Active Integrated Antenna Based Permittivity Sensing Tag

In this article, a novel negative resistance oscillator based active integrated tag antenna design is proposed for sensing material permittivity at a distance from the reader. A microstrip interdigital sensing arm acts as the terminating element of the oscillator. A broadband microstrip monopole tag antenna, working as the load element, is used to transmit the output signal of the oscillator to the reader. The frequency of oscillation varies with the permittivity of the loaded overlay dielectric substrate sample on the interdigital sensing arm. An oscillation frequency variation of 200 MHz (6.8 GHz to 7 GHz) is experimentally observed for change in relative permittivity from 1 (unloaded) to 10.2. The proposed tag is calibrated, and it can measure relative permittivity of 1 to 12 with better than 3.6% accuracy. The permittivity of the loaded material can be calculated by measuring the output signal frequency of the active tag and utilizing frequency positioning algorithm. The phase noise of the stand-alone oscillator is measured to be better than -105 dBc/Hz and -117 dBc/Hz at 500 KHz and 2 MHz frequency offsets, respectively. The design is optimized theoretically and by simulation, and its proof-of-concept working principle is demonstrated experimentally. The proposed active sensor tag provides good reading range of about 600 cm and consumes very low power (6 mW). It has low profile (40 cm x 87 cm), and it is suitable for low cost batch fabrication.