A high-efficiency, miniaturized sensor node with machined-substrate antennas for embedded wireless monitoring

This paper presents a compact, energy efficient sensor node for narrow-band embedded wireless monitoring. The communication system is based on the RF interrogation approach that employs a passive frequency doubler to return the second harmonic of the interrogation signal. The compactness and high efficiency of the node were achieved by employing 3-D, machined-substrate small antennas on the transceiver design. The node operates by receiving a signal at 2.4 GHz and re-radiating an amplitude-modulated signal at 4.8 GHz, and is optimized for an RF input power level range between -30 and -20 dBm. The node fits within a 21 mm diameter sphere (λ/6 at 2.4 GHz) and its measured efficiency is 10% at -20 dBm input power. The voltage generated by a commercial piezoelectric vibration sensor is used to modulate the return signal.