A time domain transmission measurement system for dielectric characterizations

Abstract

Delay time measurements are a commonly used technique for the characterization of dielectric materials. Especially with regard to the characterization of water-solid mixtures like soil or grain delay time measurements, e.g., time domain reflectometry offers a powerful method. However, the accuracy of reflection measurements is limited due to multiple reflections caused by inhomogenities of the environmental material of the sensor. This contribution deals with an improved sensor design based on time domain transmission (TDT) measurements. Thus, the first received impulse includes the necessary information. Multiple reflections are received at later time steps and their influence on the measurement accuracy is nearly negligible. To improve the performance and the applicability of the designed sensor, a cost-efficient TDT system is designed, which is integrated in the sensor. Additionally, a so-called "concentric reversion coupler" is used, which offers the possibility to perform TDT measurements without the necessity of external measuring ports.There are several well-established techniques available for the determination of the permittivity in homogeneous and inhomogeneous materials [1]. In particular, time domain reflectometry (TDR) measurements are an often applied method for the determination of the dielectric properties of various materials like soil, grain, or coal [2, 3]. The popularity of these reflection measurements is based on the advantage that microwave measurements like TDR are non-destructive and are available in real time. Additionally, baseband signals used for TDR measurements are highly sensitive against the water content due to the extraordinary dielectric properties of water at lower frequencies [4]. Nevertheless, the accuracy of TDR measurements is limited due to multiple reflections caused by inhomogenities of the material under test. In contrast to this, time domain transmission (TDT) measurements [5] yield accurate measurement results independent of the constitution of the surrounding material due to the fact that the first measured impulse is unique and presents the useful signal, which is only slightly influenced by multiple reflections [6]. To validate this assumption, an electromagnetic simulation inside layered materials is performed for TDR and TDT measurements. For this purpose, a two-wire line is inserted in a layered material with varying permittivities of the different layers as shown in Fig. 1. For the investigation of the influence of multiple reflections on the measurement accuracy, layer III, with a permittivity of 1 r = 20, is displaced within layer II with a permittivity of 1 r = 3. Based on the delay time, the mean value of the permittivity of the layers I-IV is determined for every position of layer III as shown in Fig. 1. Ideally, this mean value should be constant independent of the position of layer III. These results show that the accuracy of the TDR system is decreased due to multiple reflections caused by the layered surrounding material. In contrast to this, the TDT ...

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