Measurement of dielectric and magnetic properties of materials by means of a TDR probe: a preliminary theoretical investigation in the frequency domain
This paper is focused on the issue of measuring both dielectric and magnetic characteristics of a penetrable material at radio frequency through a time-domain reflectometry probe. The goal is to prove that a time-domain reflectometry probe can grant the possibility to discriminate the dielectric permittivity from the magnetic permeability of the material. This is due to the fact that the time-domain reflectometry datum depends both on the propagation velocity of the electromagnetic waves in the probed medium and on the intrinsic impedance of the probe. However, the possibility to attain such a clear discrimination is bound by the condition that the reflection coefficient is measured (or calculated) along the probe at the air–soil interface in the frequency domain. Generally, time-domain reflectometry probes measure the total (incident plus reflected) field in the time domain, and subsequently, the datum is needed to meet the condition that such claimed purpose is not just the Fourier transform of a datum collected by means of a common time-domain reflectometry probe. Rather, either a devoted hardware should be implemented or a very accurate knowledge of the incident field should be guaranteed in order to separate the reflected wave from the incident one. In this preliminary work, our survey has been restricted to a theoretical investigation in the frequency domain. In particular, our focus is set on the lossless case, and the attention is devoted to the issue of possible multiple solutions to demonstrate that this obstacle can be overcome by making the frequency step narrower or, alternatively, by narrowing the length step of the probe. Simulation results based on a bifilar transmission line model are shown.