Near-Field Microwave Imaging Using Open-Ended Circular Waveguide Probes

Near-field microwave imaging and subsurface sensing have shown significant potential in a wide range of applications, including corrosion mapping and composite structure inspection. Most of the conventional near-field imaging systems developed so far utilize an antenna (probe) scanned over the structure-under-test (SUT). The near-field interaction between the microwave signal and the SUT reveals the sought-after information about the interior of the irradiated structure. Previous systems commonly used open-ended rectangular waveguides as the near-field imaging probe. In this paper, the utility of circular waveguide probes for near-field imaging will be discussed in detail and compared with the conventional rectangular probe. The motivation behind utilizing the circular probe is the fact that its aperture field distribution yields lower side lobes. Herein, the reflection properties, near-field characteristics, sensitivity, and spatial resolution of both probes are contrasted using simulations and measurements. Furthermore, imaging results of various targets, such as flat-bottom holes, notches, as well as corrosion-under-paint samples acquired using circular and rectangular probes are presented. It will be shown that the circular probes offer key advantages in terms of resolution as well as sensitivity compared with the conventional open-ended rectangular waveguides.