A Compact Multi-Beam End-Fire Circularly Polarized Septum Antenna Array for Millimeter-Wave Applications

In this paper, a novel circularly polarized end-fire septum antenna based on substrateintegrated-waveguide (SIW) is proposed in the 60-GHz band. By introducing four symmetrical tapered curved air slots in three stacked open-ended SIW layers, the antenna can obtain an operating bandwidth from 57.2 to 63.4 GHz for both AR <; 3 dB and |S₁₁| <; -21 dB. Besides, the half-power-beam-width and 3-dB axial-ratio (AR) beam-width are all wider than 79°, and a stable gain of 7.57 dBic with a variation of 0.15 dBic can be achieved in the working band. A 4 × 4 SIW Butler matrix based on SIW with three same substrates is then designed. The modified two-layer phase shifters realized by the different path distance are employed in the folded Butler matrix. By combining the antenna element and the Butler matrix together, a four-beam antenna array with circular polarization wave is realized. The proposed array has been fabricated and measured. The measurement results show that the antenna gain up to 11.01 dBic, -10 dB impedance and isolation bandwidth from 57.8 to 62.5 GHz (except at around 58.3 GHz), stable radiation pattern and a wide angular range of ±37° can be obtained. The measured AR values of the main lobe range of four beams are all less than 5 dB from 58 to 60 GHz.