Controlling the emission of electromagnetic source

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 source inside the media will take another form in order to behave equivalently as the original case. It is demonstrated that, a source in the free space can be replaced by another source of arbitrary shape with an appropriate metamaterial coating around it without being 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 and electrically small 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.