Abstract: A system includes an identification circuit, a grid antenna, a receiver, and a processor. The identification circuit includes resonant circuits formed on a substrate within a perimeter. Identification may be based on a quantity or physical arrangement of detected resonant circuits within the perimeter. One resonant circuit provides a reference signal. Any resonant circuit may be tuned in accordance with the reference signal by the addition or subtraction of reactance formed on the substrate. A capacitance of a first group of capacitors located outside a turn of an inductor is roughly equal to a capacitance of a second group of capacitors located inside the turn. Any resonant circuit may also be tuned by affixing a resonance modifying element, for example a sticker, to the identification circuit. The grid antenna provides antenna field patterns, one for each cell location. The receiver communicates with the identification circuit via the grid antenna.