What S-parameters are
Scattering parameters describe how RF energy flows through a network in terms of incident and reflected waves. They are the standard way to specify RF component behavior.
The key terms
- S11 — input return loss, how much signal reflects at port 1.
- S21 — forward transmission, how much signal passes from port 1 to port 2.
- S12 — reverse transmission, the opposite direction.
- S22 — output return loss at port 2.
- For the engineering meaning of S11 and its VSWR equivalent, see return loss and VSWR.
Reading a datasheet
- S21 in dB tells you insertion loss or gain.
- S11 in dB tells you match quality at the input.
- Both are given versus frequency, so always check the full band.
Practical notes
- A component is matched when S11 and S22 are low, such as below -15 dB.
- For reciprocal passive parts, S21 equals S12.
- S-parameters are measured on calibrated vector network analyzers.
RFPro tip
Compare S-parameter curves, not single-frequency numbers, when selecting a component for a wideband design. Then match each block to the right family of RF components.