Design fabrication and evaluation of miniaturized passive and conformal screen printed electric field sensor for microwave Nondestructive Testing

A miniature conformal sensor for real time electric field measurement is proposed for microwave Nondestructive Testing (NDT). The sensor is an electrically short strip dipole antenna connected to a high speed Schottky diode through a high impedance parallel wire transmission line. Screen printable resistive carbon ink for the sensor transmission is formulated by dispersing fine graphite powder in colorless synthetic resin. The sheet resistivity measurement of the screen printed films was measured to select an ink formulation for sensor fabrication. The influence of strip dipole dimensions namely, half length (h) and width (w), and transmission line resistance (rs) on the sensor voltage was studied at low power levels (up to 3 W) using a square slot antenna at 915 MHz. Sensor measurements in the antenna near field were validated with 3D numerical simulations for a single sensor and 2×2 sensor array with sensor spacing (Δ/λ=0.076). The perturbation in the electric field caused by the phantom defects namely, air void, material wear and broken metal rod embedded in a 6.35 mm thick acrylic slabs was measured using a miniature screen printed sensor. Carbon ink with resin:graphite ratio of 70:30 percentage by weight was used to screen print the sensor transmission line with 0.9 MΩ/m line resistance on a 125 μm thick flexible polyethylene film. The sensitivity of the linearly polarized miniaturized sensor (h=6 mm, w=2 mm, h/λ=1.83×10-2) at 915 MHz was measured as 31.22 mV/(V/m). Good agreement between the measurements and simulations for the phantom defects indicate that an array of miniature conformal electric field sensors embedded in dielectric composites could be effectively used for in-situ monitoring in microwave NDT.