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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Wideband wide-scanning antenna arrays have been gaining popularity in the past decade due to their applicability in multiple fields and applications. Wideband arrays are desired due to their ability to combine multiple functions or services in a single aperture. The need for ultra-wideband capability is often paired to the ability to scan over a large conical region. Wide-scanning arrays enable tracking of multiple targets simultaneously, which can be important for radar systems or for communication systems with multiple users. Especially in satellite communication (Satcom) on-the-move applications, wideband wide-scanning array have a key advantage due to their ability to cover multiple bands in a single package while maintaining agile connections to multiple satellites. This type of array has been demonstrated using various technologies, but most of these use a costly and complex assembly process or have considerable cross-polarization (X-pol). One of the proposed concepts is the connected slot array with artificial dielectric layers (ADLs), which offers the required bandwidths and scanning ranges in a low-volume planar structure. Entire connected arrays can be fabricated on a single board using traditional printed circuit board manufacturing techniques, making them relatively low cost and low complexity…@en

Flat lens antennas are convenient solutions to realize highly directive antennas for millimeter wave and terahertz frequencies. Unlike the traditional three-dimensional bulky lens antennas, flat lenses are compact, low profile, and planar structures that can be manufactured with standard multi-layer technology, e.g. printed circuit board (PCB) or low temperature cofired ceramic (LTCC).

A common tradeoff in the design of flat lenses is between bandwidth and thickness. Electrically thin lenses are characterized by narrow frequency bandwidth, resulting from the phase wrapping adopted in the design. On the other hand, a wide bandwidth can be achieved by avoiding phase wrapping and using true-time-delay phase delay, but this is achieved at the cost of increased electrical thickness.

In this thesis, artificial dielectric layers (ADLs), consisting of periodic metal patches within a dielectric substrate, are proposed to realize flat lenses with large effective refractive index, which is a key property for reducing the thickness of wideband true-time-delay flat lenses. As such, ADLs are promising solution to achieve a good compromise between bandwidth and thickness.

Different aspects of ADL flat lenses are investigated in this thesis, going from the analysis to the design and experimental validation. For the analysis, a general procedure to find the permittivity profile of a gradient index (GRIN) lens is introduced. The method allows to design GRIN lenses that manipulate the phase front in different ways, by using a Geometrical Optics (GO) approach. Different cases are studied, including collimating lenses with on-axis and off-axis feeds; lenses that transform spherical wavefronts across different media; lenses changing the focal number of a quasi-optical system and Fresnel zone lenses. The design equations are validated by ray-tracing simulations in non-homogeneous media, implemented by numerical solution of the Eikonal equation.

Once the permittivity profile is defined, the continuous variation of refractive index is discretized into unit cells. Each unit cell is then implemented as an ADL stack, using ADL synthesis models developed earlier in the THz sensing group.

To validate the design procedure, an ADL flat lens with an operation band from 30 to 60 GHz is designed and fabricated using an eight-layer print circuit board (PCB) stackup. The measurement results reach good agreements with the simulation results and validate the design. The achieved performance demonstrate wideband operation, with a high taper efficiency (> 90%) and a maximum directivity of 25.5 dB. The lens is thinner than 1 wavelength within the band of operation.

The lens performance is demonstrated with a simple open-end waveguide as feed, which has high spillover losses. Diverse feeding antennas that can achieve higher aperture efficiency are also analysed by means of simulations.

Additionally, other types of GRIN lens, that manipulates the wavefront in distinct ways for different applications are investigate, to highlight the flexibility of the concept.

Future communication scenarios will require massive Multiple-input multiple-output (MIMO) by the use of multi-beam antenna systems. Maximizing the number of beams for a given antenna size is paramount given that the space allocated to the antenna is often limited. This work aims at evaluating the maximum number of beams by analyzing the SIR in different communication scenarios for planar antenna structures. The concept of ‘observable field’ is used to quantify the power received from the desired signal as well as the power associated with the interference. Due to the planarity of the considered antenna structures, the radiating domains introduce scan loss, an effect not previously modelled when considering earlier investigations based on spherical domains. Furthermore, methods of reducing interference in order to improve the SIR were investigated, i.e., the use of a tapered current distributions on the radiating apertures and null placement techniques.

