We report measurements of coherent microwave emission from a rotating relativistic electron ring (200 kV, 100 A, 200 ns) interacting with an azimuthally periodic wiggler magnetic field. Tunable radiation is observed between 12.5 and 18 GHz at power levels of -5 kW. The predicted interactions occur near the crossing points of the TMm electromagnetic waveguide modes w 2 = k'c2 + w 2 (f, m) and the beam modes w = (f + N)f2l + kj 1 vil , where u;, is the waveguide cutoff frequency, 11 is the

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... n cyclotron frequency, kl is the axial wavenumber, vj 1 is the axial drift velocity and N is the number of wiggler periods. Our observations are consistent with single mode operation in the fundamental TM 1 coaxial mode.