Development of X-band microstrip antenna for circularly polarized synthetic aperture radar onboard UAV

Recently, broadband antenna is needed to realize high resolution remote sensing images, especially synthetic aperture radar (SAR). In this research, we propose an array antenna that is the core of the X-band antenna for circularly polarized SAR (CP-SAR) onboard an unmanned aerial vehicle (UAV). The proposed antenna is designed in left-handed circularly polarized (LHCP) mode and is a compact, lightweight, low-profile microstrip antenna with printed wiring on a two-layer dielectric substrate. The dielectric substrate is a glass-fluoropolymer dielectric substrate with a thickness of 1.6 mm, a relative dielectric constant of 2.17, and a dielectric loss tangent of 0.0005. It enables high-resolution CP-SAR with a wide bandwidth of 800 MHz and a low axial ratio. For this purpose, the radiating surface of the unit element of the array antenna was designed as an ax-shaped shape with square slots. The band axial ratio was adjusted by an elliptical degenerate separation element. And the feeding method is a single proximity junction feeding to provide flexibility in the array wiring. The unit elements are arranged in two rows and two columns, and each element is fed radially from the center by a sequential rotating array. The proposed antenna was designed by simulation and fabricated by photo etching. The basic characteristics of the antenna were verified by experiments in an anechoic chamber. The design was carried out at a center frequency of 9.4 GHz with a bandwidth of 800 MHz. The unit element showed a gain of 7.9 dBic and an axial ratio of less than 3.0 dB in the simulation, while the prototype antenna showed a gain of 8.5 dBic and an axial ratio of less than 2.7 dB. For the array antenna, the simulated gain was 11.8 dBic and the axial ratio was less than 2.7 dB, and the prototype antenna had a gain of 11.6 dBic and the axial ratio was less than 1.5 dB. This antenna will be used in further study for ground test and flight test of UAV onboard SAR.