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Polarimetric radar systems are beneficial to identifying and classifying targets but require multiple transmit or receive channels with different polarizations. This leads to a high hardware effort and thus higher costs. To use a single, linear polarized radar sensor as a polarimetric system, a frequencydependent, polarization rotating reflector which can be placed in front of the radar antenna is presented. The reflector is based on a frequency selective surface (FSS) consisting of slot-excited substrate integrated waveguide (SIW) resonators. Resonator modes are analyzed and an equivalent circuit diagram to describe the filter functionality is developed. For using the described FSS as reflective structure, the design focuses on 45 • oblique incidence. Different field vectors for normal and oblique incident angles are considered and different cavity modes for these cases are analyzed. An undesired mode is suppressed by an additional plated through via hole and slot impedances are matched. Reflector designs for normal and oblique incident angles are presented for 15 GHz (K u-band) and afterward adapted to 35 GHz (K a-band). The frequency band of operation with 12% fractional bandwidth is divided into two 4.5% subbands which allows a frequency-dependent polarization rotation of a linear polarized electromagnetic wave. Investigations on the fabrication accuracies are presented and reflectors for both bands are manufactured. Measurements are performed with a vector network analyzer and results fit well to the simulated curves. In the band of polarization rotation reflection, the matching is better than −13 dB and dielectric losses of less than 1 dB are achieved.

Sensors used for Security purposes have to cover the non-invasive inspection of persons, baggage and letters with the aim to detect weapons, explosives and chemical or biological threat material. Currently, emphasis is placed on system concepts and technologies for this type of applications, employing millimeterwave-, submillimeterwave- and terahertz sensors. This is based on the capability of these frequency bands to look through textiles and the possibility to achieve a geometric resolution which is sufficient to resolve critical items within the necessary range. Using multiple frequencies promises to give more detailed information about the structure of the observed objects. Furthermore, to overcome the limitations of passive millimeter- and submillimeterwave sensors which depend on indirect illumination, systems using miniaturized mmw-radar modules are applied as well. This paper describes two approaches for the detection of concealed weapons, the first using a millimeterwave radiometer on a scanner and the second employing a miniaturized radar module based on a synthetic aperture method