We report on observations of type U bursts, simultaneously detected by the Very Large Array at 1.446 GHz and by the broad-band spectrometer PHOENIX (ETH Zurich) in the 1.1-1.7 GHz frequency band. The VLA was operated with a time resolution of 1.66 s, while PHOENIX used a time resolution of 40 ms. A sequence of approximately 6 inverted-U bursts with similar morphology occurred between 1355-1359 UT, on 1989 August 13, accompanied by a soft X-ray flare of GOES class C6.6. The peak flux was 7 SFU above background. The narrow bandwidth of almost-equal-to 10% and the weak polarization in the sense of the ordinary mode is consistent with harmonic plasma emission. The turnover frequency of 1.4 GHz at the top of the U bursts then corresponds to a plasma density of 6 x 10(9) cm-3, while the footpoints at more than 1.7 GHz are rooted in densities greater than approximately 10(10) cm-3. Extrapolations of the coronal magnetic field, assuming a potential configuration, indicate that the VLA 20 cm source demarcates an isodensity level. The source covers a wide angle of diverging magnetic field lines whose footpoints originate close to a magnetic intrusion of negative polarity into the main sunspot group of the active region (NOAA/USAF 5629) with dominant positive polarity. The centroid of the 20 cm U-burst emission, which corresponds to the turnover frequency of the type U bursts and remains stationary during all U bursts, coincides with the apex of extrapolated potential field lines at a height of almost-equal-to 130,000 km. We interpret the U bursts in terms of electron beams propagating in closed coronal loops. However, the viability of the electron beam model requires: (1) a high ambient temperature in the relevant coronal loops (T greater than or similar to 7 x 10(6) K); (2) magnetic field strengths higher than those implied by potential field extrapolations; i.e., the existence of nonpotential fields. The exciter speed of three type U bursts is estimated to be v = 42,000-64,000 km s-1.
