Cables, Connectors & Grounding — the Dominant Radiators
On most products that fail radiated emissions, the board is fine — the cables are the antenna. A tiny common-mode voltage drives current onto a metre of cable, and that cable radiates far better than any trace. Control the cables and you control RE.
A radiator's efficiency scales with its size relative to wavelength. A 5 cm trace is tiny at 200 MHz (λ ≈ 1.5 m); a 1 m cable is a resonant half-wave. So even a few millivolts of common-mode noise (Module 6) coupled onto a cable produces far more field than the same noise on the PCB. That's why the RE sources module calls cables the usual prime suspect — and why the current probe (Module 3) is your fastest RE debug tool.
A cable acts as an efficient antenna near its resonant frequencies. A cable driven against a ground plane behaves like a monopole (quarter-wave); a floating cable like a dipole (half-wave). Drag the slider to see where a given length resonates.
A shielded cable only works if its shield is bonded all the way around (360°) to the chassis at entry. A pigtail — a short wire from the shield to a ground pin — adds inductance that ruins high-frequency shield performance and often makes emissions worse than no shield at all above ~100 MHz.