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This paper presents detailed spectra for electromagnetic emission from a strong relativistic beam-plasma instability. Some emission appears early in the beam voltage pulse, apparently from reflexing. This subsides as beam propagation improves, and emission moves to high frequencies. This is compatible with a collective emission process from electrostatic waves built up in the chamber by reflection from the walls. Noise levels suggest about 100 emitting sites switching on and off randomly. The emitters comprise about 1 percent of the plasma volume and survive a time comparable to the lifetime of solitons subject to ion damping.