Pattern Reconfigurable Wideband Loop Antenna for Thorax Imaging

A unidirectional loop antenna that can achieve wideband pattern reconfigurability from -40° to +40° in the azimuth plane is presented. The antenna is designed to fulfill the multislice (level) scanning requirements of electromagnetic imaging systems for thorax imaging. To overcome the need for positioning of several antenna arrays, and hence eliminate the mutual coupling related complications, a square loop antenna with reconfigurable pattern is designed. To create a unidirectional radiation, the loop is loaded with capacitive gaps, which convert its radiation mechanism to that of two virtual dipole arrays with quadrature phase excitation. By utilizing this feature, the location of the gaps is varied on the loop's structure to form virtual dipole arrays in different directions, thus rotating the radiation pattern without physically moving the structure of the antenna. As a proof of concept, six gaps were created on the loop and each gap is loaded with a p-i-n diode to electronically switch between the positions of the designed gaps, thus enabling changing the radiation direction. The proposed antenna can achieve a compact size of 0.32λ × 0.32λ × 0.002λ (λ is the wavelength of the lowest resonance of the antenna) and a wide fractional bandwidth of 32% at 0.8-1.15 GHz, with a peak gain and front-to-back ratio of 2.1 dBi and 8 dB, respectively. The antenna is successfully tested on a thorax imaging platform to detect small volume of water (5 mL) inside lungs as an emulation of early pulmonary edema.