Technical White Paper: Low-Probability Of Intercept Of Ultra-Low-Power, Near-Field Signals

By Hans G. Schantz, Q-Track Corporation

Long considered a technical backwater, low-power, near-field systems are emerging as the preferred solution for a variety of useful applications. These applications include short-range communication, radio frequency identification (RFID), and real-time locating systems (RTLS). This paper explores the ultralow-power and thus low-probability of intercept nature of near-field signals operating within the AM broadcast band under Part 15 constraints.

The performance of a near-field link depends in large part upon regulatory constraints. The FCC authorizes unlicensed operation in the AM broadcast band under a couple of provisions within Part 15. Part 15.209 allows operation in the band 0.490-1.705MHz provided the field strength does not exceed 24000/f(kHz)^µV/m [1]. At a typical operating frequency of 1.295 MHz, the field strength limit is 18.5^µV/m. measured at 30m. Alternatively, Part 15.219 allows a transmit power of 100mW (+20dBm) into an antenna smaller than 3m [2]. These two limits work out to be roughly equivalent in many cases.

The radiated power density (S) results from the transmitted power (PTX) being distributed over the spherical surface at range r (4pr²) as modified by the gain of the transmit antenna relative to an isotropic radiator (GTX).