Advances in periodic structures and manufacturing technologies for mm-wave antennas

Resumen

The arrival of 5G services demands the use of an increasing bandwidth, which leads to better use of the electromagnetic spectrum. 5G mass deployment requires solutions for working in upper frequency bands with low cost manufacturing. Printed substrate technologies, as stripline or microstrip, exhibit high transmission losses at high frequencies due to the presence of dielectric. At high frequencies, printed technologies have been replaced by waveguide. This technology is based on the propagation of electromagnetic waves in a hollow metallic cavity, which supposes very low transmission losses. However, materials and manufacturing mechanisms are expensive and the resulting structures are heavy and bulky. This makes waveguides incompatible with the development and mass production of small and low-cost technology. In recent years, various technologies have been emerged, such as gap waveguide (GW), higher symmetries (HS) or additive manufacturing (AM), that present a solution for new millimetre-wave devices. These new technologies are becoming globally important because they offer good performance in terms of low weight, low profile and low losses, and a low-cost fabrication on a massive scale compared to other PCB technologies or classical waveguides. Gap waveguide technology is based on fully metal pieces with several rows of periodic pins that prevent leakage in a certain frequency band when a metal plate is placed above them a distance below a quarter wavelength. A groove or a ridge can be inserted between those pins to propagate fields in the same way that classic waveguides, with the advantage that GW do not need good electrical contact between plates. Higher symmetries consist of periodical structures with symmetry operations such as screw symmetry (a periodical rotation), or glide symmetry (a mirroring and displacement). Especially this last type of symmetry is of great interest because it can provide EGB (Electromagnetic Bandgap) properties or increase the refractive index in a multitude of different transmission lines. The fabrication of the resulting structures in a complex design based on GW or HS can be costly using traditional CNC machining methods. In millimetre-wave bands, GW pins tend to be small and with a reduced separation between them, which increases the price. For that reason, it is interesting to consider additive manufacturing or 3D-printing for this type of structures, given that the time and cost of manufacturing do not depend on the complexity of the part, but on its size. The increasing introduction of new materials with anisotropic properties that can be modified by applying a voltage has also been documented. Among these materials, liquid crystals (LC) stand out, which were studied in optical applications. LC can have a great importance in the creation of new electrically reconfigurable antennas or RF devices. The objective of this thesis is to study in depth these novel technologies and materials and apply them in real prototypes of antennas and RF devices in millimetre-wave bands. Specifically, in chapter I a detailed introduction of each of the technologies used during the development of the thesis is carried out. In Chapter II, a comparison between available technologies such as rectangular waveguide, substrate integrated waveguide, microstrip and gap waveguide technologies is carried out in terms of transmission losses between 10 GHz to 100 GHz. Chapter III shows a study of the properties of 3D printing and the effect of metallization of the resulting plastic parts at Ka-band. Different prototypes implemented in GW were designed, manufactured and experimentally validated. Chapter IV focuses on a more complex GW design for a Radial Line Slot Array (RLSA) antenna fed with a 3D-printed Butler matrix for a monopulse radar at 94 GHz. 3D printing technology, together with a liquid crystal mixture developed specifically for microwave frequencies, were used for the design and manufacturing of an electrically tunable phase shifter in chapter V. Chapter VI contains the validation of several prototypes based on higher symmetries that seek to enhance different aspects: stop-band and refractive index modifications and attenuation and phase shift mechanical reconfigurability. These higher symmetries are also applied in Chapter VII for enhanced gap waveguide EGB or increase of the beam steering variation with frequency in leaky-wave antennas for automotive applications. Finally, conclusion and future work are drawn in Chapter VIII.