Question
Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?
Answer Options
- A) S11
- B) S12
- C) S21
- D) S22
Correct Answer: A
Explanation
S-parameters are used to quantify signal interactions within a two-port RF network. In addition to forward transmission (gain, S_{21}), it is critical to measure the signal reflected back to the source from the input port. This reflection indicates an impedance mismatch, which is directly related to VSWR.
The parameter S_{11} represents the signal reflected back into Port 1 (input) relative to the signal fed into Port 1. This value is mathematically equivalent to the input reflection coefficient, and from it, the Voltage Standing Wave Ratio (VSWR) and return loss can be calculated. It is a critical metric for determining the quality of the impedance match at the device’s input.
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