Broadband Millimeter-Wave Endfire Circularly Polarized Array With a Low-Profile Feeding Structure

In this communication, a broadband endfire circularly polarized (CP) antenna with a low-profile feeding structure is proposed for millimeter-wave (mmW) applications. The array element consists of a horizontally oriented printed electric dipole and a vertically aligned tapered slot radiator, exciting two orthogonal radiated electric-field components simultaneously. The amplitude and phase relationships between these two field components can be adjusted by balancing the response of the tapered slot and dipole radiators, thereby producing a CP radiated wave. Importantly, a parasitic director is introduced near the printed dipole to compensate for the gain degradation of the dipole element at higher frequencies, thus significantly expanding the axial ratio (AR) bandwidth of the CP element. Compared with other reported endfire CP antennas, the proposed antenna owns a lower profile of feeding substrate (0.508 mm
$\sim 0.056~\lambda _{0}$
), making it amenable for convenient integration with planar front-end circuits. As a demonstration, a
$1\times8$
array prototype is designed and measured, exhibiting an impedance bandwidth of 42.1% (27.45–42.1 GHz) and a 3 dB AR bandwidth of 35.8% (27.5–39.5 GHz). The demonstrated CP array is a promising candidate for wideband mmW applications due to its advantages of wideband, ease of integration with planar circuits, and low fabrication cost.