Design of a Polarization Rotating SIW-based Reflector for Polarimetric Radar Application

Polarimetric radar systems are beneficial for 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 frequency dependent, 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 (Ku-band) and afterwards adapted to 35 GHz (Ka-band). The frequency band of operation with 12% fractional bandwidth is divided into two 45% sub bands 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.