Planar lenses based on metasurfaces or resonant elements are typically narrowband due to phase wrapping, which is strongly frequency-dependent. On the contrary, true-time-delay (TTD) planar lenses, which do not resort to phase wrapping, can achieve large bandwidths. One convenient way to design wideband TTD lenses is by means of artificial dielectric layers (ADLs), which are stacks of sub-wavelength-period patch arrays embedded in a host medium to increase its effective permittivity to values higher than commercially available dielectrics. The procedure to retrieve the effective refractive index for a given ADL stratification is reported, as well as the synthesis of the multi-section transformers in ADL technology. Trade-offs including bandwidth, focal ratio, lens diameter, and thickness, are discussed and related to the manufacturing constraints of artificial dielectrics, such as the number of metal layers, maximum achievable effective permittivity, and smallest features realizable in printed circuit board technology. An example of design is also presented, operating from 70 to 140 GHz with a lens diameter of 11.5 wavelengths at the highest frequency. A modeling method for the analysis of a flat lens based on ADLs is presented. This consists of a combined geometrical optics (GO) / physical optics (PO) approach, where each GO ray is studied as a plane wave problem to evaluate the reflection/transmission through each unit cell of the lens. This analysis is extended to consider the bending of the rays through the lens and propagation through multiple unit cells.

Phased arrays have emerged as a key solution for 5G base stations, to provide higher capacity by means of directive and electronically steerable beams. However, the implementation of workable low-cost antenna arrays for base stations is very challenging, because of the requirements on the total frequency and angular coverage of 5G systems. When many frequency bands are used for different services, having a single narrowband antenna for each sub-band becomes unfeasible for cost and space occupation. For this reason wideband arrays that can cover simultaneously multiple bands are gaining interest. Moreover, a large field of view is required, i.e. scanning at least from -60° to +60°, so that only a few array panels can cover the entire azimuthal angle of 360°. This thesis work has been performed in the framework of a collaboration between Terahertz Sensing Group and HUAWEI, aiming at the development of a wideband phased array for base station. Two array designs are targeted, one covering 6-8 GHz and another for 2-8 GHz. Both designs are required to achieve wide scanning capability up to 60° in all azimuth planes. This thesis focuses on the design of the feeding structure of the array unit cell and the corporate feeding networks. The unit cell feeding structure is based on integrated coaxial lines connected to microstrips or striplines. The performance is analysed first for the stand-alone transition and then for the same feed together with the connected slot element. The entire unit cell including the feed has comparable matching performance with the ideal one without feeding structure. An alternative feed design is also presented, where the input is realized with a coaxial SMP connector. The design of a corporate feeding network for the array is introduced. This consists of two designs of 1-to-32 power dividers, one implemented with microstrip transmission lines and the other with striplines. Such dividers are meant to feed in phase an entire row or column of the array. The highlights of this design is wide bandwidth, which covers two octaves by means of wideband multi-section and tapered impedance transformers, and the compactness, since the entire divider has to fit in an area of 480 mm × 15 mm. Moreover a novel feeding strategy is proposed to simplify the complexity and the costs of the unit cell. The new approach is based on replacing the coaxial lines with integrated parallel plate waveguides (PPW). Two examples of unit cell design are presented: one consists in a single-polarized array of connected slot, covering the band from 70 GHz to 140 GHz, for automotive radars; another example is referring to a dual-polarized connected slot array covering the same 2-8 GHz range, for wideband base stations. The design procedure of a PPW and cavity is easier than that of the integrated coaxial, as the feed and the radiating slot design are better decoupled. The number of layers of the ADL is also reduced with respect to the integrate coaxial, because part of the impedance transformation is implemented in the PPW.

The interest for phased array antennas has increased over the past years along with the need for wide bandwidth arrays. Connected arrays are becoming more prevalent as a well suited solution to achieve the desired bandwidth. In order to increase the bandwidth and the scan angle of this type of array, horizontal artificial dielectric supersaturates have been used with good results. However, those superstrates come with an increase in cross polarization. The cross polarization of connected arrays with isotropic dielectric substrates and superstrates is thoroughly investigated in this thesis, along with an analysis on the cross polarization of the connected array with artificial dielectrics. In order to reduce the cross polarization as a result of that superstrate, a novel artificial dielectric is introduced which implements vertical vias to form a wire medium. For this medium a method of moments analysis is performed and a closed form expression is found. Additionally, the wire medium is implemented in the horizontal artificial dielectric to form the top-hat loaded wire medium as a solution to the restricted height between layers of the artificial dielectric slab. The wire medium increases only the z-component, since the wire is oriented vertically, of the effective permittivity without impacting the other effective constitutive parameters. The increase of the refraction index for transverse magnetic wave reduces the cross polarization level and aids in the matching for E-plane scanning, while maintaining a decay over the scan angle. However, a trade off between the reduction in cross polarization and the E-plane scanning is observed. This trade off is a result of the large z-component of the effective permittivity which promotes the propagation of the TM0 surface wave mode. The effectiveness of this novel structure is shown by considered a satellite communication application for simultaneous operation in the Ku- and Ka-transmit bands with a scan angle up to 60 degrees . A design for this application is performed with good matching, -10dB for broadside and -6dB for scanning in both E- and H-plane, and with a low cross polarization level of < −10dB. An evolved design with the manufacturing necessities, such as bonding layers and support layers, is obtained which exhibits a cross polarization level < −15dB with the use of the introduced wire medium, however, the layer height restriction set by the manufacturer and the feeding structure still provide a challenge at the frequency bands which are considered.

