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UWB radar is the most promising radar system for the future. In addition, by combining the UWB and array signal processing, one can obtain 3-D images of the objects for classification and identification, which is very useful in many applications. To achieve high-resolution real-time 3-D imaging radar, two essential items are missing in the current technology: dedicated antenna systems for sparse C-scan acquisition and fast high-quality imaging algorithms. In this thesis we have focused on the development of dedicated sparse antenna systems, including the sparse array topology development and antenna element development, while imaging algorithm is out of scope of this research. The main conclusions and their novelties are presented as follows: 1. Novel microwave ASP antenna: In this thesis a novel ASP antenna working from 10 GHz to 18 GHz has been developed. This antenna is aimed to be used for the time-domain UWB imaging array. The analysis demonstrated that the antenna not only has sufficient -10 dB impedance bandwidth from 9.95 GHz to 20 GHz, but also has good radiation characteristic within the impedance bandwidth. The antenna has gain of 5 to 10 dBi. The FBR is larger than 10 dB, and the -3 dB beamwidth is about ±30° is both E- and H-plane. The antenna has about 100 ps group delay and the impulse response has 1/e pulse width of about 200 ps. The analysis of radiated pulse distortion with respect to angle demonstrated that the pulse is very similar within the -3 dB beamwidth. This demonstrated that the antenna has small distortion and short after-time ringing, which makes the antenna suitable for time-domain application. The antenna coupling behavior analysis showed that the coupling between elements is small. Therefore, no severe performance degradation was expected when this antenna operates in the sparse MIMO array environment. 2. Investigation of LTCC technology: LTCC technology has been selected to manufacture mm-wave antennas to be integrated with MMICs. The multilayer nature of the LTCC technology makes it suitable for system-in-package. However, LTCC technology is a relatively new and not-yet standardized technology and LTCC material normally possesses high dielectric constant, which makes the design of UWB antenna difficult. As a result, we have investigated the properties of LTCC material and its impact on UWB antenna design. We have also explored the manufacturing limitations of the LTCC technology, and proposed solutions to overcome these limitations. The LTCC processing variations have also been studied. It reveals that the variation of substrate height has significant influence to the antenna resonance frequency, while the variation of relative permittivity has very small impact on the antenna reflection coefficient. 3. K-band LTCC antenna: A novel K-band antenna using the LTCC technology has been developed. This antenna is a differentially-fed, multi-staircase shielded elliptical dipole UWB antenna. The antenna has a novel differential feeding which enables it to be directly integrated with differential MMICs. The multi-staircase shield reduces the antenna back radiation and improves the antenna forward radiation, while keeping the antenna impedance bandwidth large. The antenna has a -7.5 dB impedance bandwidth from 24 GHz to 30 GHz, with a gain of approximately 5 dBi to 7 dBi. Thanks to the presence of the shield, the antenna radiation patterns are stable within the operating bandwidth, and the 3 dB beamwidth at the E-plane is of about 60° and 30° at the H-plane. 4. M-band LTCC antenna: A novel differentially-fed UWB antenna working at M-band using LTCC technology has been developed. The antenna is based on ASP type of antenna with novel differential feeding structure. With this feeding structure the antenna can achieve a -10 dB impedance bandwidth from 50 GHz to 78 GHz. The high dielectric constant of LTCC material induces severe surface wave which substantially degrade the antenna radiation characteristics. Although the multi-staircase shield proposed for the K-band antenna can solve this problem, it is far too complex to realize in the M-band. A novel simplified rectangular shield has been proposed to solve this. This shield does not have complex structure but can successfully confine the surface wave, thus improving the antenna radiation characteristics. The gain of the M-band antenna is from 3.5 dBi to 8 dBi from 50 GHz to 62 GHz, and the -10 dB beamwidth is at least from -45° to 45° for both E- and H-planes. 5. Element coupling investigation for imaging array: The influence of element coupling to the quality of image has been investigated. The antenna cross-talk does not pose severe threat to the image quality, because it can be eliminated by time-gating technique. The most influential coupling is the scattering coupling, which acquires fewer attentions in the antenna community. This type of coupling will alter the antenna receive sensitivity function. If at certain frequency the coupling is stronger than others, then the sensitivity function will have a spike at that frequency. This spike will cause long after-time ringing, masking small objects behind a large object. Another profound influence of scattering coupling to the image is that the scattering coupling will cause increase of sidelobe level, which increases the clutters. 6. 2-D sparse MIMO antenna array topology: Investigation demonstrated that MIMO array concept can achieve 2-D sparse real-aperture array for fine cross-range resolution and low sidelobe imaging system. The design of the 2-D MIMO array has been break down to two steps. The first step is to design a 1-D MIMO array with desired PSF properties. The next step is expand this 1-D MIMO array into 2-D array by firstly lay the designed 1-D array on two orthogonal axes, and then use the rotational 1-D array analysis to obtain the 2-D array. Two 2-D arrays based on the same 1-D array have been developed using this approach and manufactured. The measured results of small objects demonstrated that both arrays were capable to image small objects. It was also found that the sidelobe level is one of crucial specifications of the array, which should be specified by system designers in order to achieve proper performance of the whole imaging system. The 3-D imaging results proved high potential of using microwave 2-D UWB sparse MIMO array in real-time short-range high-resolution imaging applications.

