This paper provides the essential tools for deriving quickly the quasi-thermal noise spectrum or the impedance of a given electric antenna near the plasma frequency, for calibration or diagnosis in space plasmas. We give simple analytical expressions and numerical results for either wire or sphere dipoles in an isotropic plasma with one or two Maxwellian electron populations. We include the contribution of the particles collected and/or emitted by the antenna surface. We also indicate some modifications brought about by using more complicated antenna geometries, and a drifting or a magnetized plasma. Finally, we give some conclusions for antenna design or data interpretation in plasma wave experiments. ¿ American Geophysical Union 1989