5. Conclusions

A framework for the design and optimization of large dual-linear polarized reflectarray antennas has been presented. It is based on the generalized intersection approach (IA) algorithm, which is used for both a phase-only synthesis to obtain the initial design and a direct optimization of the reflectarray layout for a subsequent optimization of the cross-polarization performance. The IA employs the Levenberg-Marquardt algorithm in the backward projector and a method of moments based on local periodicity (MoM-LP) to accurately characterize the electromagnetic response of the unit cell.

In order to demonstrate the capabilities of the proposed framework, two examples for advanced wireless communication applications are provided. For the two designs, a unit cell that consists in two sets of parallel dipoles is employed. Each set of dipoles controls the phase shift of a linear polarization. The first example is a shaped-beam reflectarray for future 5G base stations, radiating a sectored beam in azimuth and a squared-cosecant beam in elevation. A circular reflectarray with a diameter of 182 mm and comprised of 912 elements was proposed. The shapedbeam reflectarray achieves a maximum gain of 19.6 dBi and a SLL better than 20 dB. In addition, the gain smoothly decreases its value in elevation a total of 15 dB over a tilt of 50°. This feature makes the synthesis of a squared-cosecant pattern a challenging task. Finally, the maximum value of the crosspolar pattern was reduced more than 8.5 dB for both linear polarizations after a direct layout optimization using MoM-LP directly in the optimization loop, while maintaining the copolar pattern within requirements.

The second example consists of a 1.1-metre reflectarray for direct-to-home (DTH) broadcasting application. A Southern Asia coverage footprint that emulates the requirements of the SES-12 satellite has been selected. This coverage presents two zones with different gain requirements: one comprising India, Nepal, Bhutan and Bangladesh with a 30 dBi of minimum copolar gain requirement and another for Pakistan and Afghanistan with a requirement of 26 dBi. After a phase-only

Reflectarray Pattern Optimization for Advanced Wireless Communications DOI: http://dx.doi.org/10.5772/intechopen.88909

synthesis, the layout of the reflectarray was obtained using a zero-finding routine and simulated with a MoM-LP tool. The minimum copolar gain achieved in both linear polarizations is better than 31 dBi for zone 1, while it is better than 28 dBi for zone 2. Then, a direct optimization of the layout with MoM-LP was carried out to improve the cross-polarization performance. Both the minimum crosspolar discrimination and crosspolar isolation improved at least 6 dB for both zones and linear polarizations while keeping the minimum copolar gain within requirements.

The results shown here demonstrate the versatility of the proposed framework for the design and optimization of reflectarrays, as well as the feasibility of this type of antenna for advanced wireless communications.
