Emerging demand for small and dependable IoT devices necessitates accurate measurement and certification of integrated wireless components. The current work is concerned with the performance measurement and reliability testing of GPS and GSM multi-band antennas designed on a single printed circuit board (PCB) with IoT applications. The research focuses on accurate impedance matching, return loss, and radiation efficiency characterization by means of calibrated vector network analyzer (VNA) measurements. Methods of measurement uncertainty reduction and repeatability improvement were incorporated in an effort to provide evidence of good measurement of antenna coexistence and miniaturization performance. Experimental measurements demonstrate stable GPS and GSM operation with less than –10 dB return loss and more than 70% efficiency, ensuring design robustness and measurement stability. This metrological view guarantees traceability, reliability, and precision in antenna performance data, hence the conclusions are applicable to scientific, industrial, and wearable IoT applications.