A Compact Ultrawideband Circularly Polarized Antenna Based on Miniaturized Phase Shifter

In this article, a compact wideband circularly polarized (CP) antenna based on a miniaturized phase shifter with ultrawideband operation is proposed. The proposed antenna is comprised of a pair of compact orthogonal ultrawideband Vivaldi antennas and a miniaturized phase shifter. To achieve wideband impedance matching and miniaturization, parasitic radiation structures, metal coupled plates, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma $ </tex-math></inline-formula>-type balun with high-impedance transmission lines are designed. After careful optimizations, the final antenna dimensions are only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.36 {\times }0.36 \times 0.34\lambda ^{3}_{l}$ </tex-math></inline-formula>, where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{l}$ </tex-math></inline-formula> is the free-space wavelength at the lowest operating frequency. Additionally, the miniaturized 90° wideband phase shifter in the antenna is designed by employing a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Pi $ </tex-math></inline-formula>-type network and a negative group delay (NGD) network with extremely compact dimensions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.071 \times 0.047\lambda ^{2}_{l}$ </tex-math></inline-formula>. The simulated results indicate that the antenna exhibits a reflection coefficient below −10 dB within 0.32–1.2 GHz (3.75:1) and a 3-dB axial ratio (AR) bandwidth within 0.32–1.15 GHz (3.59:1). Finally, a prototype is fabricated and reasonable agreement is achieved between the simulated and measured results.