How this is calculated. Hammerstad's equations give the characteristic impedance
of a microstrip from the ratio
u = w/h and the effective dielectric constant
Ereff, which sits between 1 and
Er because the field is
partly in the board and partly in air. For
u ≤ 1,
Z₀ = (60/√Ereff) · ln(8/u + u/4); above that,
Z₀ = 120π / (√Ereff · (u + 1.393 + 0.667·ln(u + 1.444))). Accuracy is
around 1 % for
0.05 < w/h < 20.
Two cautions. Er is not a constant —
FR-4 is quoted near 4.3 at 1 MHz and falls towards 4.0 at GHz frequencies, and it varies by
weave and resin content. And the number that matters is your fabricator's stackup, not this one:
if impedance is controlled, send them the target and let them adjust the width.
This width-to-height ratio is outside the range where Hammerstad's equations are accurate — treat the result as indicative and use a field solver or your fabricator's stackup tool.