A relevant part of reader-tag communication consists of tag’s backscattering of the continuous wave coming from the reader. During this task, the microchip acts as a programmable switching device which connects or disconnects the antenna to a Zmod load (typically an open circuit or high impedance for low state, and a short circuit or the antenna conjugate impedance, for the high state). During the data transfer, the RFID system can be considered as a monostatic radar and therefore it can be characterized by the radar cross-section (RCS)σT. As detailed discussed and experimentally verified in], the RCS of many classes of tag antennas can be easily related to the RFID antenna gain and input impedance as

 (6)

The RCS of the chipless tag can be then measured as the S11 variation at the port of an illuminating antenna in both an anechoic chamber and a real scenario. The measurements are generally repeated for different impedances Zmod. Although this is an indirect measure, which does not give any direct information about input impedance and the activation range, it is nevertheless a reader-independent characterization.

Figure 1: Copper prototype of a NSSP antenna designed for a muscle-like box. Coaxial (SMA) connector is in the reverse side of the image plane.