Return Current Does Not Take the Shortest Path

At DC, current returns by the path of least resistance. Above roughly 100 kHz it does not: it returns by the path of least *impedance*, and since impedance is dominated by inductance at those frequencies, that means the path that encloses the smallest loop.

Why it hugs the trace

The return path that minimises loop inductance is the one directly beneath the signal trace. So the current spreads out under the trace and follows it, even where a shorter route across the plane exists.

What that means for your layout

A gap in the plane forces the return current around it, which enlarges the loop, which raises inductance and radiates. Two traces crossing the same gap then share a return path and couple into each other.

The practical rule: keep cross-unders short, or add a return strap.

Placeholder article — illustrative until the PCB series is published.