  The inverse electromagnetic source scattering problem from multi-frequency
sparse electric far field patterns is considered. The underlying source is a
combination of electric dipoles and magnetic dipoles. We show that the
locations and the polarization strengths of the dipoles can be uniquely
determined by the multi-frequency electric far field patterns at sparse
observation directions. The unique arguments rely on some geometrical
discussions and ingenious integrals of the electric far field patterns with
properly chosen functions. Motivated by the uniqueness proof, we introduce two
indicator functions for locating the magnetic dipoles and the electric dipoles,
respectively. Having located all the dipoles, the formulas for computing the
corresponding polarization strengths are proposed. Finally, some numerical
examples are presented to show the validity and robustness of the proposed
algorithm.
