  A comparative study of the magnetic properties of shunted and unshunted
two-dimensional Josephson junction arrays (2D-JJA) is presented. Using a
single-plaquette approximation of the 2D-JJA model, we were able to
successfully fit all our experimental data (for the temperature, AC and DC
field dependencies of susceptibility) and demonstrate that the dynamic
reentrance of AC susceptibility is directly linked to the value of the
Stewart-McCumber parameter \beta_C. Based on extensive numerical simulations, a
phase diagram \beta_C vs \beta_L is plotted which demarcates the border between
the reentrant and non-reentrant behavior in the arrays.
