Design of a Circularly Polarized Antenna Array Using Aperture Coupling Feeding Network for MIMO Imaging Radar Applications

Abstract This work presents the design, optimization, fabrication, and measurement of a $$2 \times 2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> circularly polarized microstrip patch antenna array intended as an elementary radiator for 76–77 GHz MIMO radar prototype validation. Circular polarization (CP) is achieved using a sequential rotation phase technique with four linearly polarized patches arranged on a square grid and excited with $$90^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>90</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> progressive phase shifts. Aperture-coupled feeding is employed to isolate the radiating patches from the feed network, enhancing pattern stability and impedance bandwidth. A series–parallel microstrip network is synthesized to deliver equal power to all elements with the required phase progression, using a $$32.5~\Omega $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>32.5</mml:mn> <mml:mspace/> <mml:mi>Ω</mml:mi> </mml:mrow> </mml:math> quarter-wave transformer for $$50~\Omega $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>50</mml:mn> <mml:mspace/> <mml:mi>Ω</mml:mi> </mml:mrow> </mml:math> input matching. The antenna is implemented on Rogers 3003 substrates with truncated-corner patches to improve axial ratio and matching. Measurements using a vector network analyzer (VNA) with mmWave frequency converters confirm a $$-10$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:math> dB impedance bandwidth of 10.9 GHz, good matching at 76.5 GHz ( $$S_{11} \approx -23$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>S</mml:mi> <mml:mn>11</mml:mn> </mml:msub> <mml:mo>≈</mml:mo> <mml:mo>-</mml:mo> <mml:mn>23</mml:mn> </mml:mrow> </mml:math> dB), peak realized gain of 10.9 dBi, and axial ratio below 3 dB over 74–77.5 GHz at boresight. Radiation patterns exhibit broadside coverage with $$\sim 40^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:msup> <mml:mn>40</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> half-power beamwidth in both principal planes, suitable for imaging radar. Sensitivity analysis shows robust performance against feed-line width tolerances and $$\pm 0.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>±</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> variations in substrate permittivity, with maintained gain and circular polarization across the target band. The results validate the proposed element as a compact, peak realized gain of 10.9 dBi at 76.5 GHz, as a compact CP radiator for MIMO antenna array prototype measurements and anechoic chamber validation.