Solar radio U or J-bursts are believed to be signatures of electron beams propagating along closed magnetic loops. Although, like type III bursts they are generated by electron beams, J-bursts are rather rare events. J-bursts with a turnover around 40 MHz imply large magnetic loops on the scale of a solar radius. The density in such loops is normally too low for EUV or X-ray observations. Using LOFAR between 30-80 MHz we image a sequence of three J-bursts with a continuous frequency coverage, a huge improvement over the previous observations. From the centroids of the radio sources we obtain a fit to a magnetic loop of 1 solar radii in altitude and around 1.4 solar radii in length from base to apex. The density within the magnetic loop appears much higher and varying slower than the density models inferred from type III radio bursts. Such conditions could be the reason why we do not observe as many J-bursts or U-bursts as we would expect from the closed magnetic flux in the low corona.