A comparison of phase-shift and one-port coaxial cell permittivity measurements for GPR applications

The coarse and loose nature of unbound granular road materials presents a number of challenges for conventional permittivity characterisation approaches. An alternative that appears better suited to these materials involves measuring the phase-shift at discrete frequencies through a sample of known thickness. To validate this approach against more established methods, a comparison is required on materials that can be easily measured using either method. To this end phase-shift measurements were undertaken on a range of solid dielectric slabs including various types of stone, plastic and an artificial material. Permittivity predictions from this method were then compared to results from a one-port coaxial cell. As an additional comparison, and to better understand the results, the phase-shift test setup was also modelled using GPRMax software. To improve the predictions, reverberations within the test apparatus were minimized by isolating the direct wave using time-domain Blackman windowing. However, the narrow window necessary for this particular test setup also degraded the ability to detect frequency-dependent permittivity changes. Overall the phase-shift approach produced real relative permittivity predictions similar to that from the one-port coaxial cell. Despite limitations in the current approach, the results validate the phase-shift approach as a simple and rapid method of characterizing the permittivity of larger dielectric material samples of constant thickness.