A Broadband Circularly Polarized Antenna Using Characteristic Mode Analysis

A low-profile and broadband circularly polarized (CP) antenna is presented. The CP antenna element comprises several patch cells and an L-shaped strip with three branches on the same layer. The patch cells and the L-shaped strip excite four linear polarized modes, while three branches shift these modes to suitable frequencies using characteristic mode analysis. Furthermore, a two-port antenna is constructed by mirroring the CP antenna element, where the middle patch cells can further broaden the bandwidth and cancel opposite currents to provide 27.5 dB isolation. Finally, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> antenna array realizes wide bandwidth and high gain. The fabricated CP antenna element, the two-port antenna, and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> antenna array achieve wide bandwidths of 19.5%, 21.1%, and 33.6% (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert S_{11}\vert \lt -10$ </tex-math></inline-formula> dB, axial ratio <3 dB), respectively. The measured isolation for the two-port CP antenna exceeds 27.5 dB. Besides, these antennas maintain a lower profile of 0.036 free-space wavelengths at the center frequency of 3.60 GHz and are a suitable candidate for future 5G applications.