EMC Question of the Week: December 10, 2018

EM plane wave incident on a slab of shielding material

In a plane wave shielding effectiveness measurement, the incident power is 13 dBm and the transmitted power is 3 dBm. In this case, the shielding effectiveness of the material is

  1. 10 dBm
  2. -10 dBm
  3. 10 dB
  4. 20 dB

Answer

The correct answer is "c". The plane wave shielding effectiveness is the ratio of the incident power to the transmitted power. In this case, it is 13 dBm - 3 dBm = 10 dB. Note that the first two options express a particular power level instead of a shielding effectiveness ratio. Also, note that it wouldn't matter whether we defined the shielding effectiveness as the ratio of the field strengths or the ratio of the power levels; the result expressed in decibels would be the same. 

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