RF Grounding Techniques for PCB Design

Why RF Grounding Is Different from DC Grounding

At DC and low frequencies, any conductor connecting to "ground" is effectively at 0V. At RF frequencies, every millimeter of conductor has measurable inductance (~1 nH/mm) and every via has measurable inductance (~1 nH/mm of via length). Poor RF grounding creates unexpected resonances, ground bounce, and signal degradation that can't be solved by adding more bypass capacitors.

The Solid Ground Plane Rule

  Critical rule: Never cut slots, holes, or split the ground plane
  directly beneath an RF signal trace.

  A slot under a 50 Ω microstrip trace at 5 GHz:
  - Doubles the effective substrate height
  - Changes Z₀ from 50 Ω to ~70 Ω (VSWR = 1.4:1 — unexpected loss!)
  - Slot length > λ/20 at 5 GHz = 1.6 mm → radiates as unintended antenna

  Always use a solid, continuous ground layer directly below RF layers.

Via Fencing Around RF Transmission Lines

Return Current Path Management

RF signal current flows on the top of the microstrip trace, and the return current flows on the surface of the ground plane directly below. Anything that interrupts the ground plane directly below the signal trace forces the return current to take a longer path, creating a current loop that radiates as an unintended antenna and adds loss.

  Good: ground plane completely solid below RF trace
  Bad:  power plane between RF trace and ground (forces current around power islands)
  Worse: another RF trace crossing perpendicular at same layer without ground between them

  Rule: Route power/digital on different layers, never sharing ground return path with RF

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