The center-driven insulated antenna end-loaded with a bare quarter-wave section is investigated theoretically and experimentally. This antenna is analyzed in terms of the theory of the bare antenna and that of the completely insulated antenna in a relatively dense ambient medium. A simple transmission-line-like solution is obtained. Input admittances computed from the theoretical formula have been shown to agree well with measured results with air-insulated monopoles in both salt and fresh water. For an electrically short antenna the magnitude of the current distributions for a terminated insulated antenna can be kept quite constant, while that of the completely insulated antenna with the same physical length is triangular. This feature of the electrically short antenna provides a more useful current distribution and driving-point impedance. By adjusting the radius of the insulation, it is possible to match the characteristic impedance of the insulated antenna to the apparent input admittance of the monopole antenna to obtain a nearly pure traveling wave even for a longer insulated antenna.