase or in dB.
• B. S11 is forward transmission; S21 is input reflection.
• C. S11 is the reflection phase; S21 is the reflection magnitude.
• D. S11 and S21 are both reflection coefficients.

Answer: (A)

S-parameters describe how RF signals scatter at the ports of a network. Specifically, S11 is the reflection coefficient at the input port: it tells you how much of the incident wave is reflected back toward the source when the output port is matched. In other words, it measures input return, and its complex value has both magnitude and phase. S21 is the forward transmission coefficient: it tells you how much of the input signal from port 1 actually makes it to port 2, again with a complex value for amplitude and phase. You can express these either as magnitude and phase (the raw complex S-parameters) or in decibels. For S11, the decibel form is the return loss, typically written as -20 log10(|S11|). For S21, the decibel form is 20 log10(|S21|), representing insertion loss (if less than 0 dB) or gain (if greater than 0 dB). This is why the statement that S11 is input return loss and S21 is forward transmission, expressed as magnitude/phase or in dB, is the best description.

S-parameters describe how RF signals scatter at the ports of a network. Specifically, S11 is the reflection coefficient at the input port: it tells you how much of the incident wave is reflected back toward the source when the output port is matched. In other words, it measures input return, and its complex value has both magnitude and phase. S21 is the forward transmission coefficient: it tells you how much of the input signal from port 1 actually makes it to port 2, again with a complex value for amplitude and phase.

You can express these either as magnitude and phase (the raw complex S-parameters) or in decibels. For S11, the decibel form is the return loss, typically written as -20 log10(|S11|). For S21, the decibel form is 20 log10(|S21|), representing insertion loss (if less than 0 dB) or gain (if greater than 0 dB). This is why the statement that S11 is input return loss and S21 is forward transmission, expressed as magnitude/phase or in dB, is the best description.