This article presents a thorough study of spherical probe-corrected phaseless near-field measurements with the two-scans technique. Such technique is based on retrieving the antenna under test radiation pattern from the measurement of the near-field amplitude signals on two spheres of different radii. The postprocessing of this type of measurements results in a highly nonlinear algorithm, prone to get trapped in local minima and provide incorrect solutions when the measurement conditions are not properly selected. Through a series of numerical simulations, the influence of different measurement parameters on the phaseless technique is analyzed. It will be shown how both the relative and absolute values of the measurement spheres highly affect the convergence of the phase retrieval algorithm. The type of AUT and its radiation pattern characteristic also play a fundamental role in the feasibility of phaseless measurements. Other parameters such as sampling rate, noise, probe correction, polar truncation, and measurement offsets are also investigated. The conducted study allows to extract a set of guidelines to improve the accuracy of phaseless spherical near-field algorithms. In addition, purely phaseless antenna measurement examples are given to demonstrate the algorithm capabilities and limitations, and to validate the developed numerical investigations.

This paper presents a general technique for the farfield transformation of near-fields measured over arbitrary surfaces and multimode probe correction in an efficient and accurate way. The use of not only one but several spherical wave expansions to model the antenna under test fields allows to incorporate full probe correction for arbitrary orientations with low computation cost. Combining this approach with a hierarchical subdomain decomposition strategy, the transformation problem is solved with a low computational complexity. In this method, the fields produced by neighbor subdomains are gradually aggregated and interpolated following a multilevel scheme, leading to a good scalability with frequency compared with other matrix-based transformation methods. Numerical and experimental results are provided to show the efficiency and capabilities of the proposed algorithm.

This article proposes an under-sampling scheme in φ for spherical near-field antenna measurements. Reducing the number of samples along one of the spherical coordinates allows to decrease the number of positioner steps. Therefore, the savings in the number of samples are almost directly translated into measurement time savings. Under-sampling usually leads to aliasing; however, it can be controlled by exploiting the properties of the spherical wave expansion. The axial symmetry of conventional antennas leads to narrow azimuthal spectra, where the influence of this aliasing is small. By designing an estimator of the error introduced by the aliasing, the required number of φ samples for a given accuracy can be determined, without any a priori knowledge about the antenna under test. This allows the application of this technique for any kind of antenna regardless of its level of axial symmetry, obtaining different values of measurement time savings depending on the level of symmetry. Several examples using the numerical and measured data are presented, showing significant reductions in the measured samples for different antennas. In addition, the algorithm is successfully tested against antennas with poor axial symmetry to demonstrate its robustness.

In spherical near-field measurements, the number of measured samples grows with the radius of the minimum sphere enclosing the antenna under test(AUT). This leads to unnecessary long acquisition times for antennas mounted in offset positions with respect to the measurement sphere. In this communication, a general technique is proposed to reduce the sampling rate requirements of offset mounted antennas. The approach is based on centering the Spherical Wave Expansion(SWE) over the AUT by a shift in the coordinate system to compensate for the offset. The purpose of this shift is to reduce the number of significant spherical waves required to represent the AUT field. In the subsequent steps of the algorithm, the coordinate system is brought to its original location to preserve the efficiency and probe-correction capabilities of the traditional spherical near-field to far-field transformation technique. This is performed by a proper translation of the SWE coefficients between both coordinate systems. Higher order probe correction is also supported intrinsically. The proposed algorithm is numerically tested to assess its performance. Electromagnetic simulation and anechoic chamber tests are used to validate it, showing reductions in measurement times with low transformation errors.

This letter presents a dual-band Smart Electromagnetic Skin (SES) for millimeter-wave 5G applications, which provides simultaneous operation at the 28/39-GHz bands with independent reflection performance at each band (i.e., different incidence and reflection angles). The SES consists of a two-layer reflectarray panel of 202.5-mm side, which produces two independent reflected beams in dual-polarization when it is illuminated from two different feeders (one for each operating band). A prototype of the SES has been fabricated and tested, and the measured radiation patterns agree well with the simulations. The 2-dB gain bandwidth exceeds 14.3% at 28 GHz (26-30 GHz) and 5% at 39 GHz (38-40 GHz). The dual-band SES will contribute to enhance 5G communications by producing two independent reflected beams when it is illuminated from two faraway base stations, each operating at a different frequency.