Electric field emissions at frequencies of (n+1/2) fce have been generated in a large-scale electron beam experiment in the giant NASA vacuum facility in Ohio. These emissions arise when a contrastreaming beam configuration exists, the primary beam consisting of monoenergetic electrons (50 eV to 5 keV) and the other beam of lower-energy 'backscattered' secondary electrons. It is suggested that the same mechanism could also be the source of the observed 3/2fce emissions at auroral latitudes. In the absence of the beam-beam instability, weak oscillations were observed at the plasma frequency. In the latter case no significant modifications of the primary beam velocity distribution occurred, and the beam configuration is adequately described by single-particle motion in the ambient magnetic field.