The knowledge of the electric and magnetic properties of materials over a broadband frequency range is an essential requirement for accurate modelling and design in several engineering applications. Such applications span printed circuit board design, electromagnetic shielding, biomedical research and determination of EM radiation hazards (Deshpande et al. (1997); Li et al. (2011); Murata et al. (2005)). The electric and magnetic properties of materials usually depend on several factors: frequency, temperature, linearity, isotropy, homogeneity, and so on. The dispersive behaviour exhibited by these materials can be represented by a complex relative permittivity and magnetic permeability which depend on frequency as e(ω) = e′(ω)− je′′(ω) (1) μ(ω) = μ′(ω)− jμ′′(ω) (2) being ω the angular frequency, e′, μ′ the real parts and e′′, μ′′ the imaginary parts of the complex relative permittivity and magnetic permeability, respectively. The real part takes the ability of the medium to store electrical (or magnetic) energy into account, the imaginary part the dielectric (or magnetic) energy losses. The interaction of incident electromagnetic fields with a material can be successfully investigated only when accurate information on the complex permittivity and magnetic permeability is attained. For example, from the knowledge of the frequency dependence of the complex relative permittivity and magnetic permeability of a material, the shielding effectiveness of a structure made of that material can be predicted; similarly, signal interconnects can be accurately designed when the frequency dependence of the dielectric substrate is known; from dielectric property information of tissues the spatial distribution of an incident electromagnetic field and the absorbed power can be accurately determined. Although the complex relative permittivity and magnetic permeability are quantities not directly measurable, they are reconstructed from the measurement of a sensor reflection coefficient or scattering parameters, which can be obtained with a number of different techniques proposed and developed over the last decades (Afsar et al. (1986); Baker-Jarvis et al. (1995); Faircloth et al. (2006); Ghodgaonkar et al. (1990); Queffelec et al. (1994)). Some of these techniques are: open-ended coaxial probe, free-space measurement, cavity resonator, parallel plate capacitor, transmission-line techniques (microstrip, waveguide, etc.); they may be in time domain or frequency domain and make use of probes with one or two ports. No technique is all-embracing as each 1