Millimetre wave lens and transmitarray antennas for scan loss mitigation

Beam steering impairments adversely affect antenna performance at wider steering angles. Scan loss degrades the antenna gain, and hence the link budget. To address this problem, antennas designs based on phased arrays, lenses, and transmitarrays are proposed. Millimetre wave beamforming within 5G cell sectors is considered as an application scenario. Feed networks for an 8-element phased array, operating at 28 GHz, were designed using unequal power dividers. A Taylor amplitude distribution was applied to reduce the sidelobe level to -15.2 dB at boresight. Prototypes were fabricated in microstrip, using meanders to steer the beam. Cascaded Fresnel lenses were placed around the array, to enhance the gain. By tilting the lenses to align with the steered beam, the lenses increased the gain by 3.19 dB at ±52°, and by a further 1.5 dB when repositioned in simulation. Asymmetric amplitude distributions were applied to the array to prevent the main lobe from splitting. Diffraction theory was used to analyse the focusing properties of the lens arrangement. The fabricated prototype exhibited a bandwidth of 1.75 GHz. Antennas were designed and simulated for line-of-sight MIMO scenarios. An envelope correlation coefficient below 0.0356 was maintained for both designs. 2D SISO beam steering was also simulated. Achievable data rates were estimated from the antenna parameters, and the effect of interference was evaluated. Scan loss was mitigated for the two antenna rows within the focal region. A conformal transmitarray was designed, using 1-bit unit cells based on crossed-slots. A unit cell placement rule was proposed to reduce the number of electronically reconfigurable cells by 59%. A measured gain of 12.5 dBi and a simulated total efficiency of 75% were obtained at boresight and the maximum steering angle of 53°. By combining reconfigurable lenses with phased arrays, the focusing directivity is able to mitigate scan loss