We present a dual-polarized connected array of slots with an artificial dielectric layer (ADL) radome for mobile communication applications operating in the sub-6 GHz and the upper 6 GHz bands of 5G. The radiating slots are combined with two interchangeable ADL radomes with different thicknesses, targeting the bands 6-8 and 2-8 GHz, respectively. This highlights the main property of the ADL radome, which realizes an impedance transformer whose bandwidth is proportional to the height of the structure. Moreover, the ADL anisotropy allows for wide scanning, up to 60° in the main planes for both radomes, without scan blindness. An $8\,\, \times \,\, 8$ prototype array has been manufactured and tested with the two ADL radomes. The measured results of the active voltage standing wave ratio (VSWR) and the radiation patterns are reported to validate the design.

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The expectation is that XG communications will have a disruptive impact in their applications in smart cities, industrial automation, agriculture, e-health, smart grids, domotics, and autonomous driving. However, such scenarios imply the availability of technology that cannot be met resorting to incremental changes in present techniques. Specifically, it is now apparent that the ultrawideband massive MIMO in both the microwave and sub-THz bands will be needed.

By exploiting larger bandwidths, unprecedented data rates will be achieved for MIMO applications in the microwave band. Before this work, wideband arrays have been considered unsuitable for MIMO applications due to the high levels of inter-elements mutual coupling. However, in this work it has been proven that if the entire array is coherently excited, multiple orthogonal beams can be generated, regardless of the inter-element mutual coupling. The orthogonality levels depend solely on the beam overlap and, therefore, on the beam width, the side lobes, and the position of the nulls.

Moreover, a wideband phased array has been designed for sub-8 GHz MIMO communications. The array is realized in the form of a dual-polarized connected slot array with interchangeable Artificial Dielectric Layers (ADLs) radome. This allows the array to scan up to 60° in every azimuthal cut while being matched between 6 and 8GHz with the first radome, and 2 and 8 GHz with the second one. Finally, an 8x8 prototype is manufactured and tested.

Due to the long wavelength at microwave bands, the antenna size is the largest constraint when it comes to MIMO applications, especially for wideband operations. To this aim, a new metric is developed to assess the signal and the interference of MIMO antennas constrained within a given volume. This allows us to compare the performance of an intended antenna design or even a realized prototype to the one of the maximum gain antenna located within the given volume. By means of these concepts, it is possible to link the MIMO performance with the antenna size to optimize the space.

By exploiting larger bandwidths, unprecedented data rates will be achieved for MIMO applications in the microwave band.

The second road to high data rates for MIMO application is the use of higher frequencies (sub-THz) communications. The main hindrance to integrated antennas for this regime is the technological challenge due to microfabrication and the integration with the electronics. This calls for accurate simulations that enable optimal designs.

For this purpose, an integral equation solver was developed to study dielectric lenses together with their feeds. At high frequency, the thickness of the metal plays an important role and cannot be neglected. Due to the different scales involved in the feed and in the lens, the required computational effort might be prohibitive. Therefore, a method has been devised to combine the numerical solution with analytical results to enable large-scale simulations. The impedance of an integrated antenna can be seen as composed of the reactance of the feed, the impedance of the feed radiating in a semi-infinite space without reflections, and an impedance associated with the reflections. While the former two can be evaluated analytically or with fast numerical simulations, the latter requires a time-intensive full-wave simulation of the entire problem. However, this can be simplified by synthesizing a much coarser feed, which radiates equivalently into the semi-infinite medium. Having this much coarser discretization it allows us to simplify the simulation of the entire problem and to isolate the reflections conveniently. Then, all the components can be combined together, and the input impedance can be estimated accurately.
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The calculation of the transmission line Green’s function (TL GF) of an infinite metal line of rectangular cross section embedded in a generalized stratification is presented. The proposed procedure is quasi-analytical and extends prior models to account effectively for the nonzero thickness of the conductors. From Green’s function of an infinite dipole, an equivalent network is derived to represent the current propagating along the dipole and the reactive loading due to the feeding gap dimensions. The method is validated with numerical simulations and available experimental results. Examples of dipoles in free space, microstrips, and leaky dipoles are shown.@en

A spectral domain analysis aimed at studying dipoles radiating in layered media is presented. The model allows for the modeling of the non-zero metal thickness and the reactance associated with a Δ-gap excitation. The solution can be efficiently calculated with a semi-analytical procedure, and it is linked to an equivalent circuit representation. The results show an accuracy up to par with commercial solvers regarding the input impedance and the radiated far field patterns.

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We present a study on beam coupling in radiating structures that support multiple simultaneous beams. The formation of multiple beams is relevant in modern wireless communication applications when diverse data streams can be sent from a single transmitter to users located in different directions. General expressions are provided that relate the coupling between radiated beams to the associated current distributions on the radiating aperture. When expanding the currents in terms of basis functions, the beam coupling can be also written in terms of coupling contributions due to each pair of basis functions. The analysis is applied to antenna arrays with different levels of mutual coupling between the array elements. More specifically, arrays of resonant elements, characterized by very low mutual coupling, are compared with arrays of connected elements, which are strongly coupled. We show that, even for a very high level of mutual coupling between individual array elements, it is possible to obtain orthogonal beams if the total current distributions associated with the beams are uncoupled.

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A Volumetric Method of Moments accelerated by means of iterative solvers combined with FFT matrix-vector products has been presented. The method allows analyzing small lens antennas with their integrated feeds efficiently since different structures and geometries can be studied with the same discretization, just by adding the material characteristics in post-processing.@en

The thermal emission from finite-size bodies is directly investigated without resorting to reciprocity. Specifically, an integral equation representing the electromagnetic field distribution on a conductive body under investigation is proposed. The source of the electromagnetic field is classic as it is an extension of Johnson noise sources to volumetric problems. The solution of the integral equation allows one to study the radiometric properties for geometries that are smaller than the investigated wavelengths, and in observation points, both in the near and in the far-field. The limits of validity of the formulation are clarified.@en

The number of independent links that can be hosted by an antenna platform for Line-of-Sight (LoS) communications is limited by its physical size and the interference between the beams associated with different users. For large-size platforms, the interference can be reduced by compromising the aperture efficiency, and this trade-off is the metric to quantify the effective use of the platform. This metric fails for antenna platforms that are not electrically large, for which the aperture efficiency is no longer a useful parameter. Here we resort to the concept of the Observable Field, related to the maximum theoretical directivity, to estimate the potential number of independent links supported by moderate-size platforms. This allows the introduction of coupling coefficients between the beams associated with the observable portion of the incident field and the beams associated with the receiving antennas. These coefficients are bounded to unity for any platform dimension, unlike the aperture efficiency, and they are maximized when the antenna pattern is equal to the pattern predicted by the Observable Field. Accordingly, selecting beams dictated by the Observable Field constitutes a benchmark for the effective use of the volume. Any antenna design can be compared to this benchmark to assess its merits.

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