EMC Question of the Week: March 20, 2023
![cable with 4 wires twisted together](/images/QOTW/Four-wire_harness01.jpg)
How many independent TEM modes (i.e., differential modes) can propagate in an unshielded wire harness with 4 wires?
- 2
- 3
- 4
- 5
Answer
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.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at