Asymmetric Stripline Impedance Calculator

A symmetric stripline sits exactly halfway between two reference planes. Real stack-ups almost never do that, because prepreg and core come in fixed thicknesses and the layer arrangement rarely divides evenly. What you actually build is an offset or asymmetric stripline: dielectric $h_1$ to the nearer plane, $h_2$ to the farther one.

The starting point is Cohn's superposition. The trace sees two half-structures — one bounded by the near plane, one by the far plane. Mirror each half about its own plane and you get an ordinary symmetric stripline of spacing $2h$, whose capacitance is twice that of the half it came from. The two halves share one conductor, so their capacitances add:

using the exact conformal-mapping solution for each half rather than the familiar logarithmic approximation, which carries a few percent of its own error and needs a second branch for wide traces.

Why superposition alone is not enough

Superposition quietly assumes a magnetic wall on the trace plane outside the trace, so no field may wrap around the trace edge from one half to the other. Writing $C = W/h_1 + W/h_2 + F$, it always returns the symmetric fringing $F = 8\ln 2/\pi$ no matter how offset the trace is — while the true fringing grows sharply as the trace nears a plane. That single omission is the entire error, and it reaches 14% at extreme offset.

This calculator adds the missing fringing back, using a correction fitted to a method-of-moments field solver. Two properties are preserved exactly: set $h_1 = h_2$ and the result collapses bit-for-bit to the symmetric stripline; push either gap toward zero and the impedance correctly goes to zero. Several published approximations fail both checks, returning values above the symmetric case or refusing to collapse at all.

Where the return current goes

The practical consequence of asymmetry is not the impedance shift — that is easily compensated by adjusting trace width. It is the return current distribution. Return current splits between the two planes roughly in inverse proportion to the dielectric distance, so with a 3:1 asymmetry the near plane carries something like three quarters of it. Any split, void, or antipad cluster in that near plane forces the return to detour, and the resulting loop area shows up as crosstalk and radiated emission.

Accuracy

Because the zero-thickness building block is the exact conformal-mapping solution, it holds at every trace width — no narrow/wide branch and no discontinuity where the two would meet. Against a method-of-moments field solver the result stays within 0.7% across the whole practical range of trace widths and offsets. Copper thickness is handled by Cohn's fringing increment, which is exact for wide traces and vanishes correctly at zero thickness.