Resonance-Based microwave technique for body implant sensing

There is an increasing need for safe and simple techniques for sensing devices and prostheses implanted inside the human body. Microwave wireless inspection may be an appropriate technique for it. The implanted device may have specific characteristics that allow to distinguish it from its environment. A new sensing technique based on the principle of differential resonance is proposed and its basic parameters are discussed. This technique allows to use the implant as a signal scattering device and to detect changes produced in the implant based on the corresponding change in its scattering signature. The technique is first tested with a canonic human phantom and then applied to a real in vivo clinical experiment to detect coronary stents implanted in swine animalsPeer ReviewedPostprint (published version

Terahertz integrated antenna arrays for imaging applications

Terahertz is the portion of the spectrum that covers a frequency range between 300 GHz - 3 THz. This frequency band has proven its potential for imaging applications thanks to the good compromise between spatial resolution and penetration; however, this push towards high frequencies contains many technological difficulties in all the subsystems involved in the signal generation, transmission and detection. The power budget restrictions and high losses that sources and receivers currently suffer at these frequencies require systems with a high level of integration among all the devices and components of the systems and subsystems. Therefore, the antennas needed for these systems require to be integrated within the same fabrication processes and technologies as the sensing and power converting devices that are used at their terminals. This doctoral thesis has focused on the development of integrated antenna arrays at Terahertz frequencies for imaging applications, for both near-field and focal-plane geometries, with a special emphasis on the technologies and the fabrication capabilities that can be potentially used and are currently available. The current imaging systems require large arrays of antennas in order to achieve the high-speed image acquisition that is required in most THz applications. This fact increases considerably the difficulty and complexity to achieve highly integrated and efficient antennas. This thesis has characterized and analyzed these difficulties and provided solutions to the development of antenna arrays at millimeter and submillimeter wave frequencies. The first part of this thesis has focused on the study of a planar antenna array, called retina, for a specific near-field imaging system based on the Modulated Scatterer Technique (MST) at millimeter and submillimeter-wave frequencies. This system has been selected for its capabilities to perform high-speed imaging and because it does not require a high frequency distribution line network. However, it is hindered by many technological difficulties: the selection of an antenna geometry that achieves high efficiency, the selection of the adequate active element and its integration with the antenna. In this thesis, these challenges have been addressed and studied in-depth, and a design methodology that integrates all the different aspects of the system has been developed. Because planar antennas at millimeter and submillimeter wave frequencies suffer from high losses due to the surface wave modes inside substrate, these losses have been analyzed and quantified for different antennas, and an antenna geometry that reduces significantly this problem has been developed. Different switching technologies currently or potentially available at these high frequencies have been considered in order to study and analyze their capabilities and their integration possibilities: PIN diodes, Schottky diodes and RF-MEMS. These technologies have been studied through the development of three retina prototypes, which have been fabricated using high precision fabrication processes such as laser micromachining and photolithographic. Different measurement set ups were fabricated and assembled to validate the different premises presented. The second part of the thesis is devoted to the study of integrated Focal Plane Arrays (FPA). The development of FPA at submillimeter wave frequencies is highly on demand due to the enormous progress in designing integrated heterodyne receivers. These receivers integrate arrays of submillimeter-wave diode-based mixers and multipliers with Monolithically Integrated Circuit (MIC) amplifiers on the same wafer stack. For this stackable multi-pixel terahertz camera technology to work, a leaky wave antenna with silicon micro-lenses has been developed, which allows wafer level integration compatible with silicon micro-fabrication techniques for bulk array manufacturing and has high directivity in order to illuminate a reflector efficiently. Detailed and thorough design guidelines for this antenna are presented. Two antenna prototypes were built in order to evaluate the two fabrication possibilities: advanced laser micro-fabrication and photolithographic fabrication. A study of the aberrations of the lens has been developed in order to evaluate the performance of the lens profile fabricated. Moreover, a set of radiation pattern measurements of the fabricated prototypes was performed in order to evaluate the performance of the antenna and its possibilities to be used as a FPA

RFID multiantenna systems for wireless communications and sensing

Many scientific, industrial and medical applications require the measurement of different physical parameters in order to collect information about the spatially distributed status of some process. Very often this information needs to be collected remotely, either due to the spatial dispersion of the measurement points or due to their inaccessibility. A wireless embedded self-powered sensor may be a convenient solution to be placed at these inaccessible locations. This thesis is devoted to study the analytical relation governing the electromagnetic coupling between a reader and a embeddable self-powered sensor, based on radio frequency identification (RFID) technology, which is capable of wirelessly retrieving the status of physical parameters at a remote and inaccessible location. The physical parameter to be sensed may be the electromagnetic (EM) field existing at that location (primary measurement) or the indirect measurement of other parameters such as the temperature, humidity, etc. (secondary measurement). Given the simplicity of the RFID solution (highly embeddable properties, scavenging capabilities, penetration and radio coverage characteristics, etc.) the measurement can be done at a single location, or it can be extended to a set of measuring locations (an array or grid of sensors). The analytical relation is based on a reciprocity formulation studying the modulation of the scattered field by the embedded sensor in relation with the incident field, and allows to define a set of quality parameters of interest for the optimum design of the sensors. Particular attention is given to the scavenging circuitry as well as to the antenna design relevant to the sensing objective. In RFID tags, the existence of an RF harvesting section is an improvement with respect to conventional scattering field probes since it removes the need of DC biasing lines or optical fibers to modulate the sensor. However, this harvesting section introduces non-linearities in the response of the sensor, which requires a proper correction to use them as EM-field probes, although the characterization of the non-linearities of the RFID tag cannot be directly done using a conventional vector network analyzer (VNA), due to the requirements of an RFID protocol excitation. Due to this, this thesis proposes an alternative measurement approach that allows to characterize the different scattering states used for the modulation, in particular its non-linear behavior. In addittion, and taking this characterization as the starting point, this thesis proposes a new measurement setup for EM-field measurements based on the use of multiple tones to enlarge the available dynamic range, which is experimentally demonstrated in the measurement of a radiation pattern, as well as in imaging applications. The RFID-based sensor response is electromagnetically sensitive to the dielectric properties of its close environment. However, the governing formulation for the response of the probe mixes together a set of different contributions, the path-loss, the antenna impedance, the loads impedance, etc. As a consequence, it is not possible to isolate each contribution from the others using the information available with a conventional RFID sensor. This thesis mathematically proposes and experimentally develops a modification of the modulation scheme to introduce a new set of multi-load scattering states that increases the information available in the response and properly isolate each term. Moreover, this thesis goes a step forward and introduces a new scattering state of the probe sensitive to temperature variations that do not depend on the environment characteristics. This new configuration enables robust environmental sensing in addition to EM-field measurements, and sensing variations of the dielectric properties of the environment