In the context of the Bachelor Graduation Project at the Delft University of Technology Department of Electrical Engineering, we have been tasked with a project to design prototype of a bi-static radar. This thesis describes the waveform design of a bi-static radar system and its implementation with Software Design Radio, SDR for short. The radar system can be mounted on delivery drones and used to detect other drones and obstacles to avoid collisions. The thesis focuses mainly on the radar waveform design. In addition, the thesis provides analysis of the radar system performance and system implementation with commercial off the shelf SDR. A proof of concept has been realised by means of simulations using the open source software GNU Radio Companion.

To solve extant complications with standard wafer-probing techniques, such as probe-to-probe coupling and probe-tip deterioration, a novel probe tip device has been designed and verified by means of 3D EM-simulation for the 220-325 GHz frequency band. The new probing technique uses an on-wafer cavity-backed slot to couple the signal to a tapered structure in an open-ended waveguide which is held above the cavity, and acts as a fully shielded transition from the transverse electromagnetic mode of the planar stripline to the fundamental transverse electric mode of the waveguide. The on-wafer structure used for the transition is limited to the back-end-of-line of the process, and is shielded from the ill-characterised substrate. The transition achieves an insertion loss of <1.8 dB across the entire 220-325 GHz band and does not require galvanic contact with the die. The layout for the on-wafer structure has been designed for a 0.25 µm SiGe process and a simplified version has been used for a simulated sample measurement using the thru-reflect-line calibration algorithm. Due to coupling to nearby structures, the novel probing technique show significant improvement over the standard coplanar probes only when a chip area greater than the cross-section of the open-ended waveguide is reserved. Simulated measurement of an independent passive structure show a reduction of the average worst case bound from 0.041 to 0.017 in the 220-325 GHz band.

Typical antenna arrays are designed such that the active element pattern is symmetric around the broadside direction. However, applications exist, for example in satellite communication, where a symmetric pattern is not needed or even unwanted. This angular selectivity can be achieved using asymmetric elements. However, it is known that for well sampled infinite arrays the asymmetry of the active element pattern disappears. Although designs of under-sampled antenna arrays achieving an asymmetric active element pattern have been presented in literature, the fundamental properties of this type of arrays in terms of radiation characteristics have not been investigated in detail. This thesis studies the asymmetry in the active element pattern of a finite linear array of asymmetric elements. To this end an in-house method of moments code is developed in Matlab to simulate tilted dipoles in free space and in the proximity of a ground plane. The dependency of the asymmetry of the active element pattern on the inter-element distance, the skew angle of the elements and the number of elements in the array is analyzed and design rules are derived. Using entire domain basis functions, closed form expressions for spectral integrals and the periodicity of the array the implemented code enables the simulation of large arrays in a much shorter time compared to commercially available software, such as CST.

Regarding the choice of antenna element, a dipole bent into a Z-shape is proposed as an alternative for a tilted dipole. This type of dipole can be defined to have an equivalent radiation pattern to that of a tilted dipole. This shape of dipole can be implemented using standard PCB technology using horizontal metal strips and vertical vias. The Z-shaped dipoles are analyzed using a method of moments code based on horizontal and vertical dipoles. The spectral Green's function of stratified media can be included in the spectral domain expressions to account for the presence of dielectric slabs in realistic designs.

In this work, the analytical models to describe artificial dielectric layers (ADLs), when a lateral shift between layers is present, is presented. The alternate lateral displacement between layers is an important parameter to engineer the desired effective electromagnetic properties of the equivalent homogeneous material realized with the ADLs. More specifically, the equivalent dielectric constants that can be realized by alternatively shifting the layers are higher compared to the aligned case. Closed-form expressions are derived for the equivalent layer reactance that includes the higher-order interaction between shifted layers. Furthermore, the effect of finite conductivity of the metal patches is studied and included to the extended versions of the formulas. The given analytical formulas can be used to derive an equivalent circuit model that describes the scattering parameters of a plane wave impinging on a slab composed by an arbitrary finite number of metal layers. To aid the design of artificial dielectric slabs, the effective permittivity and permeability tensors are also retrieved from the scattering parameters.