Fresu EMC Academy · Reference Card 01

The Propagation
Channel

The foundational framework for EMI control through PCB layout. Energy propagates in the dielectric. Copper acts as boundaries. Your job as a designer is to engineer the channel — before the first trace is placed.

What a properly bounded channel looks like

A signal channel requires exactly two paired conducting boundaries with dielectric between them

Signal conductor

Trace / routing layer — first boundary

Boundary 1

Dielectric

This is where the fields actually propagate — not the copper

Signal medium

Return Reference Plane (RRP)

Second boundary — contains and channels the fields back

Boundary 2

⚡

"The dielectric is where the fields propagate and the copper acts as the boundaries to contain and channel the fields." A single layer cannot contain the fields. Two paired boundaries are always required.

The mental shift

❌ What we are taught in school

The signal is in the copper

Electrons flow through the conductor. The copper trace carries the signal. Ground is zero volts. Current returns through the ground plane.

✓ What actually happens

The signal is in the dielectric

Fields propagate in the space between conductors. Copper is the boundary. The RRP is not a ground — it is the second conductor of the transmission line.

The 4 ways the channel breaks in layout

Failure 01

Missing or wrong RRP pairing

Signal layer referenced to a power plane instead of a dedicated RRP. The power plane has splits — the channel is broken wherever a split occurs.

→ Always pair: signal with RRP, power with RRP

Failure 02

Splits and gaps in the RRP

Any cut, slot, or gap in the return reference plane forces fields to find an alternative path. The gap becomes a slot antenna. Emissions follow.

→ No splits. No gaps. The RRP must be solid and continuous.

Failure 03

Layer transitions without RRV

When a signal changes layer, the channel must also transition. Without an adjacent Return Reference Via (RRV), the field channel breaks at every layer change.

→ Every signal via needs at least one adjacent RRV

Failure 04

Uncontrolled field environment

Copper pours, polygon islands, nearby conductors — all modify the electric field distribution. They change the impedance the channel sees, creating discontinuities.

→ Every conductor placed affects every nearby channel

The vocabulary

Dielectric

The material between conductors where EM fields actually propagate. Not an insulator — the signal medium.

Boundary

What copper layers actually are. They contain and channel the fields. They do not carry the signal.

RRP

Return Reference Plane. Not a "ground plane" — the second conductor that closes the transmission line.

RRV

Return Reference Via. Placed adjacent to every signal via to maintain the channel through layer transitions.

Propagation channel

The engineered path — dielectric space bounded by two conductors — where the signal propagates.

Cavity

Space between conducting layers with its own resonant frequency. Must be controlled via stitching via density.

"When you understand the signals as electromagnetic fields rather than just current flow, then everything starts to make sense — you can think about correct layer placement, the importance of dielectric thickness and conductor spacing, and why controlled impedance matters."

— Dario Fresu, PCB Design Review for EMC and EMI Control