Kavitha Arunachalam

Inverse problem solutions in NDE can be broadly classified as model-based approach and system-based approach. In model-based approach an accurate forward model is used in an iterative framework to provide a defect shape that minimizes the error between the measured signal and a simulated signal. However this approach results in repeated executions of a three dimensional forward model in each iteration, making it computationally demanding. This paper presents a direct approach to inversion using principles of time reversal. The feasibility of the approach is demonstrated via application to microwave NDE data. A two-dimensional finite difference time domain model for simulating the propagation of forward and time reversed wave fields is first developed. The key advantage of the approach is that it provides a model-based inversion method that is not iterative. Simulation and experimental results validating the approach are presented. © 2011 American Institute of Physics.

A miniaturised frequency selective surface (FSS) with λ0/7 x λ0/7 unit cell at the pass band centre frequency is presented for millimetre wave remote sensing. A resonant corner convoluted square ring (CCSR) is loaded with non-resonant cross dipole (CD) for tri-band spectral response. Rectangular patches integrated in the CCSR are used to optimise the coupling between the CD and CCSR to reject 50-60 GHz (B1) and 170-195 GHz (B3), and transmit 87-91 GHz (B2). The FSS fabricated on 175 μm thick quartz with 3721 unit cells indicates polarisation and angle stable transmission response.

The feasibility of non-contact and non-destructive evaluation (NDE) of planar aerospace dielectric composites using microwave is examined in this paper. Free space microwave measurement set up includes spot focusing horns to gather the scattering parameters of the composites with the help of (Gated-thru reflect line) G-TRL calibration. The dielectric properties are studied from the measured scattering parameters to characterize the material's defective and non-defective region. Particulate reinforced composite with an air gap of 0.1 mm thickness and 10 mm width is used for validation of the technique.

A hybrid microwave NDE technique employing near field sensor for electric field measurement in plane wave regime is proposed in this paper. A custom made spot focusing horn antenna has been used to illuminate the sample with defect. A graphene based electric field sensor is employed to measure the electric field with less field perturbation. Thin dielectric composites of thickness less than quarter wavelength has been inspected using this method. Samples of size 300 mm × 300 mm with machined defect radius of 10 mm and 5 mm with 0.5 mm depth were measured using the proposed hybrid method. The hybrid technique has a higher spatial resolution and insignificant perturbation to the electric field compared to the conventional methods.

A wide band dual arm slot spiral antenna operating over 600 MHz to 3000 MHz was designed and tested for a Stepped Frequency Ground Penetrating Radar (SFGPR) application. This antenna is low profile, planar, uses a simple current element as the balanced feed. The influence of the receiver Intermediate Frequency (IF) Band Width (BW) on the Signal to Noise Ratio (SNR) for antenna operation under SFGPR mode is investigated for a metal rod buried inside dry sand.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

A non-contact microwave sensor is presented for in situ process monitoring of nuclear waste glass melts inside a cold crucible induction melting (CCIM) furnace. The level and thermal steady state of the molten glass inside a 1400 °C CCIM furnace were measured during 6-hour long vitrification process using a corrugated horn antenna operating over 20–24 GHz designed for high temperature measurement. The non-contact in situ microwave measurements indicate the ability to measure absolute level, identify thermal steady state of the glass melt, and remotely monitor the vitrification process for safe immobilization of the liquid radioactive waste.

This paper presents the development and characterization of a single reference Dicke radiometer operating over 1.2 GHz-1.4 GHz for clinical applications. A variable hot noise source is designed using a peltier module for calibrating the radiometer. System characterization was carried out using a temperature controlled test load to obtain the calibration curve of the device. The accuracy of the calibrated single-reference Dicke radiometer is reported for a matched test load at thermal steady state.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

This article presents a compact precision free-space microwave measurement setup with a choice of three dielectric lenses to tailor the antenna focal plane characteristics for extracting complex dielectric permittivity of small samples. Custom designed spot-focusing horn antenna pairs were used to achieve a compact setup with antenna separation distance, 2f l : 4λ c-8λ c and focal spot size, fs : 1λ c-1.5λ c , where λ c is the wavelength at center frequency. Using the compact free-space setup, relative complex permittivity (j) was extracted over 8-12 GHz for low-and high-loss dielectrics with lateral dimensions, 3.3 λ c× 3.3λ c and 10 λ c × 10 λ c. For large materials under test (MUTs), i.e., 10 λ c × 10 λ c , measurement accuracy in dielectric constant, Δ% was <0.65% and ≤1.14% for low-and high-loss dielectrics, respectively. For smaller MUTs ( 3.3 λ c × 3.3 λ c ), Δ % was <0.89% and ≤2.29% for low-and high-loss MUTs, respectively. The error in loss tangent ( Δ) varied over 0.002-0.016 and 0.015-0.056 for large ( 10 λ c × 10 λ c ) and small MUTs ( 3.3 λ c × 3.3 λ c ), respectively. For large MUTs, biconvex lens pair with the smallest f-s ( 1 λ c ) and f-l ( 4 λ c ) among the three lenses yielded the best accuracy in dielectric constant due to tight field focusing at the focal plane. The plano-convex lens pair yielded the best accuracy in loss tangent for large MUTs due to slow variation in the phase of the local plane wave. By tailoring antenna focal plane characteristics, a compact free-space setup that is 6×-10× smaller than the classical setup for handling MUTs that are 1/5th of the size used in classical setup is demonstrated without compromising the measurement accuracy.