Ultrawideband Circularly Polarized Antenna for Near-Field SAR Imaging Applications

In this article, an ultrawide band circularly polarized crossed dipole antenna is proposed for near-field synthetic aperture radar imaging. The presented antenna comprises two orthogonal dipoles with elliptical arms vacated by rotated elliptical slot. The slots and a modified feeding configuration enhance both impedance and axial ratio bandwidth. The antenna has 4.75:1 impedance bandwidth (1.6-7.6 GHz) and 4.1:1 axial ratio bandwidth (1.8-7.1 GHz). To reduce the coupling in quasi-monostatic imaging scenarios, a planar circularly symmetric structure is used that decreases coupling about 10 dB. Due to importance of antenna time-domain behavior in imaging systems, transient response is evaluated in near-fields and also far-fields, which shows a good fidelity factor. The electric field distribution in a region of near-field area is almost uniform in different frequencies that improve imaging. The far-field features like radiation pattern, gain, and axial ratio are also satisfactory. Finally, an oil well monitoring system using the proposed antenna is simulated and based on the results, existed perforations in the well wall are detected. Compared to previous antennas, this antenna has several advantages such as higher impedance and axial ratio bandwidth, planar structure, low front to side ratio which mitigates mutual coupling, and satisfactory transient response.