Circularly Polarized S/C Dual-Band Antenna for Nonlinear Detection

Nonlinear radars detect nonlinear (harmonic or intermodulation) responses excited from semiconductor junctions within electronic targets, allowing for the detection of small targets that are otherwise difficult to detect. To properly detect the nonlinear responses from targets, which are typically much lower than the responses at the fundamental frequency, the transmit, and receive antennas in a nonlinear radar, must ensure high isolation between the transmit and receive ports to suppress unwanted harmonics from the transmit side interfering with target responses. In this letter, a circularly polarized dual-band shared-aperture antenna, applicable to nonlinear radar, is presented. The S-band part of the antenna has a center frequency of 3 GHz for transmit, while the C-band part is designed to operate at the second-harmonic band of 6 GHz for receive. The proposed antenna is designed to exhibit high isolation between the transmit and receive ports in both bands. The characteristics of the proposed antenna are demonstrated via simulation and measurement, where it shows good circular polarization (CP) characteristics at both S-and C-bands with high isolation between ports. Furthermore, the performance of the proposed antenna for nonlinear detection is verified through an experiment where a nonlinear target is detected.