  Context. Solar radio bursts originate mainly from high energy electrons
accelerated in solar eruptions like solar flares, jets, and coronal mass
ejections. A subcategory of solar radio bursts with short time duration may be
used as a proxy to understand the wave generation and propagation within the
corona. Aims. Complete case studies of the source size, position and kinematics
of short term bursts are very rare due to instrumental limitations. A
comprehensive multi-frequency spectroscopic and imaging study was carried out
of a clear example of a solar type IIIb-III pair. Methods. In this work, the
source of the radio burst was imaged with the interferometric mode, using the
remote baselines of the LOw Frequency ARray (LOFAR). A detailed analysis of the
fine structures in the spectrum and of the radio source motion with imaging was
conducted. Results. The study shows how the fundamental and harmonic components
have a significantly different source motion. The apparent source of the
fundamental emission at 26.56MHz displaces away from the solar disk center at
about 4 times the speed of light, while the apparent source of the harmonic
emission at the same frequency shows a speed of < 0.02c. The source size of the
harmonic emission, observed in this case, is smaller than that in previous
studies, indicating the importance of the use of the remote baselines.
