Conducted emission is caused by current in the conductor (cable, wire, PCB trace)
This is a classic mistake, and I cannot recall how many hours I spent trying to educate the customers that conducted emission is not about the current in the wire.
There are many books and presentations explaining field theory and how high frequency current travel on the surface of the conductor, not inside the conductor, so I am not trying to repeat that. What I want to discuss here is that conducted emission is just a test method to measure the EM performance of a device under test (DUT). The actual emission of the unit is much more complex and difficult to picture.
The electromagnetic field propagates using every possible way it could. In an ideal world, If there’s no conductor near the DUT, then there’s just radiation. However, if there’re conductors nearby, the field would use the conductor as a propagation path. Often the conductors present low impedance path for the field to travel, therefore using the conductors is a preferred method. If we measure the surface current of these conductors, we can see current and we call it common-mode current.
An analogy would be. In a busy airport, there are many travellers walking with their heavy suitcase. If there’s a moving walkway provided, passengers with heavy suitcases will prefer to immediately use the moving walkaway rather than dragging the suitcases themselves. We can imagine the passengers as electromagnetic field, when a low energy path (like a moving walkway) is provided, field will use it without doubt. The moving walkway is like conductors. When passengers walk on it, we can picture it as current flowing on a conductor. You still have some passengers drag the suitcase on the ground, they represent the radiated field.