  Solar radio U-bursts are generated by electron beams traveling along closed
magnetic loops in the solar corona. Low-frequency ($<$ 100 MHz) U-bursts serve
as powerful diagnostic tools for studying large-sized coronal loops that extend
into the middle corona. However, the positive frequency drift component
(descending leg) of U-bursts has received less attention in previous studies,
as the descending radio flux is weak. In this study, we utilized LOFAR
interferometric solar imaging data from a U-burst that has a significant
descending leg component, observed between 10 to 90 MHz on June 5th, 2020. By
analyzing the radio source centroid positions, we determined the beam
velocities and physical parameters of a large coronal magnetic loop that
reached just about 1.3 $\rm{R_{\odot}}$ in altitude. At this altitude, we found
the plasma temperature to be around 1.1 MK, the plasma pressure around 0.20
$\rm{mdyn,cm^{-2}}$, and the minimum magnetic field strength around 0.07 G. The
similarity in physical properties determined from the image suggests a
symmetric loop. The average electron beam velocity on the ascending leg was
found to be 0.21 c, while it was 0.14 c on the descending leg. This apparent
deceleration is attributed to a decrease in the range of electron energies that
resonate with Langmuir waves, likely due to the positive background plasma
density gradient along the downward loop leg.
