A sensing system which comprises a material (30) formed of a matrix and a plurality of non-insulating particles (40) substantially equally spaced within the matrix such that the material has coherent electrical periodicity in at least one dimension; and a receiver (10), the receiver arranged to receive a source RF signal and a returned RF signal, the source RF signal being reflected by the non-insulating particles to produce the returned RF signal. A change in the position of one or more of the non-insulating particles causes the returned RF signal to change, such that a change in a property of the material can be determined from the returned RF signal.