A novel concept to mitigate the unintended transmission and reception of signals at the third harmonic of dielectric rod antennas is proposed. To suppress the third-harmonic radiation, an additive-manufactured photonic bandgap material is used in the dielectric rod antenna. The antenna has been designed to operate at the frequency of 5 GHz and exhibit a bandgap at the third-harmonic frequency of 15 GHz. The design of the material is explained via the band diagram of the bandgap material and imperfections introduced due to the additive manufacturing (AM) process considered. Numerical simulations of dielectric rod antenna prototypes fed via a rectangular waveguide (RWG) with and without harmonic suppression are carried out to confirm the operational principle. The design proposed is verified experimentally. The manufactured antenna is characterized in terms of its input reflection coefficient and far-field radiation properties. The obtained experimental results agree well with predictions obtained through simulations and confirm the third-harmonic suppression capability of the antenna.

The feasibility of using hybrid additive manufacturing (AM) to produce low-cost, phased array antennas for SatCom applications is investigated in this work. A 4x2 planar array comprised of dielectric resonator antennas (DRAs) operating in the fundamental mode within the K-band SatCom downlink band (17.7-21.2 GHz) has been designed, fabricated and measured. The impact of print settings on the material properties is assessed and incorporated into the antenna design process. Manufactured prototypes of the single element and the planar array geometry are experimentally verified in terms of their scattering parame-ters and far-field radiation patterns. The potential of combining hybrid AM and DRA technologies for future mmWave phased array developments is highlighted by the presented results.

Today, state-of-the-art device manufacturing for high-frequency (HF) electromagnetic (EM) applications is primarily dominated by subtractive techniques, including milling, laser cutting, electric discharge machining, and chemical etching, among others. While these methods are well established and enable remarkable accuracy, they are fundamentally constrained in terms of the geometric complexity they can achieve. The growing demand for more compact, efficient, and high-bandwidth EM devices challenges these limitations. Consequently, there has been a surge in alternative manufacturing techniques that leverage three-dimensional space to unlock additional design freedoms. The present work explores the unique capabilities of extrusion-based additive manufacturing (AM) to realize complex periodic dielectric structures for modern high-frequency EM applications. Specifically, this thesis investigates the homogenization, characterization, and practical implementation of periodic dielectric structures—also referred to as dielectric crystals—for advanced EM applications at microwave and millimeter-wave frequencies.
The thesis opens with a comprehensive review of AM applications in HF electromagnetics, highlighting significant potential while also identifying critical gaps in the current state of the art. These gaps guide the original contributions of the presented research, summarized below.

First, a comprehensive overview and comparison of homogenization and experimental characterization techniques for evaluating dielectric material properties is provided. While literature offers treatments on numerous analytical, numerical, and experimental methods, there is a lack of a systematic comparison tailored specifically to AM periodic dielectric structures. This thesis bridges that gap by rigorously comparing selected homogenization methods against experimental characterization results. It provides an analysis of the advantages and limitations of each approach and identifies their suitability for analysing AM periodic dielectrics.

Second, the impact of additive fabrication on the anisotropy of dielectric material properties is studied. The phenomenon of mechanical anisotropy resulting from the layer-by-layer additive fabrication approach is well-documented in literature, while its implications for dielectric properties remain unexplored. The presented research addresses this critical gap by thoroughly investigating the effective permittivity tensor. For this purpose, an analytical model is presented and its predictions validated through numerical simulations and experimental characterization using various materials and printing parameters. The results confirm significant anisotropy with negative birefringence, aligning well with theoretical predictions.

Third, although previous studies have utilized volumetric infill variations to influence the effective dielectric properties of periodic structures, the fundamental connection between crystallographic theory and dielectric behavior has remained largely unexplored. This thesis systematically investigates how lattice symmetries affect the effective permittivity tensor. A detailed parametric study of selected Bravais lattices and crystal centerings is conducted, demonstrating precise control over dielectric anisotropy. These findings are confirmed experimentally through careful characterization of several lattice structures exhibiting distinct uniaxial and biaxial anisotropic properties.

Fourth, spatially modulated dielectric structures have mostly been employed in graded-index (GRIN) lenses for long-wavelength applications. This work expands significantly beyond this scope, exploring new applications in both the long- and short-wavelength operating regimes. For long-wavelength scenarios, cylindrical heterogeneous dielectric resonator antennas (DRAs) with radially and vertically varying permittivity profiles are investigated. Experimental results demonstrate that spatial modulation enhances both the impedance and axial ratio bandwidth compared to homogeneous DRAs. Furthermore, heterogeneous dielectric slabs within rectangular waveguides are explored, demonstrating practical control over transmission and reflection characteristics, exemplified through the realization of a third-order Chebyshev bandpass filter.
Regarding short-wavelength applications, this research explores 3D periodic dielectric bandgap materials. A thorough numerical and experimental verification of the bandgap properties of woodpile structures is conducted, culminating in a novel dielectric rod antenna that significantly suppresses third harmonic radiation through combined long- and short-wavelength concepts. Collectively, these examples demonstrate the vast potential for innovative device performance enabled by spatially modulated periodic dielectric structures.

Fifth, integrating conductive and dielectric materials in a single AM process is highly advantageous yet underutilized, primarily due to the prohibitive cost of specialized equipment. Addressing this limitation, the thesis introduces an accessible, open-source inspired approach to hybrid AM. A custom-built AM system incorporating a micro-dispensing head for conductive ink deposition is successfully demonstrated through the fabrication of a fully additive-manufactured dielectric resonator phased array antenna operating at $20$\si{\giga\hertz}. This practical demonstration confirms the viability and effectiveness of hybrid AM technology for advanced antenna applications.

