The characteristic impedance of a transmission line depends on conductor geometry. It defines the ratio between the E and H field intensities1 that propagate along that line. On a circuit board, the very close spacing between traces and from traces to conducting planes limits the field energy that leaves (radiates from) any one transmission line. For traces that are between conducting planes, there is essentially no board radiation. On a typical outer layer, there may be hundreds of transmission lines in operation at the same time. If a small fraction of the transported energy leaves each transmission line, the result can be a significant radiated field.