Novel reflector antennas in millimeter wave sensing systems for on-the-move security imaging

Résumé

In response to the increasing threat of terrorism, personnel surveillance at security checkpoints, such as airports, train and bus stations, is becoming increasingly important. Millimeter-wave systems, usually defined within the 30–300 GHz frequency band are the most promising solutions because millimeter-waves are non-ionizing and, therefore, pose no known health hazard at moderate power levels. These systems can also have very high resolution due to their relatively short wavelength (1–10 mm), and they allow through-clothing imaging. The most common millimeter wave systems are close range ones, although today, long distance detection systems are becoming increasingly relevant, since they allow the identification of potential threats between crowds (for example, queues at an airport, protest demonstrations, sports stadiums, etc.). Achieving long distance detection is a difficult challenge, although it has been shown to be a promising technique for the detection of under clothing hidden objects. The architecture of the antenna systems used for long distance detection are based on configurations of multiple reflecting surfaces that explore a certain region of the space. The system that will be used as a starting point in this Thesis is an Bifocal Elliptical Gregorian Reflector System (BEGRS), capable of generating images in real time, at a distance of 8 m and with a field of view of 50 cm $\times$ 90 cm focusing a 300 GHz beam. A bandwidth of 27 GHz will be used to ensure good range resolution. The baseline of this design consists of two faced reflecting surfaces with a common focus. The system operation responds to a monostatic configuration, so the recovered signal will mainly correspond to the signal received due to the specular reflection from the target. This is one of the biggest limitations of this type of systems. When the measurement system works with complex geometry targets (curved planes, etc.), it is very likely that the direction of the specular reflection deviates sufficiently so that the system is unable to recover the response of the target in a particular area. This causes the received signal level to be well below the noise floor and because of this, certain areas of the target will be invisible to this system configuration. In order to recover as much target information as possible, new configurations, that using the existing system allow us to increase the amount of retrieved information, are sought. The development of this type of alternative architectures will be the greatest contribution of this Thesis. The first proposed solution is capable of recovering invisible target areas for the monostatic configuration of the BEGRS. A multi-bistatic architecture formed by a combination of the initial reflector antenna system with multiple secondary transmitters which illuminate the target from different angles, thus being able to recover the target areas that the initial monostatic configuration is unable to recover, is proposed. The recovered image is generated by a combination of the monostatic and the multiple bistatic recovered images. In this way, the amount of retrieved information is noticeably higher. The multi-bistatic configuration allows more accurate reconstructions, but still the monostatic system presents an important limitation. Due to its geometry, system exploration at a fixed distance (in this case 8 m) is done, which is an inconvenience due to the nature of its application scenarios. These types of systems are intended for its use among crowds, so it is unlikely that a human target will be static. The ability to explore at different distances is really interesting. This is the second contribution of this work, the introduction of a small system architecture modification that allows multiple distance exploration, thus allowing moving objects reconstruction. At this point, an existing and operational THz imaging system performance has been improved. On the one hand it has been possible to increase the amount of system collected information and on the other, the range detection has been extended. Using the target observation from different illumination points philosophy to maximize the amount of recovered information, a new low-cost multistatic near field architecture has been developed. The main idea is the use of multiple transmitters and receivers that explore the target from different angles, thus maximizing the amount of received information. This is the third contribution of this work, the design and construction of a multistatic near-field imaging system that, although it does not use the conformation of reflective antennas used so far, it draws on everything learned from it, both from the monostatic configuration and from the multi-bistatic one. This new configuration allows to obtain on-the-move images and also has a much higher reconfigurability capacity than previous configurations. Each of the previous configurations has been experimentally validated. In a previous stet, a physical optics simulator (PO), capable of predicting the behavior of each of the elements of the antenna system, has been used. The ad-hoc simulator predicts the electromagnetic interaction between all system surfaces. In addition, it calculates the electromagnetic response of the target that is received back in the system. The simulator is also able to calculate the response of different nature targets (metallic, dielectric materials, etc.), something that today arouses great interest. A large amount of information about its composition, its properties and even its size can be extracted from the response of the dielectric materials. As the last contribution of this Thesis, an algorithm capable of characterizing dielectric materials has been developed.