Intense relativistic electron beams fired into air at varying pressures display a wide range of microwave signatures. These experiments held beam current, energy, and pulse length constant while varying gas pressure. Our observing window is 10 to 40 GHz. At low pressures (<10 mTorr) exponential spectra result, consistent with beam reflexing or virtual cathode oscillations. Above 20 mTorr the spectrum flattens and suggests collective emission at the beam‐generated plasma frequencies. Power falls linearly with pressure above 20 mTorr, until electron‐Neutral collisions damp the emission at a few Torr. However, weak 10 GHz emission appears at full atmospheric pressure.