See Ground Return & Common Impedance Coupling
Differential Mode Configuration
Assuming 1A is propagated from the source to the load using I1 to represent the current flow. The 1A current must return to the source represented by I2. If I1 = I2 then we have a perfectly balanced transmission line system, no loss in the network.
The EM filed that exists in the outgoing path will couple inductively to the RF return path (AC transmission while DC will always travel in the lowest rsistance path I2). Magnetic flux between these two transmission lines will cancel each other out, being of equal value and opposite in dirrection. Assuming that the spacing between opposite conductors is very small, there should be no radiated emissions. Differential-mode radiation is caused by the flow of RF current loops within a system 's structure.
Common Mode Configuration
Assuming tht 50% of the transmitted current is consumed within the load, it leaves 50% of current that must be returned to its source. The Kirchhoff's Law states that the sum of all currents withinn a transmission line must equal zero.We have 50% loss.
I'2 represents the a virtual return path through free space or metallic interconnect. Not all desired return current will flow in I2 due to inductance or loss in transmission line. The remaining of the desired return current will flow in I'2. A negative current flow will exist in I2, travelling in opposite direction to satisfy Ampere's Law. The undesired (negative) current flow in I2 is that portion that contributes to common-mode currents.
Common mode radiation results from unintentional voltage drops caused by a circuit rising above the 0V reference.
Cables connected to the affected reference system will act as dipole antenna when stimulated with a voltage source.
The only solution to resolve CM radiation is reducing the common path impedance for the return current.
The total magnitude of imbalance in a DM transmission line system becomes the the total magnitude of CM current.
RF loss within a system or transmission line will result in CM energy, and this CM current is the reason for EMI problems.