This paper describes an ultra-small wireless sensor node (WSN) with a solar battery for the 300-MHz band. In Japan, this band is regulated for extremely low-power wireless systems, require no license. To reduce the total size of the WSN, all elements, which are made of different materials, and their integration must be as small as possible. To realize an electrically small dipole antenna, we consider two methods: to mitigate the impedance decrease with an asymmetric structure, and to design a wireless IС to drive low impedance. We fabricated two types of WSNs consisting of a solar battery, charging capacitor, ultra-low-power pulse modulation wireless IС, electrically small-size dipole antenna, and vibration sensor. The WSNs are mounted on small printed circuit boards (PCBs) and the PCBs are stacked three-dimensionally to integrate devices made of different materials with reduced fabrication cost. As a result, the total volumes are reduced to 1-cm cubic with 1/10 wavelength λ of asymmetric dipole antenna and 5-mm cubic with 1/20 λ symmetric one. It achieves data transmission by only solar energy charged in capacitor.
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