We present the results of a study of spatial structure of sources of microwave and millimeter bursts with simple spiky time profiles at 17 and 34 GHz, similar to those found to be common at 3 mm wavelength. These bursts are of short duration, with fast 2-4 s rise time to peak, followed by a rapid exponential decay. When mapped at high spatial resolution with the Nobeyama Radio Heliograph (NoRH), the radio images show direct evidence that the radio sources are compact bipolar loops: source sizes are less than 5" and three of the five events studied show closely spaced oppositely polarized components in the circular polarization maps. All five events are located directly over magnetic neutral lines in the photosphere. The soft X-ray behavior is not entirely consistent with the Neupert effect in these events, since all five events show a rise in the soft X-ray flux well before any nonthermal electrons are present in the corona and the ratio of peak soft X-ray flux to peak radio 17 GHz flux may vary by many orders of magnitude from one event to the next. The abrupt time profiles of these events and their physical properties are consistent with a single-loop scenario in which magnetic energy release and acceleration of nonthermal electrons are confined to a compact localized region.