EMC Question of the Week: July 1, 2024

TEM Cell

Most TEM cells used for EM emissions and immunity testing have a characteristic impedance equal to

  1. 50 Ω
  2. 75 Ω
  3. 100 Ω
  4. 377 Ω


The best answer is “a". Most TEM cells have a 50-Ω characteristic impedance in order to match the input impedance of the test equipment and connecting cables. Other than that, there's no compelling reason for choosing 50 Ω. The ratio of the electric field strength to the magnetic field strength for any plane wave propagating in a transmission line with an air dielectric is about 377 Ω (i.e., equal to the intrinsic impedance of air). This is completely independent of the transmission line's characteristic impedance.   

Some large TEM cells used for immunity testing have characteristic impedances greater than 50 Ω in order to maximize the space between conductors of a given width. These work just as well as long as the impedance of the matching termination and connecting cables is matched to the TEM cell impedance.  

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