We searched for the emission of microwave radiation in the Ku band generated
by a 95 keV electron beam in air. We unequivocally detected the radiation, and
measured its yield and angular dependence. Both the emitted power and its
angular pattern are well described by a model, where microwave photons are
generated via bremsstrahlung in the free-electron atomic-nucleus collisions,
during the slowdown of the electrons. As a consequence, the radiation is not
isotropic but peaked in the forward direction. The emission yield scales
proportionally with the number of electrons. This contrasts a previous claim
that the yield scales with the number squared, due to coherence. With a Monte
Carlo simulation we extrapolate our results to the Ultra High Energy Cosmic Ray
energy range.Comment: 5 pages, 4 figures. Accepted for publication in Phys.Rev.
