This paper presents a compact, energy-efficient, passive, and wireless vibration sensor node for embedded monitoring. The narrowband radio frequency (RF) communication system employs a frequency doubler on the sensor node to return the second harmonic of the interrogation signal. The compactness and high efficiency of the node are achieved by employing 3-D, machined-substrate small antennas on the transceiver. The node operates by receiving a signal at 2.4 GHz and reradiating a modulated signal at 4.8 GHz, and is optimized for an RF input power 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. A commercial piezoelectric vibration sensor and mechanical switches are used to modulate the return signal. Experimental tests show that vibrations of <;0.1-Hz frequency and 0.01-g amplitude can be detected. The communication range is greater than 60 m using a 43-dBm effective isotropic radiated power transmitter.