This study presents a methodology to investigate the complex permittivity of heterogeneous samples like concrete, mortars and cement pastes by using an open-ended coaxial probe. The probe is associated with portable Vector Network Analyser (VNA) allowing in-situ measurements. Another advantage of the proposed methodology is that the complex permittivity of concrete can be assessed from the measurement of the reflection parameter S11 alone which is innovating in comparison with common devices. This can be achieved by modelling the electrical equivalent circuit of the coaxial probe and the dielectric sample with a capacitive model briefly presented in this paper. A calibration procedure on materials with known permittivity developed to estimate the input parameters of the capacitive model is presented in this paper meanwhile all the details of measurements. Measurements performed on various cementitious materials (cement paste, mortar and concrete) at different degrees of water saturation (from almost dry to fully saturated) show that the probe is able to characterize their dispersive behavior. It is also demonstrated that the probe is able to measure both real and imaginary permittivity of cementitious materials in an extended range of moisture. Relationships are proposed between real or imaginary permittivity and volume water content.