Effective Aperture Determination for Efficient Vehicle-Mounted Antennas Measurement via Spherical Wave Expansion Power Spectrum Analysis

With the advancement of wireless technology within the automotive industry, vehicle antenna measurement has garnered increasing attention, as antenna system performance exerts critical influences on wireless communication performance. In spherical near-filed (SNF) automotive measurement, the assignment of minimum sphere radius (MSR) is of paramount significance in reducing test duration. Current industrial practice typically presumes the aperture equivalent to the entire vehicle, consequently assigning the minimum sphere to enclose the entire vehicle structure. Such a sampling scheme, however, is often redundant since regions distant from the antenna experience weak illumination and contribute negligibly to radiation, particularly at higher frequencies. Thus, determining the effective aperture becomes essential for MSR reduction and enhanced testing efficiency. To this end, this paper investigated the effective aperture of vehicle-mounted antenna (VMA) to reduce the test duration. The spherical mode coefficients (SMCs) of the VMA are derived, and their power spectrum is analyzed. Leveraging the correlation between source distribution and spectral power characteristics, modal truncation error was evaluated to ascertain the effective aperture dimensions. A GNSS antenna operating at 1.94 GHz is designed, fabricated, and mounted atop the front and rear of a vehicle model. Numerical results are provided to demonstrate the proposed methodology, and measurement outcomes are presented to validate its efficacy.