  The Sun is the source of different types of radio bursts that are associated
with solar flares, for example. Among the most frequently observed phenomena
are type III solar bursts. Their radio images at low frequencies (below 100
MHz) are relatively poorly studied due to the limitations of legacy radio
telescopes. We study the general characteristics of types IIIb and U with stria
structure solar radio bursts in the frequency range of 20 - 80 MHz, in
particular the source size and evolution in different altitudes, as well as the
velocity and energy of electron beams responsible for their generation. In this
work types IIIb and U with stria structure radio bursts are analyzed using data
from the LOFAR telescope including dynamic spectra and imaging observations, as
well as data taken in the X-ray range (GOES and RHESSI satellites) and in the
extreme ultraviolet (SDO satellite). In this study we determined the source
size limited by the actual shape of the contour at particular frequencies of
type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80
MHz. Two of the bursts seem to appear at roughly the same place in the studied
active region and their source sizes are similar. It is different in the case
of another burst, which seems to be related to another part of the magnetic
field structure in this active region. The velocities of the electron beams
responsible for the generation of the three bursts studied here were also found
to be different.
