EMC Question of the Week: March 22, 2021

Variety of cut electrical cables stacked in a pile

What property of a two-conductor transmission line is strongly correlated to the radiated emissions from that transmission line? 

  1. characteristic impedance
  2. electrical balance
  3. differential-mode voltage
  4. common-mode current


The best answer is “d.” The radiated emissions from a transmission line, such as a twisted wire pair or coaxial cable, is generally proportional to the common-mode current on the cable. Differential signal currents and voltages are unlikely to contribute measurably to radiated emissions unless a change in electrical balance converts some of that signal power to a common-mode current.

There is no correlation at all between characteristic impedance and the radiation from a transmission line. The same is true of electrical balance. 

Radiated emissions due to differential-mode currents on parallel wires are not zero, but they are rarely high enough to matter in typical applications. Radiated emissions from differential currents on twisted wire pairs and coaxial cables are virtually non-existent. 

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