A Method to Characterize the Frequency-Dependent Structural Coefficient of Antennas: Application to Millimeter Wave Identification

The passive UHF RFID technology, well known for the logistics and security applications, is more and more considered for sensor networks and Internet of Things (IoT). Recent works have demonstrated the possibility to exploit the main advantages of RFID in the millimeter-wave domain, giving rise to the millimeter wave identification (MMID) technology. This new technology uses the large available ISM band from 57 to 66 GHz in Europe. In backscatter communications, such as RFID or MMID, the radar cross section (RCS) is a fundamental parameter. The antenna RCS is defined as a function of its load and its structural coefficient. The structural coefficient that depends on the antenna geometry is often approximated to unity in RFID because tag antennas are usually thin dipoles compared to the wavelength. However, in MMID this approximation is no longer suitable due to the use of high directive antennas. This letter aims to propose a method to determine the structural coefficient of any types of antenna. The proposed approach is illustrated through the study of a horn antenna and a slotted waveguide antenna array in WR15 waveguide (from 50 to 75 GHz). Theoretical, simulation, and measurement results are compared and demonstrate the interest of the method.