The emission by field desorption of a beam of ions from the atomic scale apex of a Au nanotip formed on the axis of a W base tip was extensively analyzed. Emission patterns, currents versus time, voltage and temperature, and energy distributions of the ion beams from the atomic size apex were measured. The data were analyzed using known theories of field desorption. We discuss the formation of the nanotip, the supply function, and the ionization process in order to have a qualitative model of the characteristics of the source.