An Amplitude-Only Measurement Technique for the Evaluation of the Complex Permittivity of Composite Materials in the Mic
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AN AMPLITUDE-ONLY MEASUREMENT TECHNIQUE FOR THE EVALUATION OF THE COMPLEX PERMITTIVITY OF COMPOSITE MATERIALS IN THE MICROWAVE RANGE. B.BISCEGLIA', A.CILIBERTO'", G.d'AMBROSIO*, G.FERRARA", R.MASSA', C. SABATINO**. Bisceglia, d'Ambrosio, Massa: Univ. di Napoli, Dip. Ing. Elettronica, via Claudio I - 80125 Napoli, Italy. Ciliberto, Sabatino: Aeritalia SAIPA-GAT, v.le dell'Aeronautica, Pomigliano d'Ar co, Italy. "Ferrara,Univ. di Catania, Ist. di Informatica e Telecomunicazioni, v.le A. Doria, I - 95125 Catania, Italy. ABSTRACT The basic transmission line and rectangular waveguide equations, with reference to different loading conditions, are firstly reviewed. Amplitude and phase, amplitudeonly, and phase-only permittivity measurement techniques are briefly discussed. Two very naive graphical and numerical calculation procedures are presented, together with the results of measurements on some composite materials. INTRODUCTION Composite materials have electrical properties that are strongly dependent on the particular composition and structure, and may vary over a very wide range. The knowledge of such properties is very important not only for electrical (microwave) processing purposes, but also for diagnostic and non destructive testing purposes. Aeritalia SAIPA-GAT and the University of Naples (Dep. of Electronic Eng.) started a research program on microwave non destructive test techniques, and the first step was the evaluation of the permittivity of a number of composite materials ill the 6 - 17 GHz range. To this end the most commonly used measurement techniques were reviewed and it was decided to proceed according to a new very simple procedure based on amplitudeonly measurements. The results obtained allow to go on further, studying microwave non destructive test techniques on composite materials.
z zi
kzZ(O) Z2
0 Fig.l: Transmission line scheme.
Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
z
498
BASIC EQUATIONS Let us consider (Fig.1) a z axis and a lossless transmission line (characteristic impedance Z 1 and propagation constant kl,, both real) from z = -oo to z = --4, a lossy line (characteristic impedance Z 2 and propagation constant k 2 , both com-plex) from z = -t to z = 0, and a load impedance Z(0). The input impedance i(-e) of the lossy line can be calculated from the Voltage Standing Wave Ratio (VSWR-=S, measured on the lossless line) and the position (z = -t - A) of the voltage star.ding wave minimum closest to z = -t: = Z, 1 + iStg(klzA) S + itg(ki.A) assuming, here and in the following, a e-" time dependence [1]. By the other hand the same Z(-f) depends on the lossy line parameters (Z 2. k 2 .) according to the following relationship :
i(.-e)
= Z• Z(0) - iZ 2 tg(k 2•5 ) Z 2 - iZ(0)tg(k2 ,)"
Once the loading conditions (Z(0)) are given, and the z(-t) is known from the measurement of S and A, equation (2) can be used for calculating the lossy line! parameters (Z 2 , k 2.). As long as the loading conditions (Z(0)) are concerned, let us consider the following ones. 1)
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