ABSTRACT
A frequent situation, in telecommunication engineering, is that an e.m. field is either determined experimentally, or given as a design specification, while its sources J 0 and M 0, on the other hand, are not known. How to determine these sources is a trivial problem if the field {E, H} is known everywhere in a region, V, which contains the sources. In that case, J 0 and M 0 can be computed using Maxwell’s equations. On the contrary, if one knows the field {E, H} only in a part of the region V where the field is defined, and wishes to extract from this partial knowledge some information on sources located elsewhere, the problem is not trivial at all. For example, to design a transmitting antenna, the typical specification one starts from is the field intensity at the receiving antennas. Starting from these data one must design a distribution of imposed currents which sustains the specified field at a distance. In order to be able to proceed in this way, we must first go through the opposite path. At least, this will give us the ability to distinguish physically acceptable specifications from those which can never be satisfied. This is in fact the purpose of this chapter.
