SDR-Based Location and Interrogation of Harmonic Transponders

In this work, a software defined radio system is presented that both locates and interrogates harmonic transponders, with the objectives of accurately tracking the position of and measuring the forward link loss to the transponder, respectively. Using only the backscattered response, monopulse direction finding is implemented to obtain the position and channel losses are determined by using an amplitude modulated interrogation signal. Experimental results show that the directional angle can be found within ± 2° and the forward path loss within ± 0.5 dB. The work is motivated by an application in which harmonic transponders are deployed as soil moisture sensors and where the change in soil moisture can be determined by measuring the change in channel loss.