This paper presents a complete and detailed description of the fabrication and measurement of the electromagnetic properties of water-based semi-solid phantoms with emphasis on the analysis of the time evolution of the complex permittivity of several samples stored in different conditions. A known recipe for a 2/3 muscle equivalent phantom is used as test material, and the several phantom sample properties are measured with an in-house developed coaxial probe technique. It is shown that the storing condition is of paramount importance to extend the lifetime of a given phantom. This behavior stems from the way the storing condition affects the water evaporation rate of the sample. In particular, while an unprotected sample can preserve its electromagnetic properties only for a few days, a very well-sealed one can last at least up to a year.
Keywords: 2/3 muscle; BAN; coaxial probe technique; complex permittivity measurement; physical phantom.