Contrast Enhancement of 76.5 GHz-Band Millimeter-Wave Images Using Near-Field Scattering for Non-Destructive Detection of Concrete Surface Cracks

Akihiko HIRATA  Makoto NAKASHIZUKA  Koji SUIZU  Yoshikazu SUDO  

IEICE TRANSACTIONS on Electronics   Vol.E103-C   No.5   pp.216-224
Publication Date: 2020/05/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.2019ECP5034
Type of Manuscript: PAPER
Category: Microwaves, Millimeter-Waves
millimeter-wave radar,  imaging systems,  millimeter-waves,  radar systems,  radio propagation,  

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This paper presents non-destructive millimeter-wave (MMW) imaging of sub-millimeter-wide cracks on a concrete surface covered with paper. We measured the near-field scattering of 76.5 GHz-MMW signals at concrete surface cracks for detection of the sub-millimeter-wide cracks. A decrease in the received signal magnitude by near-field scattering at the fine concrete surface crack was slight, which yielded an unclear MMW image contrast of fine cracks at the concrete surface. We have found that the received signal magnitude at concrete surface crack is larger than that at the surface without a crack, when the paper thickness is almost equal to n/4 of the effective wavelength of the MMW signal in the paper (n=1, 3, 5 ...), thus, making MMW image contrast at the surface crack reversed. By calculating the difference of two MMW images obtained from different paper thickness, we were able to improve the MMW image contrast at the surface crack by up to 3.3 dB.