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We have observed subpicosecond electrical pulses to propagate on 5-pm coplanar transmission lines at velocities faster than the phase velocity in the underlying dielectric. This situation produces an electromagnetic shock wave in a manner similar to Cherenkov radiation and electro-optic Cherenkov radiation. Using time-domain spectroscopy, we have measured the strong frequency-dependent loss of energy in the propagating electrical pulse due to this radiation. However, Cherenkov has shown that, when charges move faster than the phase velocity for electromagnetic radiation in a material, radiation is emitted as an electromagnetic shock wave. ' The initial analysis of this eAect was for electric monopoles, but the physical picture holds true for higher-order moments as well.Auston has recently demonstrated that it is possible to produce an electric dipole moving faster than the appropriate phase velocity in a dielectric, and thereby to produce an electromagnetic shock wave.This dipole, which is caused by an ultrashort laser pulse driving the optical rectification eftect in a nonlinear dielectric material, has the same spatial distribution as the laser pulse.When the group velocity of light (and, consequently Fig. 1(a), which includes radiation of an electromagnetic shock wave in the form of a Cherenkov-type cone.In this Letter we present time-domain measurements of the attenuation versus frequency for the spectral components of the propagating electrical pulse, due to the radiation of the electromagnetic shock wave. This loss is so severe that after propagating only 1 mm, power at 0.8