High-Resolution UHF Near-Field Imaging Probe

Imaging probes which work at relatively low microwave frequencies in the ultrahigh frequency band (UHF) facilitate imaging through high loss dielectric materials and yield manageable imaging system complexity. However, acquiring images with resolutions on the order of few millimeters at frequencies within the UHF band requires super-resolution imaging methods and probes. In this paper, a compact near-field imaging probe working in the UHF band is proposed for high-resolution noninvasive imaging applications. The proposed probe is based on an electrically small loop loaded with a miniaturized spiral resonator. The probe is analyzed in this paper using numerical electromagnetic simulations and its equivalent circuit model is consequently developed. A prototype of the proposed probe operating at 426 MHz is fabricated and its spatial resolution as well as sensitivity are experimentally verified. A spatial resolution of around $\lambda $ /140 is demonstrated in simulations and verified experimentally. In order to highlight the utility of the probe for practical applications, images of many test samples including multilayer dielectric composite structures and corroded metallic substrate are acquired using the fabricated prototype. It will be shown that the images obtained using the proposed probe is comparable with those produced using an open-ended rectangular waveguide operating at 24 GHz. The salient features of the proposed probe include its high sensitivity and resolution while operating at relatively low frequency.