Focused Planar Electromagnetic Waves for Enhanced Near-Field Microwave Imaging With Verification Using Tapered Gradient-Index Lens Antenna

Radar-based algorithms for electromagnetic (EM) imaging are developed based on the assumption that EM has a local planar wave front inside the imaging domain. However, this might not be the case for majority of utilized antennas as the imaged object is usually located within the near-field zone of the antenna. The impact of that assumption on imaging accuracy and whether utilizing an antenna that can create a focused planar wave front inside the imaging domain improves EM imaging are investigated in torso imaging as an example. Thus, three types of antennas are used to scan the torso; 1) bio-matched loop-dipole, 2) Gradient-Index lens (GRIN), and 3) Tapered GRIN (T-GRIN) lens antenna. The proposed T-GRIN lens antennas is designed to create a focused plane wave propagation inside the torso using tapered trapezoid water-filled cavities inside a host medium. The proposed design improves penetration depth by 33% compared to conventional GRIN lens and 75% compared to the bio-matched loop-dipole antenna, in a wide fractional bandwidth of 83% at 0.7-1.7 GHz. The realized results indicate that generating focused plane wave inside the imaged object, which is realized using T-GRIN lens antenna, improves the detection accuracy by 15 % and 56% compared to conventional GRIN lens and bio-matched loop-dipole antennas, respectively. Moreover, the localization accuracy is improved by 54.5% and 100% compared to conventional GRIN lens and bio-matched loop-dipole antenna, respectively. This study highlights the importance of creating focused planar wave front within the imaging domain for improved detection and localization using microwave techniques.