Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 157

changes the magnitude and phase of the reflected HF pulse, which can be detected by
the reader.
4.3.5 Switched sensors
Surface wave transponders can also be passively recoded (Figure 4.104). As is the
case for an impedance sensor, a second interdigital transducer is used as a reflector.
External circuit elements of the interdigital transducer's busbar make it possible to
switch between the states 'short-circuited' and 'open'. This significantly changes the
acoustic transmission and reflection ρ of the transducer and thus also the magnitude
and phase of the reflected HF impulse that can be detected by the reader.