In this paper, a low-profile wide bandwidth circularly polarized microstrip antenna is proposed as element for a C-band airborne circularly polarized synthetic aperture radar sensor. Several bandwidth improvement techniques were proposed and implemented. In order to increase impedance bandwidth, the antenna is constructed using double-stacked substrate with low dielectric constant, modified radiating shape for multi-resonant frequency, and a circle-slotted parasitic patch. Generation of the circularly polarized wave employs a simple square patch with curve corner-truncation as radiating element. The asymmetric position of the feeding is attempted to improve the axial-ratio bandwidth. To avoid a complicated feed network, the antenna is fed by single-feed proximity-coupled microstrip line. The effect of copper-covering on the upper layer for decrease undesired radiation wave emitted by the feeding is also studied and presented. Measurement results show that the impedance bandwidth and axial ratio bandwidth are 20.9% (1,100 MHz) and 4.7% (250 MHz), respectively. Meanwhile the measured gain is 7 dBic at the frequency of 5.3 GHz.
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