In summary, this research underscores the transformative potential and enables new opportunities of additive manufacturing in EM device engineering. While recognizing the ongoing challenges related to material and equipment limitations, the thesis illustrates that these can be effectively managed through careful selection of homogenization, characterization, and design methodologies. Promising future directions identified include further exploration of hybrid AM for high-frequency EM applications, innovations in communications, radar, and sensing technologies, and continued advancements in integrated material processes. Ultimately, this work significantly advances additive manufacturing for electromagnetic applications, laying a strong foundation for future innovations in communication, radar, and sensing technologies.

The design, fabrication, and experimental validation of a gradient-index (GRIN) lens operating at 20 GHz is presented. Different realizations with and without matching layers at the input and output interfaces are compared. The lens design employs a semi-analytical approach to compute the desired permittivity distribution, which is realized using a periodic dielectric structure with a spatially modulated volumetric infill. The lens is manufactured via a multi-tool 3D printer utilizing two different dielectric materials for the core and matching layers, respectively. The lens design with and without the matching layer is experimentally verified. The comparison highlights the critical role of impedance matching at the interfaces, with the lens exhibiting superior performance when matching layers are incorporated. This work demonstrates the potential of multi-dielectric 3D printing for producing mmWave components, suggesting its applicability in future high-performance antenna systems.

This study explores the feasibility of using a hybrid additive manufacturing (AM) to design and produce low-cost, wideband phased array antennas for SatCom applications. We demonstrate the design, fabrication and experimental verificatino of an eight-element linear array comprised of multi-mode dielectric resonator antennas (DRAs) providing full coverage of the K-band SatCom downlink bandwidth (17.7-21.2 GHz). The impact of print settings on material properties is assessed and incorporated into the antenna design process. The manufactured prototype is experimentally verified via impedance and far-field measurements. Furthermore, beam steering capabilities are demonstrated using a commercially available integrated circuit and a simple calibration procedure. The phased array antenna achieves full coverage of the intended band with an input reflectino coefficient below -10 dB. The averaged embedded element pattern demonstrates a realized gain of approximately 3 dB and a half-power beamwidth of 81 degrees. These results highlight the potential of combining hybrid AM and DRA technologies for future mmWave phased array development.

Additive manufacturing (AM) is increasingly recognized as an enabling technology for novel devices with increased functionality and bandwidth for microwave applications. However, due to the layer-by-layer build approach employed by most AM techniques, internal patterns are introduced to printed materials, which cause an effective anisotropy in their material parameters. An electrostatic model inspired by parallel plate capacitors, describing the dielectric anisotropy of materials manufactured with fused filament fabrication (FFF) AM techniques is proposed in this work. The accuracy of the model's predictions about the permittivity tensor of printed materials is investigated via numerical simulations. Furthermore, permittivity tensor measurements are carried out for samples printed with various materials and print settings. Measurement data is fitted to the model via a least squares approach, and excellent agreement is observed. Novel conclusions about the impact of material and print parameters on the effective permittivity of additively manufactured materials, improving the accuracy in modeling and designing additively manufactured microwave devices are drawn.

The utilization of low-cost hybrid additive manufacturing equipment for creating a fully integrated patch antenna system is demonstrated. A miniaturized stacked dual-band patch antenna for GNSS applications with integrated feed, amplifier, matching, and bias network is considered. Design and manufacturing challenges due to the peculiarities of hybrid additive manufacturing, like anisotropic substrates, thermal curing steps, and poor layer adhesion between different materials, are discussed, and solutions are proposed. In this work, the passive part of the antenna is manufactured, and its measured performance is compared to results obtained via numerical simulations. An excellent agreement between simulation and measurement of the passive antenna is observed. The results of the fully integrated and active antenna will be demonstrated at the conference.

Additive manufactured structured dielectrics with engineered permittivity tensors are promising tools for novel microwave components and are drawing increasing attention from researchers. However, design modeling and experimental verification of anisotropic materials are challenging and have not yet been thoroughly explored in the literature. In this work, a design approach based on superimposed spatial harmonics for the design of anisotropic lattices called dielectric crystals is used. Furthermore, the plane wave expansion method (PWEM) is identified as a powerful tool for modeling the effective permittivity tensor. A wideband material characterization measurement setup based on rectangular waveguides is utilized for experimental verification. Experiments with uniaxial anisotropic dielectric crystals are carried out and are shown to be in satisfying agreement with our theoretical modeling.

The utilization of additive manufacturing (AM) to engineer the permittivity profile of dielectric resonator antennas (DRAs) is considered. For the first time, the capabilities of AM are exploited to create continuously swept permittivity profiles and applied to cylindrical DRAs. The spatial variant lattices (SVL) synthesis algorithm is implemented to create the desired permittivity profiles from a single material, and resulting geometries are manufactured using a high-permittivity material in a fused deposition modeling AM process. Three individual antennas for global navigation satellite system bands are designed and manufactured, two inhomogeneous DRAs with continuous permittivity profiles along the radial and vertical axis, and one homogeneous DRA for comparison. The manufactured antennas are characterized by impedance, realized gain, and axial ratio. Experimental results agree well with simulations and show increased impedance-, gain-, and axial-ratio bandwidths for both inhomogeneous antennas compared to the homogeneous one.