We address the problem of extracting the maximum amount of information on an electromagnetic field over a domain DO from field sample measurements on a domain DI, with a priori information on the source (or scatterer). The problem is faced in two steps. In the first one, the source reconstruction is dealt with by taking into account the available a priori information and the optimal probe positioning is determined as that optimizing the singular value dynamics of the involved linear radiation operator. The second step consists of reconstructing the field on DO as that radiated by the retrieved source. An extensive numerical analysis highlights the performance of the approach.