A PCB does not conduct electricity. It guides electromagnetic fields. Until you internalise this distinction, you will keep designing for the wrong physics — and EMC testing will keep exposing the gap.
The standard mental model of a PCB trace is a conductor that carries current from point A to point B. This model works for DC and low-frequency circuits. It breaks down at the frequencies where EMC problems live — and understanding why is the foundation of designing for low EMI.
At high frequencies, a PCB trace and its return path together form a transmission line. The signal energy is not stored in the conductor — it propagates in the electromagnetic field in the dielectric between the trace and the reference plane. The conductors guide the field. They do not carry the energy.
Imagine a single conductor carrying current, viewed in cross-section. Surrounding it are two fields: the magnetic field (lines of force encircling the conductor, represented by Φ) and the electric field (lines of force radiating outward from the conductor, represented by Ψ). Together they form the electromagnetic field associated with this current.
With only one conductor, the electric field lines radiate outward to infinity. The field is unconstrained. This is an efficient radiator. This is what you want to avoid on a PCB.
When a return conductor is introduced — a reference plane beneath the trace — the field geometry changes fundamentally. The electric field lines, instead of radiating to infinity, terminate on the return conductor. The magnetic field lines are similarly constrained by the presence of the return path. The result is a field that is largely contained in the dielectric between the trace and the plane.
This is the operating principle of a controlled-impedance transmission line. The closer the trace is to the reference plane, the more tightly the field is confined. A tightly confined field radiates less. This is why adjacent ground planes are not a preference — they are a field containment requirement.
The reference plane is not a ground connection. It is the second conductor of a transmission line, and its proximity determines how well you contain the electromagnetic field.
Once you adopt the field model, design decisions that previously seemed arbitrary become physically motivated. Stack-up selection is about how tightly adjacent planes contain the field. Component placement is about minimising the area over which fields must propagate between source and load. Filter placement at connectors is about intercepting field propagation at the system boundary. Connector bonding to chassis is about preventing fields from propagating out of the enclosure through the connector aperture.
EMC compliance is not achieved by following a checklist. It is achieved by understanding where the electromagnetic field is generated, where it propagates, and what structures you are building to contain or redirect it. Every non-compliance can be traced to a field propagation path that was not accounted for in the design.
Begin looking at your designs not as collections of components and traces, but as three-dimensional field structures. Ask: where is the field strongest? Where does it propagate? What is the boundary between inside and outside my system, and how do fields cross that boundary? The answers will tell you where your EMC risks are — before you book the chamber.
Work directly with Dario to identify EMC risks at the design stage — before a €15,000–40,000 chamber session reveals issues that require a respin.