45 GHz Wideband Circularly Polarized Planar Antenna Array Using Inclined Slots in Modified Short-Circuited SIW

This paper presents a O-band planar circularly polarized (CP) antenna array with a wide bandwidth, in terms of both impedance matching and axial ratio (AR). A centered 45°-inclined slot in the broad wall of a substrate-integrated waveguide (SIW) with a modified short-circuited termination is first introduced to realize a wideband 45° linearly polarized radiation, incorporated with a pair of inductive vias for impedance matching. Then, orthogonally integrating two proposed slots with an SIW 90° directional coupler, a wideband CP element is designed and its beam exhibits a good AR over a relatively wide angular region in one direction. Following this, a two-layer SIW is employed to design a CP antenna array. Four CP elements are arranged in a rotational manner in the top layer, and they are sequentially fed by a four-way power divider in the bottom layer with an incremental phase delay of 90° to achieve a conical CP beam and higher gain. The performances of the power divider and cross-layer coupling slot are examined in the wide operating band. The proposed CP array antenna is prototyped at O-band, and measured results show that the antenna has |S ₁₁ <-13 dB, AR < 2.5 dB, and a gain of 8.2-10.5 dBic over 42.3-49 GHz (14.72%) and has a dimension of 30 mm × 30 mm × 1 mm.