A low profile, low-RF band, small antenna for underground, in-situ sensing and wireless energy-efficient transmission

A key challenge to underground, in-situ soil sensing with wireless interface is the antenna size. Smaller operating frequency supports lower path losses but enhances the wavelength and hence the size of standard monpole (8.2 cm in height at 915MHz) or rectangular microstrip patch antenna (11.5 cm × 9.3 cm at 915 MHz), which is prohibitive for underground sensors. Lowering the frequency below 915MHz is not an option as it only further enhances the antenna size. To circumvent the size problem, a composite right-left handed (CRLH) microstrip patch antenna for wireless transmission at 915 MHz that doubles up as an underground, sensing element (external capacitor) has been designed and fabricated. The combined antenna/sensing CRLH patch integrates with the previously implemented on-board, multi-frequency dielectric based impedance sensor for soil moisture and nitrate determination and provides almost 93% reduction over the standard microstrip antenna size. As a proof of concept, the input impedance of the CRLH sensor, surrounded by the soil containing moisture and nitrate ions, is measured at multiple frequencies in the lab setting. It is shown that the change in moisture and nitrate can be successfully detected using the sensor. The small profile of the proposed antenna (3 cm × 2 cm), that is almost 93% smaller, makes it ideal for compact packaging.