There are many problems with agricultural technology around the world, such as extensive irrigation which causes the waste of water. To significantly improve the efficiency of irrigation water, an antenna sensor, with high sensitivity and wide range, is proposed to measure the soil water content (SWC) wirelessly. When the SWC changes, the resonant frequency of the antenna will shift. By reading the resonant frequency of the antenna sensor, the SWC can be measured and transmitted wirelessly. The antenna sensor has a wider detection range of SWC, ranging from 10% to 44%, and a novel metamaterial structure electromagnetic bandgap (EBG) is designed and its function in improving sensitivity is verified. Furthermore, by sputtering 60-nm-thick SiO2 film on the surface of the antenna patch, its sensitivity has also been greatly improved. When the SiO2 film is sputtered on the antenna sensor, its sensitivity is 7.2 MHz/% and 7.6 MHz/% when the SWC is 10%–29% and 30%–44%, respectively. In summary, this work contributes to methods and instruments for wireless measurement of the SWC.