  Based on a recent formulation of the V-parameter of a photonic crystal fiber
we provide numerically based empirical expressions for this quantity only
dependent on the two structural parameters - the air hole diameter and the
hole-to-hole center spacing. Based on the unique relation between the
V-parameter and the equivalent mode field radius we identify how the parameter
space for these fibers is restricted in order for the fibers to remain single
mode while still having a guided mode confined to the core region.
