When routing RF traces, the temptation is often to keep them compact to save board space. But in RF design “close” can quickly become “coupled.”

Two parallel traces placed too close will interact electromagnetically. That coupling can affect impedance, increase crosstalk, and degrade isolation between signal paths.

The higher the frequency, the more pronounced the effect, so be extra careful above 6GHz.

A good rule of thumb is to keep trace separation at least 3x the trace width.