The input admittance or impedance and the effective height of small metal antennas, with a finite gap, located in a dispersive environment, is obtained in an analytical form by properly renormalizing a modal series. The equivalent network of the antenna turns out to be composed of physically realizable elements, thus allowing the computation of the transient response by using Laplace transform techniques. The extension to the case of an inhomogeneous or a uniaxial anisotropic environment is also suggested.