An analysis of the radiation characteristics of a vertical current inside a dielectric-layer over ground is carried out. The general solution for the fields is given for the case of a vertical current which is independent of the vertical (z) coordinate. An "equivalence-principle" for the replacement of the vertical current by an appropriate surface current is presented. Expressions for the complex input power and for the power which goes into the lowest order transverse magnetic (TM)-type surface-wave mode are derived. In addition, graphs for the case of a short vertical dipole inside a dielectric-layer over ground for the radiation pattern, radiation resistance, and surface-wave resistance are presented. The calculations described above can be applied to the analysis of vertical feeds and short circuits in microstrip antennas.