The key to a basic understanding of common-mode currents is to recognize that the word "common" is used to describe currents flowing in the same (common) direction on both transmission line conductors. This is in contrast to the optimal situation (with no common-mode currents), where transmission line conductors will have currents flowing in opposite directions exactly balanced.
Common-mode currents appear on transmission lines due to asymmetry in the antenna system. For example, common-mode currents appear when feeding a balanced antenna such as a dipole with unbalanced feed line (coaxial cable).
Good DX and 73, NJ2X
Written by: NJ2X, Michael Maher