EMC Question of the Week: October 28, 2024

Equations defining the S-parameters for a 2-port linear system

Which 2-port S-parameter value is consistent with a vector network analyzer measurement of a lossless cable that connects Port 1 to Port 2? 

  1. |S­11| = 0 dB
  2. |S­11| = 3 dB
  3. |S­21| = 0 dB
  4. |S­12| = -∞ dB

Answer

The correct answer is “c.” All of the incident voltage should be transmitted from the input to the output of the cable. The ratio of the output voltage to the input voltage is 1. Expressed in decibels that's 0 dB.

|S11| = 0 dB indicates that all of the power is reflected (e.g., the cable is shorted or not connected on one end). |S11| = 3 dB indicates that the reflected power is greater than the incident power, which can't happen. |S12| = -∞ dB indicates that the cable did not allow any power to pass through it.

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