If current always flows in loops, how is it possible for a signal to propagate if the signal has not yet reached the load and the conduction current path has not yet been established?
The key is in the displacement current.
The easiest way to understand this is by considering how current flows in a capacitor when charged with an AC source.
In reality, there is no conduction of current in the dielectric between the capacitor plates, as this would contradict the definition of a dielectric.
However, it appears that a conduction current is produced by the displacement of the bound charges in the dielectric, whose orientation (displacement) follows the applied electric field on the capacitor plates.
If we then consider how the electric field is applied to the conductors in a PCB, we can see that the same concepts apply here.
As the signal wavefront propagates, a displacement current is established.
This displacement current is essentially the bridge between the forward current and the return current, which are both parts of the conduction current.
In this way, the current loop is allowed to form and follows the propagation of the signal edge.
The problem here is that this current loop is responsible for many radiated emissions test failures because their strength is related to the area that this loop encloses.
So, watch out for these current loops.
I hope this helps,
Dario
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