Using an impedance-modulated reflector for passive communication

The idea of using modulated reflectors for communicating is not new. Stockman (1948) considered this possibility, envisioning however that the modulation would need to be achieved by mechanical means, stating that "modulation (of a reflector antenna) usually requires mechanical oscillation of large masses, joined into a rigid system by members of insufficient stiffness; thus the upper modulation cutoff frequency becomes unduly low." We discuss the alternative of using electronic modulation of its terminating impedance to vary the reflectance of a passive-node antenna (PNA) and demonstrate its practical feasibility in a real system. The system described was developed specifically to use an impedance-modulated reflector (IMR) as the PNA in a medium range (up to 10s of km), low power (a few watts at the active node), slow-scan video surveillance application using compression technology to permit it to utilize narrowband satellite and telephone channels. An advantage of an IMR in this application is that it is inherently covert and therefore hard to detect. The passive-node hardware can also be made small, lightweight, rugged, reliable and power parsimonious (only a small amount of power is needed to operate the ancillary electronics). The active node in our system uses a Doppler radar.