EMC Question of the Week: March 20, 2023

cable with 4 wires twisted together

How many independent TEM modes (i.e., differential modes) can propagate in an unshielded wire harness with 4 wires? 

  1. 2
  2. 3
  3. 4
  4. 5


The best answer is “b.” A transmission line with N conductors can support N-1 independent propagating (or differential) modes. It can also support a non-propagating common-mode current. The common-mode current does not carry power from one end to the other, but it can be a significant source of radiated emissions. 

While signal integrity engineers tend to focus on the propagating modes, EMC engineers are generally much more concerned with the common mode.

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