Design and implementation of a self-calibrating, compact micro strip sensor for in-situ dielectric spectroscopy and data transmission

A compact, on-board, self-calibrating, micro strip sensor is presented. The sensor can make accurate multi-frequency measurements of complex permittivity in real time and transmit this information wirelessly by using the same sensor as a micro strip patch antenna. Such multi-frequency measurements in a multi-phase mixture like soil are used to estimate the concentration of individual constituents like bulk-soil, water, and various nutrients in soil. The sensor architecture comprises of a programmable phase locked loop (PLL) which sweeps through the frequency band of 3-40 MHz. The signal generated by the PLL is allowed to reflect from the micro strip patch which is surrounded by the dielectric medium under test (such as soil or food). The amplitude and phase of incident and reflected signals are captured and impedance due to the surrounding dielectric mixture is calculated. This impedance value is used to estimate the dielectric constant by mapping the input impedance of the micro strip sensor to different surrounding dielectric constant values. The sensor has an inbuilt self-calibrating mechanism which makes it useful for remote, underground and hand held applications.