Millimeter-wave Series-fed Microstrip Antenna Arrays

In recent years,with the rapid development of wireless communications,the systems have been put forward requirements of high speed,large capacity and broad band,and millimeter-wave spectrum has attracted more and more attention as a new spectrum resource.Millimeter-wave antenna arrays have become a hot topic and are being extensively investigated for the fifth-generation mobile communication systems(5G)and radar systems.Series-fed microstrip antenna arrays are widely used in the millimeter-wave systems because they offer unique advantages of having a low profile,being light weight,inexpensive to manufacture,and minimized feed line length to reduce line losses.Firstly,based on the patch width tapering method,16-element end-fed microstrip antenna array has been designed at the 77 GHz band.Simulation results show that a 16-element series-fed microstrip linear array has a impedance bandwidth of 1.9 GHz and can achieve the sidelobe level(SLL)of-16.8 dB with half power beamwidth(HPBW)of 5.8°.The measurement results show that a linear array in the planar array has a impedance bandwidth of 2.6 GHz and can realize the SLL of-11.0 dB with HPBW of 5.4°.The impedance bandwidth and beamwidth meet the specification,however,the deterioration of SLL is severe.It has been observed that the patch width tapering method fail to realize the low SLL in the millimeter-wave band.Next,a design strategy for millimeter-wave low sidelobe level(SLL)series-fed microstrip antenna arrays is proposed.Using the design strategy based on the combination of the differential evolution algorithm(DEA)and the full-wave high-frequency structure simulator,the antenna parameters are optimized simultaneously.Using the DEA,constraint conditions of the HPBW,the direction of the antenna,inter-element spacing(IES)between array elements,resonant lengths and relative widths of array elements are both taken into consideration,then optimization results of SLL which meet the constraints will be obtained.Mutual coupling between radiating elements,loss from the feeding line and surface wave effects are taken into consideration in this strategy,so the results are more accurate and the strategy is more suitable for millimeter-wave band.Finally,a quasi-equal IES structure is proposed and by adopting the joint optmization,center-fed and end-fed antenna arrays of quasi-equal IES are designed at the 38 GHz band.Measurement results show that the 10-element center-fed and end-fed antenna arrays of quasi-equal IES can achieve the SLL of-20.2 dB with the HPB W of 8.0°and-21.7 dB with the HPB W of 7.6°,respectively.Using the proposed joint optimization design strategy,the series-fed antenna arrays can achieve lower SLL,narrower HPB W,and shorter length in the millimeter-wave band than the ones using the traditional design methods in the case of high gain.In the case of same HPBW,the SLLs of the series-fed antenna arrays of quasi-equal inter-element spacing move closer to the SLL of equal IES Chebyshev arrays.