A new 1.4-GHz soil moisture sensor

Moisture content determination in agriculture and civil engineering is a common process which needs special sensors with high accuracy, durability and compatibility with the measurement environment. A new 1.4-GHz soil moisture sensor using microstrip transmission line is presented. The proposed sensor consists of two separate parts: (1) A sensor head, which is a microstrip transmission line to be placed in the soil, and (2) an electronic transceiver which sends a sinusoidal wave into the sensor head at one end of the transmission line, and receives the traveled wave from the other end. Transmitter is basically a Colpitts oscillator, and the receiver is a phase detector that measures the phase shift due to velocity variation caused by the moisture content of soil. At a certain frequency, the velocity of a microwave traveling through a media depends on the permittivity of that media. The proposed sensor is implemented and tested on one sample of typical soil. The main advantages of the proposed sensor are its high accuracy, quickness of measurement, its low cost and ease of implementation. Since the sensor has low power consumption, it can be recommended for low-power applications such as wireless sensor network.