마이크로파를 이용한 도장 두께 측정과 전력선 통신용 적응형 비접촉식 커플러 설계

Abstract

This thesis describes contactless methods that use microwaves to measure paint thickness on a metal surface, and the design of a variable inductive coupler for power line communication. To measure the paint thickness, I used horn antennas and vector network analyzers to measure the phase of the reflection coefficient for normally incident microwaves on 400–2000 m thick dielectric layers. For the plane-wave approximation, I used the SOLT calibration and time-domain gating to eliminate all but the first reflected wave from the dielectric layer and, then, I used a transfer function modification to obtain the reference planes without air region. The reference plane can be the dielectric surface or the metal surface. I used electromagnetic frequency f = 15.5 GHz in the case of the dielectric reference plane and f = 55 GHz in the case of the metal reference plane. The thickness resolution is periodic and yields phase-sensitive regions at (2n + 1)/4 and phase-insensitive regions at n/2. The experimental results are in good agreement with the theoretical results in the phase-sensitive regions but not in the phase-insensitive regions. I propose a variable inductive coupler consisting of a toroidal coil, a capacitor, and a signal loop for variable powerline impedance. A toroidal coil with a capacitor operates as a bandpass filter at resonance frequency of 20 MHz. The strong magnetic field inside the toroidal coil and impedance matching enhance the coupling between the toroidal coil and powerline, and induce communication signals to the powerline at the resonance frequency. To match the variable powerline impedances, I propose signal loops that are inserted into the toroidal coil and can achieve impedance matching by changing the coupling coefficient between the signal loop and toroidal coil. I fabricated signal loops of four different sizes to achieve adaptive impedance matching to powerline impedances of 50–200 Ω. The measurements yielded S11 of -19 to -34 dB, S21 of -0.29 to -0.69 dB, and a 3-dB bandwidth of 0.41–1.18 MHz. The measured data rate was 6.7 Mbps for a 12 m powerline.