Characteristic impedance Z₀ for a symmetric (centered) stripline — a conductor sandwiched between two ground planes in a homogeneous dielectric. Uses the IPC-2141A formula with conductor-thickness correction.
Stripline confines the field entirely within the dielectric, giving εeff = εr (no air fringing) and excellent isolation from neighboring traces and external fields. This makes it the preferred topology for inner-layer routing in multi-layer RF PCBs, especially when crosstalk or radiation control matters.
The trade-off vs microstrip: harder to access for probing, components must be brought to the surface via vias (adding parasitic inductance), and slower phase velocity due to the fully-loaded dielectric.
Z₀ = (60 / √εr) · ln(1.9·b / (0.8·W + T))
This is IPC-2141A's recommended expression for a symmetric stripline. The thickness T enters via the effective conductor width (0.8·W + T), which accounts for current crowding at the edges of a finite-thickness trace.