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Antenna elements and arrays for active millimeter wave imaging

vrijdag, 26 maart, 2010 - 16:00
Campus: Brussels Humanities, Sciences & Engineering campus
Islam Md. Saiful

This Ph.D. thesis deals with the study of the possible scenarios of integrating frequency selective
surfaces based on slot antenna arrays as quasi-optical elements in active millimeter wave imaging
systems for improving their performance. Active millimeter wave systems for detection of concealed
objects in security applications are typically operating in the W-band (75-110 GHz). This spectral
band yields a good compromise between image resolution and penetration depth through a whole set
of materials in the visible range.

First we will present an overview of the needs for active illumination techniques in mm wave
imaging systems and discuss the issues related to the coherence degree of active sources in imaging
applications. In the following chapters frequency selective surfaces on the basis of slot antenna
arrays are theoretically and experimentally studied for the purpose of reducing the coherence related
artifacts of mm wave images by diversifying the illumination patterns.

Theoretical aspects and analysis methods of FFSs are discussed and compared. Fundamental
geometrical topologies and material parameters which determine their resonance behavior are
investigated. We considered the impact of technological processes and related tolerances. For
the imaging application grounded versus non-grounded slot antenna arrays are theoretically and
experimentally compared and finally the grounded FSS has been proposed for the enhancement of
noise artifact in active mm-wave imaging due to their wider band width and functionality.

As commercial software packages such as Microwave Studio of CST can only treat infinite x infinite
arrays, there is a need for experimental work in this field for getting more in depth insight into the
peculiarities of reduced dimensions of the arrays. Only recently advanced numerical techniques have
been developed by INTEC-UG to predict theoretically the behaviour of finite × finite antenna arrays.
Experimental work is compared to these techniques.

Different applications for the FSS are proposed and experimentally tested: e.g. quasi-optical
deterministic functions such a beam splitting, beam bending, a pseudo-random diffuser, coherent
destroying diffuser and a digital generator of stochastic independent antenna phase patterns. The
latter one is based on orthogonal Hadamard phase patterns and could only be realized on the basis
artificially magnetic conductors. The W-band Artificial Magnetic Conductor has been investigated
and tested. Finally first applications in real imaging applications are discussed.