SUBARRAY DESIGN FOR C-BAND CIRCULARLY-POLARIZED SYNTHETIC APERTURE RADAR ANTENNA ONBOARD AIRBORNE

This paper presents the design and realization of a 4 × 4 broadband circularly polarized microstrip antenna as subarray element for airborne C-band circularly polarized synthetic aperture radar (CP-SAR). The main objective of this work is to optimize impedance bandwidth, axial-ratio bandwidth, gain, and radiation pattern of a CP-SAR array antenna due to the limitation in the available space for a large array antenna installation on airborne platform. Various patch separations in uniformly 2 × 2 subarray configuration have been simulated to investigate characteristics of impedance bandwidth, axial-ratio bandwidth, gain, and radiation pattern. In order to broaden the impedance bandwidth, the proposed antenna is constructed by stacking two thick substrates with low dielectric constant and dissipation factor. The measured 10-dB impedance bandwidth is 0.91 GHz (17.2%), spanning from 4.83 GHz to 6.01 GHz. A simple square patch with curve corner-truncation is applied as the main radiating patch for circularly-polarized wave generation. The radiating patch is excited by single-fed proximity coupled strip-line feeding. The improvement of axial-ratio bandwidth in 2 × 2 and 4 × 4 subarray is employed by a feeding network with serial-sequential-rotation configuration. Experimental result shows the 3-dB axial-ratio bandwidth achieved 1.18 GHz (22.17%) from 4.8 GHz to 5.71 GHz. Other characteristic parameters such as gain and radiation pattern of the 4 × 4 subarray antenna are also presented and discussed.