Proximity Effect

Proximity effect redistributes current within conductors due to magnetic fields from nearby conductors — adjacent turns, return paths, and shield plates. In tight spirals, proximity effect often exceeds skin effect alone, especially on inner turns where field from many neighbors adds.

PCB coils with narrow gap between traces and litz spirals with dense packing are proximity-limited. Simulation tools with proximity loss models or measured Rac sweeps beat naive Rdc scaling.

01Design mitigations

• Increase turn spacing where L budget allows.
• Taper inner turn width or use parallel inner segments.
• Move return conductor away from the active spiral plane.
• Avoid placing solid copper shields too close without gap.

02Asymmetric proximity

Steel chassis on one side of the receiver creates stronger proximity on facing turns — Rac becomes orientation-dependent. Test receiver on vehicle fixture, not free-hanging. DD coils can show different Rac per D leg if spacing differs.

03Diagnostics

• Hot inner turns after thermal camera scan of unpotted samples.
• Q lower than simulation with only skin effect modeled.
• Rac drops slightly if turns loosen (bad sign — mechanical issue).