  The coordinate transformation on the space that contains electromagnetic
sources is studied. We find that, not only the permittivity and permeability
tensors of the media, but also the sources inside the media will take another
form in order to behave equivalently as the original case. It is demonstrated
that, a source of arbitrary shape and position in the free space can be
replaced by an appropriately designed metamaterial coating with current
distributed on the inner surface and would not be detected by outer observers,
because the emission of the source can be controlled at will in this way. As
examples, we show how to design conformal antennas by covering the sources with
transformation media. The method proposed in this letter provides a completely
new approach to develop novel active EM devices.
