Evaluation of a MIMO Radar Concept based on a Hexagonal Antenna Configuration
Christoph Dahl, Michael Vogt, Ilona Rolfes
Kleinheubacher Tagung 2015, U.R.S.I. Landesausschuss in der Bundesrepublik Deutschland e.V., Miltenberg, Germany, Sep 28-30, 2015
Radar level measurement of bulk materials in silos is a challenging task. Due to the fact that bulk solids can heap up to complex surfaces inside the silos, a single range measurement is insufficient to determine the volume inside the silo. Imaging radar provides the opportunity to measure the entire surface profile to increase the accuracy and reliability of level measurements. In order to apply this concept in an industrial application, multiple-input multiple-output (MIMO) radar systems can be used to reduce the system cost by reducing the number of transmitting and receiving channels. A common antenna configuration is based on two linear arrays, one for transmitting and one for receiving, that are positioned perpendicular to each other. The radiation pattern of the transmitting and receiving array have fan-shaped beams, that are also perpendicular to each other. As a result, in the two way radiation pattern, a pencil shaped beam is formed. In order to improve the angular resolution and the side lobe suppression for a fixed number of antenna elements, a hexagonal antenna configuration has been analyzed. The proposed hexagonal antenna concept is based on the Gosper island fractal. The transmitting array consists of seven antennas positioned in a hexagonal shape. The receiving array is a scaled and rotated version of the transmitting array. The antenna concept can be analyzed by introducing a virtual array, that is formed by a convolution of the transmitting and receiving array. The radiation pattern of the virtual array corresponds with the two way radiation pattern. In this case, the virtual array consists of 49 elements positioned on a hexagonal grid and is shaped like the second iteration of the Gosper island fractal. The corresponding two way radiation pattern has a beam width of 13.8° and a side lobe suppression of 15.7 dB. A comparable perpendicular antenna configuration leads to a rectangular shaped virtual array with 49 elements positioned on a rectangular grid. In comparison with the perpendicular antenna concept, the hexagonal configuration has a 1.1° better angular resolution and a 3 dB better side lobe suppression. The antenna concepts have been compared in a reference scenario using a radar system working in the frequency range from 24 to 26 GHz. The virtual array of each concept has been sampled with a horn antenna by using synthetic aperture measurements. The resulting radar images show the advantages of the hexagonal antenna concept, regarding the angular resolution and side lobe suppression.