Figure 3 shows the dimensions and locations of the two identical beam steering arrays, as well as the cavity-backed slot on the metallic casing. The length and width of the cavity-backed slot are 8.8 and 1.5 mm, respectively. Because the wave reflects from the bottom of the cavity, with a depth of 4 mm, and is superimposed with the wave radiated directly from the slot, the radiation pattern is unidirectional, as desired.

Figure 4 Two 4x4 Butler matrix feed network block diagram (a) and layout (b).

The block diagram and layout of the two 4x4 BMFNs are shown in Figure 4. They are used to feed the two beam steering arrays because they provide the necessary bandwidth, beam steering capability and beamwidth. Each BMFN is comprised of four hybrid couplers, two crossovers and two pairs of phase shifters to achieve the required amplitude distribution and phase differences between the output ports. The feeding ports of the BMFNs are ports 1 through 8, and their corresponding outputs, which connect to all eight antenna elements, are ports 9 through 16. The feeding ports and output phase differences are shown in Table 1. The BMFNs are fabricated on a 0.254 mm thick Rogers 5880 substrate, with ε_r = 2.2 and tanδ = 0.0009. Eight mini-SMP connectors are used for the measurements.

RESULTS AND DISCUSSION

Figure 5 Measured |S₁₁| of the feeding ports.

The beam steering antenna arrays were simulated using HFSS Version 15, and the |S₁₁| of the fabricated prototype were measured using ground-signal-ground RF probes, prior to the assembly of the mini-SMP connectors. The measured |S₁₁| for ports 1 to 4, plotted versus frequency in Figure 5, shows a minimum 10 dB bandwidth of 2.8 GHz, from 26.2 to 29 GHz, which covers the 28 GHz FCC band of 27.5 to 28.35 GHz. For brevity, the results for ports 5 to 8 are not shown.

Antenna gains and radiation patterns were measured in a mmWave compact range. At 28 GHz, the measured performance compared with the simulations are shown in Figure 6 for the four cavity-backed slot antenna elements fed by ports 1, 2, 3 and 4, demonstrating peak gains of 10.1, 9, 9.4 and 9.8 dBi, respectively. Losses are attributed to the BMFN, with an approximate insertion loss of 1.5 dB, as well as the screw feeding structure and mini-SMP connector. With different port excitations, uniform amplitudes with different phase distributions were achieved at the output ports, enabling the array to radiate beams at angles of −22 degrees (port 3), −8 degrees (port 1), +8 degrees (port 4) and +22 degrees (port 2), a total of ±22 degrees. The fabricated prototype is shown in Figure 7.

Figure 6 Simulated vs. measured radiation patterns at 28 GHz.

CONCLUSION

Figure 7 Fabricated prototype.

A 28 GHz beam steering antenna array was successfully implemented in a metallic casing for cellular phones. As well as demonstrating good performance - return loss, gain and beam steering - the experimental results were validated by simulation. Owing to its performance, ease of integration, low fabrication cost and fitting into the restricted volume of a cellular phone, the design approach offers an attractive solution for 5G mmWave cellular phones.

ACKNOWLEDGMENTS

This work was supported by the Shanghai Eastern-Scholar Professorship Award and, in part, by the 5G antenna foundation of Huizhou Speed Wireless Technology Co. Ltd.

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