Ultra-Wideband Millimeter-Wave Bidirectional Circularly Polarized Monopole Antenna Array Using a Sequentially Rotated Feeding Technique

In this communication, an ultrawideband millimeter-wave (mm-Wave) bidirectional circularly polarized (CP) monopole antenna array with sequential rotation feeding (SRF) technique is proposed. First, a broadband CP monopole antenna element is realized by utilizing the asymmetric ground plane, C-shaped ring, and rectangular stub structure. A microstrip to coplanar waveguide (CPW) transition structure is designed to feed the antenna element. Then, the SRF technique is adopted to further improve the axial ratio (AR) bandwidth of the 2 <inline-formula> <tex-math notation="LaTeX">$\times2$ </tex-math></inline-formula> CP subarray. Finally, a <inline-formula> <tex-math notation="LaTeX">$4\times4$ </tex-math></inline-formula> CP array is designed to further enhance the gain for mm-Wave applications. Measured results show that the proposed <inline-formula> <tex-math notation="LaTeX">$4\times4$ </tex-math></inline-formula> CP array exhibits the −10 dB impedance bandwidth of 102.7%, 3 dB AR bandwidths of 100.7%/101.3% in the <inline-formula> <tex-math notation="LaTeX">$+\textit {z}/-\textit {z}$ </tex-math></inline-formula>-direction, and the peak gain of 15.6 dBic.