Vol. 135 (2019) ACTA PHYSICA POLONICA A No. 4 Special Issue of the 8th International Advances in Applied Physics and Materials Science Congress (APMAS 2018) Complex Constitutive Characterizations of Materials in the X-Band Using a Non-Destructive Technique T.A. Elwi a, * , A.J. Salim b , A N. Alkhafaji b , J.K. Ali b and A.S.A. Jalal c a Department of Communication Engineering, Al-Mammon University College, Baghdad, Iraq b Microwave Research Group, Department of Electrical Engineering, University of Technology, Baghdad, Iraq c College of Information Engineering, Al-Nahrain University, 10070, Al-Jadriya Complex, Baghdad, Iraq In this article, a measurement technique based on the Nicholson–Ross–Weir formulation to retrieve the complex permittivity, εr = ε ′ r +iεr , of materials from the measured S-parameters, S11 and S12, at the X-band, 8 GHz up to 12 GHz, is discussed. Finite element method simulation based on Comsol software package formulations is invoked to evaluate the S-parameters based on the retrieved complex permittivity and to compare them to their measurements. Then, a parametric study is conducted for the simulation to match the numerical results to their identical measurements by considering the retrieved permittivity as an initial guess. Nevertheless, the complex permittivity is measured using network/impedance material analyzer in the frequency range from 1 MHz to 1.2 GHz to be compared against the evaluated values at the X-band. A PTFE sample is considered as an example to validate the precision of the proposed method. The obtained εr is found to be about 2.04-i0.0001, which is very close to the manufacturer range. Finally, excellent agreement between the measured and simulated S-parameters is observed DOI: 10.12693/APhysPolA.135.567 PACS/topics: FEM, NRW, PTFE 1. Introduction Identifying the material properties in the microwave range has received a great interest in the aerospace and telecommunication industries [1]. During last few years, several techniques were investigated to measure the rel- ative permittivity of materials [2–5]. The most com- mon methods are cavity perturbation, free-space mea- surement, and closed waveguide techniques. The free- space technique is employed for characterizing material samples, under the test of large dimensions [2]. How- ever, this technique involves undesirable reflections from the surrounding circumstances, difficulties of impinging a plane wave in a limited area, and diffractions from the edges of the sample that lead to low measurement pre- cision [3]. Therefore, using focusing lenses in such tech- niques are applied to reduce the reflections and diffrac- tion effects [4]. In the cavity perturbation techniques or resonant cavity measurements, the precision is more accurate than the free space technique [5], but these mea- surements are only applicable for a limited frequency band, where this technique acquires very sensitive design parameters for the frequency band of interest [6]. Closed waveguide method is widely used in complex permittivity measurements of materials with wide frequency bands [7]. Despite the fact, this method has lower accuracy than the resonant cavity method [8], in which the analysis at the dominant mode is appropriate with high accuracy of re- trieving the complex permittivity. However, the accuracy * corresponding author; e-mail: taelwi82@gmail.com of extracting the complex permittivity is very sensitive to the uniformity of the cross-sectional area of the sample under test, as well as air gap defects [9]. In this study, the PTFE is used as a reference sample to validate the applied the Nicholson–Ross–Weir (NRW) technique and the finite element method (FEM) simula- tions for retrieving the complex permittivity of materi- als. The retrieved complex permittivity is obtained af- ter running the measured S11 and S12 spectra from a rectangular waveguide in the NRW formulations at the X-band. This technique is applied to a low loss mate- rial, which can be extended to other types of materials. Furthermore, FEM simulations are conducted to validate the retrieved complex permittivity of the sample from matching the measured S-parameters to the simulated results. The comparison of the simulation using FEM and measured S-parameters is provided for further con- firmation of the obtained complex permittivity. The rest of this work is organized as follows: in Sect. 2, the mate- rial preparation and experimental methodology are dis- cussed. The results are presented in Sect. 3. Finally, the paper is concluded in Sect. 4. 2. Material and methods PTFE as a reference sample, of 1 mm in thickness, is loaded inside a WR-90 rectangular waveguide followed by measurement of the S-parameters at the X-band us- ing Agilent 8720 network analyzer as seen in Fig. 1. It is worth noting that the sample must fully fit the cross- sectional area of the waveguide with a minimum air gap to avoid any additional losses due to defects. The fun- damental transverse electromagnetic (TE10) mode is the only applied mode in this measurement. The network (567)