EMC Question of the Week: September 10, 2018

Radiated Emission Plot

Approximately how much power at any given frequency is being radiated by a product that is at or near the FCC or CISPR Class B radiated emissions limit?

  1. 1 nW
  2. 1 μW
  3. 1 mW
  4. 1 W

Answer

The correct answer is "a". An average radiated power of 1 nW (1x10-9 watts) from a dipole or monopole source above a ground plane will product a field on the order of a hundred microvolts/meter at a distance of 3 meters. This is at or near (same order of magnitude) the FCC and CISPR Class B radiated emissions limit. 

Note that a precise calculation requires knowing the directivity of the radiating source as well as the frequency (since the limit is different in different frequency bands). Nevertheless, for an order-of-magnitude calculation, we can assume a free-space directivity of about 1.5 or 1.6 (the directivity of an electrically small or resonant dipole). The equation below shows the calculation of the average radiated power required to produce a field strength of 100 μV/m.
Calculation of power required to produce a 100 microvolt per meter field at a distance of 3 meters

Note that radiating 1 μW of power would result in a field approximately 60 dB above the limit. 1 mW of power would correspond to a field approximately 120 dB above the limit, and 1 W of power would be 180 dB above the limit!

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