Coherent high‐frequency oscillations near the electron plasma frequency (&ohgr;≲&ohgr;p) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath–plasma resonance and is driven by a negative rf sheath resistance associated with the electron inertia in the diodelike electron‐rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening, and copious harmonics. The fundamental instability is a bounded oscillation caused by wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres), which excludes other radiation mechanisms such as orbitrons or beam–plasma instabilities. The line broadening mechanism has been identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burstlike rf emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.