This article review the current state of the quasisteady-state theory of the high frequency electrical properties of sources (probe antennas) in a plasma in which one can neglect the collisions of the charged particles. We discuss the structure of the potential and of the electric field, the impedance and the noise; the effect on these characteristics of a d.c. magnetic field, of excitation of plasma oscillations, thermal and directional movement of particles (passage effects), of deposition of particles on the surface of sources, etc. When plasma oscillations are excited, we show that the electric field usually or generally does not decline with distance from the source, or declines considerably more slowly than in a vacuum. We demonstrate that the fundamental characteristics of ``large'' sources (of dimensions large in comparison with the spatial-dispersion parameter) can be determined in the ``cold'' plasma approximation, in which the equation for the potential is generally an equation of mixed type, while the solutions of physical interest are complex. For "small" sources, the principal effect studied is the passage of electrons in the inhomogeneous a.c. field of the source. Expressions are given for the impedance and noise of sources with account taken of passage effects. We note that finite particles should exert in a number of cases a substantial effect on the characteristics of ``small'' sources.