Drill a via through a multilayer board and you create a problem most people never notice: a copper-plated barrel punching straight through a solid power or ground plane it was never meant to touch. The fix is the antipad, a deliberate ring of absent copper etched into the plane around the hole. At its simplest the antipad is just clearance. The plated barrel needs an air-and-dielectric gap from the surrounding pour or you short signal straight to plane. But the moment you care about anything above a few hundred MHz, that void stops being a manufacturing afterthought and becomes a tuning parameter. A signal via sandwiched between reference planes is, electrically, a short coaxial line. The barrel is the inner conductor; the antipad edge is the outer shield. That geometry sets the via’s characteristic impedance: • Z is proportional to (60/sqrt of er) x ln(b/a) • a = barrel/pad radius • b = antipad radius • er = dielectric constant of the laminate Widen the antipad and you pull th...