Imaging radar has become a keen research topic in recent years. UWB technology provides many advantages to imaging radar such as fine resolution and high power efficiency. The performance of a UWB imaging radar can be further improved by applying polarimetric diversity. The polarimetric signature of objects can be used to enhance the quality of target recognition. Like any other wireless systems, antennas are key factors of radar systems. The focus of this thesis is to develop a dual polarized antenna for UWB imaging radar. An antenna element was designed for the Ku-band and an impedance bandwidth from 8 GHz to 24 GHz was achieved. An orthogonal coax-to-coplanar transition has been developed during this project and this transition is used to feed the antenna element. The antenna elements are successfully applied in two different array configurations. It is demonstrated that these sub-arrays have over 100% fractional bandwidth, good impedance matching, linear phase (almost constant group delay) and uni-directional pattern. These aspects collectively account for the novelty in design. In future, these sub-arrays will be implemented inside a complete array structure of UWB imaging radar.

Rescue dogs are commonly used during the urban search-and-rescue (USAR) operations for the initial indication on the presence of trapped victims after the collapse of man-made structures. However, dogs are not able to inform the rescue crews whether the trapped victims are alive or not and where exactly they are located. Other complementary tools, such as acoustic- and audio-visual equipment, are prone to inaccuracy, interference and inadequate range of operation. Ultra-wideband (UWB) radar is considered a promising tool for more exact assessment on the range of trapped victims. However, implementation of UWB radar for trapped-victim detection faces challenges such as low signal-tonoise ratio (SNR) conditions, interference from non-stationary clutter, residues due to amplitude instability originating in the equipment as well as narrowband radio interference. There are four commercially available UWB radar technologies and it is not clear which radar technology is the most suited one for the purpose of detecting trapped victims. There is very little available knowledge on the two target features that enable detection of a trapped human body using radar (respiratory- and cardiac motion). In need of further investigation is the choice of the optimal operational frequency band as well as assessment on the amount of attenuation of a few obstacles that represent, to various accuracy, real-life rubble. Chapter 2 introduces the reader with the basic principles of generation, sampling and pre-processing of UWB signals for the four available UWB radar technologies. It investigates the applicability of two time-domain- and the continuous-wave (CW) UWB radar technology for the purpose of trapped-victim detection, both based on the inherent properties as well as by means of an experimental verification study evaluated under as similar measurement conditions as possible. The results of both the theoretical and experimental verification indicate that the CW UWB radar technology is to prefer over the time-domain radar technologies due to generally higher dynamic range, better use of the designated spectrum and higher transmit power as well as that it enables the extraction of two target features, as opposed to only one (respiratory motion). The study is not definitive nor final and should serve as guidelines for further studies and/or system design. Chapter 3 investigates in detail the time-domain and frequency-domain behaviour of the two available target features for various body positions. It shows that the respiratory motion responses are in average 13 dB stronger than the cardiac motion responses. The position such that the chest is turned toward the receive antenna produces strongest respiratory motion responses due to larger chest displacement and reflective area than the other positions. Detectability of respiratory motion responses as function of aspect angle was investigated under line-of-sight conditions for three body positions, four bi-static angles and three antenna-pair polarisations using a single test person. It showed that there is no considerable difference in detectability among the investigated bi-static angles and co-polarised antenna pairs. However, it was concluded that cross-polarised antenna pairs should be avoided in real life as they produce significantly lower detectability values. The attenuation as function of frequency of two types of obstacles (piles of sandstone blocks and a 60-cm concrete wall) was investigated in chapter 4. The results show that the attenuation for both materials is ca 10-15 dB across the frequency range of interest. However, realistic rubble thicknesses and types of rubble can heavily increase the attenuation and thereby lower the probability of detection. Measurements involving a test person resting under 80-cm concrete rubble pile and behind two concrete walls, showed that the centre frequency well below 1 GHz gives rise to highest SNR values. Bandwidths of ca. 400 MHz centred at frequencies below 1 GHz give rise to higher SNR values than larger investigated bandwidths. On the hand, lower bandwidths result in poorer down-range resolution, which is necessary for resolving non-stationary clutter responses or multiple trapped victims. One of the fundamental tasks of this thesis is the development of a respiratory motion detection algorithm. Chapter 5 details a novel and computationally efficient algorithm which is able to improve SNR conditions and better suppress non-stationary clutter compared to an existing algorithm, assessed both experimentally and in a simulated environment. The algorithm further incorporates a threshold which aids in the decision making process by the operator. The performance of three common stationary-clutter suppression methods is investigated on a single measured data set containing respiratory motion and linear amplitude instability (linear trend). It was shown that the linear-trend removal method, which removes any potential linear trend and DC level in the slow-time dimension, is the preferred approach to stationary-clutter suppression. Narrowband interference (NBI) results in increase of noise floor and thereby worsens the probability of detection, when using stroboscopic sampling (such as in impulse radar). Chapter 6 analyses the performance of four developed methods for NBI suppression implemented in stroboscopic samplers. The most suitable method for NBI suppression in stroboscopic samplers is to filter out the NBI in the analogue domain and, after sampling, implements linear interpolation of the missing spectrum in order to avoid ringing of the backscattered waveforms from the victim, is regarded the most suitable method. It shows an improvement factor of 12.9 dB in noise reduction and manages to preserve the signal waveform and energy very well. The thesis is completed by the conclusions and recommendations for future studies.