This paper presents the study of radiation from a 2D electromagnetic band gap (EBG) structure excited by a line source. The formulation is based on the full-wave spectral analysis and is solved by the method of moments (MoM) approach. The EBG structure consists of a metal strip grating over a grounded dielectric slab (GDS). Both TM and TE cases are investigated. The array scanning method is applied in order to solve the aperiodic problem using a periodic problem, in which the line source is periodically duplicated. Highly-directive radiation patterns can be obtained when leaky waves are strongly excited in the structure. The complex propagation wave-numbers of the leaky waves are found by solving the homogenous problem using the MoM. The excitation of both proper and improper leaky waves is studied and the cases in which their radiation is dominant are presented. A parametric study on the geometrical dimensions of the EBG and their effect on the radiation pattern is presented. For validation purposes, the particular case of very wide strips (narrow slots) is compared to previously published results.