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A Circularly Polarized Microstrip Antenna using an Open Stub Coupled-Line Phase Shifter
The design, simulation, fabrication, and measurement of a proximity-fed circularly polarized microstrip antenna is presented. The two major design components are (1) a stepped impedance open stub (SIOS) coupled line 90° phase shifter, and (2) a dual probe proximity fed circular patch antenna. The 90° phase shifter is a 4-port network consisting of two microstrip lines: the SIOS with coupled line, and a reference line. The antenna is a 64.4 mm diameter circular patch printed on top of a 140 mm х 140 mm х 0.8 mm upper layer FR4 substrate. The antenna is fed by a cascaded phase shifter and Wilkinson power divider on a 170 mm х 170 mm х 0.8 mm lower layer FR4 substrate. The top side of the lower substrate contains the ground plane and the feed network is printed on the bottom side. Connecting the upper and lower substrates are feeding probes which extend from the two outputs of the phase shifter to the bottom of the antenna substrate through 16.7 mm air layer. Simulation results show a 3-dB axial ratio (AR) bandwidth of 22.2% from 1.68 GHz to 2.1 GHz, a 2-dB AR bandwidth of 19.3% from 1.69 GHz to 2.05 GHz, and a 10-dB return loss bandwidth of 60.9% from 1.6 GHz to 3.0 GHz. A prototype was fabricated and measured results show a 3-dB AR bandwidth of 38% from 1.66 GHz to 2.44 GHz, a 2-dB AR bandwidth of 14% from 1.8 GHz to 2.07 GHz, and a 10-dB return loss bandwidth of 60.9% from 1.6 GHz to 3.0 GHz.