Pulse radiation from an insulated antenna: an analog of Cherenkov radiation from a moving charged particle

Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge.