Influence of source - phantom multiple interactions on the field transmitted in a flat phantom

For many applications involving antenna systems in the vicinity of lossy scatterers, such as phantoms used for Specific Absorption Rate (SAR) assessment, it would be of great interest to know how local absorption can be related to the free-space radiated electromagnetic field. However, when the source closely approaches e. g. a phantom, complex interactions mechanisms begin to occur, leading to modifications of the nearfield effectively impinging on the phantom. Based on plane-wave expansion, this paper proposes an analysis of the contribution of the free-space field to the field transmitted in a phantom, as the antenna-phantom distance is varied. The particular cases of λ/2 dipoles at 900 and 1800 MHz close to a flat phantom are studied numerically. The results surprisingly show that the electric field topography inside the phantom is poorly distorted by multiple interactions, even for distances as short as 2.5 mm. It appears that this distortion is generally even weaker, for source-phantom distances where the modulus of the reflection coefficient at the antenna input port is high.