Precision Agriculture: Ultra-Compact Sensor and Reconfigurable Antenna for Joint Sensing and Communication

In this article, a joint sensing and communication system is presented for smart agriculture. The system integrates an ultra-compact soil moisture sensor (UCSMS) for precise sensing, along with a pattern reconfigurable antenna (PRA) for efficient transmission of information to the base station. A multiturn complementary spiral resonator (MCSR) is etched onto the ground plane of a microstrip transmission line to achieve miniaturization. The UCSMS operates at 180 MHz with a three-turn complementary spiral resonator (3-CSR), at 102 MHz with a four-turn complementary spiral resonator (4-CSR), and at 86 MHz with a five-turn complementary spiral resonator (5-CSR). Due to its low resonance frequency, the proposed UCSMS is insensitive to variations in the volume under test (VUT) of soil. A probe-fed circular patch antenna is designed in the wireless local area network (WLAN) band (2.45 GHz) with a maximum measured gain of 5.63 dBi. In addition, four varactor diodes are integrated across the slots on the bottom side of the substrate to achieve pattern reconfiguration. Six different radiation patterns have been achieved by using different bias conditions of the diodes. In the standby mode, PRA can serve as a means for wireless power transfer (WPT) or energy harvesting (EH) to store power in a battery. This stored power can then be utilized to bias the varactor diodes. The combination of UCSMS and PRA enables the realization of a joint sensing and communication system. The proposed system's planar and simple geometry, along with its high sensitivity of 2.05%, makes it suitable for smart agriculture applications. Moreover, the sensor is adaptive and capable of measuring the permittivity of various material under test (MUT) within the range of 1-23.