A The best definition of “common mode” appears in the new ARRL RFI Book. Basically, common mode is a situation where a given signal appears equally on more than one conductor in a cable, with the same voltage and the same current flowing in the same direction relative to a particular device (see Figure 2). The return path of the signal is typically outside of the affected cable.
A good example is RFI to a telephone via the line cord. The 4 (or 6 or 8, depending upon the phone system) conductors act as a multiconductor, end-fed antenna.
Figure 2—In (A) there are differential-mode signals conducted between two wires of a pair. This signal is independent from earth ground. In contrast, a common-mode signal (B) is in phase on all wires that form the conductor (this includes coaxial cable). All wires act as if they are one wire. The ground forms the return path.
From QST July 1999
