The tags’ impedance has been measured by means of a Vector Network Analyzer, VNA (Anritsu MS2024A) probe connected to the slot mid-point through a bazooka balun having the purpose to prevent unbalanced currents from the probe to flow on the outer surface of the coaxial cable [14].
An approximately λ/4 metal sleeve, shorted at one termination encapsulates the coaxial probe (Fig. 8). The input impedance measured by the VNA will be hence the tag impedance itself without artifacts. As a proof, Fig. 9, shows a comparison between the simulated tag without cable and balun, and the also simulated impedance in the measurement condition. As visible the input impedance of the tag plus the balun, estimated by FDTD, is practically unchanged, at least in the RFID band, with respect to the standalone antenna.