Adaptive Millimeter-Wave and THz Antenna Devices Based on Dielectric Elastomer Actuators

Dynamic reconfiguration of antenna devices is becoming a prime need in emerging telecommunication and remote sensing systems operating in the portion of the electromagnetic (EM) spectrum spanning from millimeter-wave (MMW) to terahertz. Different techniques and materials are currently available for the implementation of a given EM reconfiguration in antenna systems at microwaves andMMW, such as semiconductors, RF-MEMS, Liquid Crystal, and ferroelectrics. However, a common feature to all these technologies is the significant increased loss and complexity with regard to the devices fixed counterpart. Both loss and complexity further increase when high-performance reconfiguration capabilities are addressed atMMWand above, constituting a major limitation to the future deployment of dynamically controllable systems. Advanced performance, low-complexity and low-cost are, therefore, the cornerstones in the development of new technologies for antenna reconfiguration. In this framework, the main objective of this thesis is to contribute to the advancement of low-cost and efficient technologies enabling antenna reconfiguration capabilities fromMMWto THz frequencies. Within this scope, it is proposed the analysis, design and implementation of mechanically reconfigurable devices using dielectric elastomer actuators (DEAs). DEAs are an emerging technology that possesses unique properties, and represents a potential alternative to conventionalmechanical reconfiguration schemes. DEAs are especially attractive for their large strain combined with low-costmaterials and fabrication, analog control and near-zero DC power consumption. These characteristics make them particularly suited to the realization of low-cost and low-loss reconfigurable antennas. The high potential of DEAs for the realization of adaptive devices is experimentally validated by the development of different prototypes operating atMMWand THz frequencies: i) a very low-loss true-time-delay phase shifter operating at Ka-band; ii) a Ka-band reconfigurable reflectarray with 1-D beam-scanning capability; iii) a stretchable and beam-scanning THz reflectarray exhibiting the uncommon potential for the implementation of conformal or reconfigurable devices based on mechanical stretching. The designs and concepts demonstrated in this thesis pave the way for the evolution of new DEA-based reconfigurable devices resulting in low-cost, low loss and compact structures.

Miniaturized Microwave Cavities for Industrial Applications

Maddalena Violetti

Microwave cavities, or microwave resonators, are used in a variety of applications, including filters, oscillators, frequency meters, and tuned amplifiers. The ever-increasing demand for compact and/or portable telecommunication systems drives the need for further miniaturization of such structures for improved integration. In this thesis, new miniature microwave cavity solutions are proposed, which are useful in at least two scopes of application. In the first scope, the new devices are used as microwave front-ends of a short-range radar system for on-line blade tip monitoring in gas turbines (i.e. land-based large-frame turbines, aero-derivatives and aero-engines). In general, microwave sensors are advantageous for blade tip sensing: contrary to other existing techniques, they can survive to high temperatures for extended period of operation, unaffected by contaminants, like gases and dirt. Nonetheless, the microwave front-end of such systems is of difficult implementation, due to the strict thermal-mechanical constraints of the specific field of application (i.e. a large temperature change, the presence of vibrations, dirt and combustion by-products) and the tight requirements on the size of candidate probe designs. Antenna-based solutions are currently used as tip monitoring probes, with the disadvantage of being complex structures with radiation characteristics that are not entirely well suited for short range sensing. As a valid alternative, we propose two cavity-based probe solutions, namely a miniature waveguide resonator probe and a coaxial resonator probe, which represent an elegant compromise between robustness, miniaturization and simplicity of operation. Tests in laboratory and on real turbine engines demonstrated the suitability of such devices for the aimed application and several improved characteristics with respect to other comparable techniques, like antenna-based and eddy-current sensors. In the second scope, the new proposed solutions are used as microwave resonators in miniature rubidium atomic clocks. Attempts to miniaturize such kind of systems are usually limited by the dimensional constraints of standard microwave cavity resonators (MWR) , which account for a large part of the volume of the physics package. We propose a new miniature MWR based on a loop-gap resonator structure, also referred to as the μ-LGR. This cavity design meets the desired field configuration for the atomic clock operation and achieves an interesting size-reduction. A laboratory experiment of an atomic clock integrating the μ-LGR achieved unmatched results of stability compared to other microcell clocks. Finally, we propose a miniature planar MWR for the integration of 2D microfabricated cells.

EPFL2012

Antennas for low-cost Ka-band ground terminal devices

Joana Rita Alves dos Santos Silva

The ever-growing demand for high-speed data links together with the increasing congestion in the traditional microwave spectrum are pushing for the exploitation of higher microwave and millimeter-wave (mm-wave) frequencies, including the so-called Ka band. Major investments came along, especially in the satellite communications industry, raising the technological con-straints imposed on electronic equipment, mainly concerning performance, compactness and both reduced cost and weight. The great antenna size reduction achieved at Ka band has boosted the research towards satel-lite-on-the-move applications (SOTM), aiming the development of ground terminal antennas for both commercial and personal use. Such antennas must work simultaneously at downlink and uplink Ka bands with circular polarization and also have beam steering capabilities to keep a steady connection with the satellite. In addition, the target is also achieving a cost-effective low-profile antenna. The present thesis is focused on the design of feed antennas to be integrated in mechanical beam steering systems for the aforementioned application. Three different designs are here proposed and discussed. The first one, a wideband ridged horn antenna, was tested standalone and with a dielectric lens antenna. Once good results were achieved in both sce-narios, reducing its height would be the next goal. Thus, the second design is a cavity backed patch antenna. A comparison between both feed antennas is performed, highlighting the pros and cons of both solutions, either standalone or with the same dielectric lens antenna. Here, the first studies with a planar lens antenna are shown. Finally, the third device consists of a ridged cavity antenna with a cross-slot on its top aperture. This time, the feed was successful-ly tested with a transmitarray which allows achieving a more compact antenna system than the first one here presented. This thesis also analyzes two different manufacturing techniques, traditional milling technique and an innovative additive manufacturing (AM) technique based on metallized polymers called stereolithography (SLA). The present AM-SLA prototypes clearly illustrate the strong potential of this technology and pushes for its further assessment.

EPFL2017

Miniaturized Microwave Cavities for Industrial Applications

Maddalena Violetti

EPFL2012

Antennas for low-cost Ka-band ground terminal devices

Joana Rita Alves dos Santos Silva

EPFL2017