Periodically-Stub-Loaded Microstrip Line Wideband Circularly Polarized Millimeter Wave MIMO Antenna

This paper presents a wideband eight-port multiple-input-multiple-output (MIMO) circularly polarized antenna system for mm-wave applications. The proposed antenna element is an L-shape microstrip line with a series fishtail stubs on one side. The spacing between these stubs is adjusted to be a guided wavelength at the center frequency. These fishtail stubs are used to cancel the anti-phase magnetic currents to enhance the radiated fields from the microstrip line. A 90° phase difference between the fields of the two arms of the L-shape is obtained by shifting the feeding point from the center of the corner by one eighth guided wavelength. This 90° phase difference introduces the required circular polarization. The proposed antenna covers the frequency range from 27.5 up to 31 GHz, which is suitable for mm-wave N261 5G-band which cover the range from 27.5 to 28.35 GHz. These antenna elements are arranged on the four corners of a rectangular substrate to configure the proposed MIMO antenna system. Different parameters of this MIMO antenna are investigated by using numerical simulation and verified by experimental results.