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.

In this work we present a microwave sensor for noncontact monitoring of liquid level at high temperatures. The sensor is a high gain, directional conical lensed horn antenna with narrow beam width (BW) designed for operation over 10 GHz - 15 GHz. Sensor design and optimization was carried out using 3D finite element method based electromagnetic (EM) simulation software HFSS®. A rectangular to circular waveguide feed was designed to convert TE10 to TE11 mode for wave propagation in the conical horn. Swept frequency simulations were carried out to optimize antenna flare angle and length to achieve better than -10 dB return loss (S11), standing wave ratio (SWR) less than 2.0, 20° half power BW (HPBW) and 15 dB gain over 10 GHz - 15 GHz. The sensor was fabricated using Aluminum and was characterized in an anechoic test box using a vector network analyzer (E5071C, Agilent Technologies, USA). Experimental results of noncontact level detection are presented for boiling water in a metal canister.

An electric field sensor is fabricated on a 125 micron thick flexible dielectric substrate for electromagnetic (EM) nondestructive material inspection at 915 MHz. The sensor consists of an electrically short dipole antenna and a radio frequency (RF) diode detector connected to a pair of high impedance screen printed carbon lines. The DC component of the rectified diode voltage conveyed across the high impedance lines is measured using a data acquisition circuit. Sensor measurements are validated with simulated data for a conformal patch antenna operating at 915 MHz. Sensor performance for EM nondestructive testing (NDT) is evaluated using phantom defects in low loss dielectric slabs. Preliminary results indicate sensor utility for EM NDT and support further testing on realistic defects.

Targeted heating with minimal dose to neighboring tissues is possible with intra cavitary microwave applicators as they can treat tumors within/or nearby body cavities. Here we present an intra cavitary applicator for hyperthermia treatment of gynecological cancers at 434 MHz. A 3D numerical model of the applicator with conformal patch antenna in muscle tissue is studied for rectangular patch, variations of bow tie and spiral antennas. Antenna performance is evaluated in terms of size, return loss, bandwidth, specific absorption rate (SAR) and effective field surface (EFS). Fish tailed bow tie and spiral patches exhibited ≤-25 dB return loss and ≥25 MHz bandwidth compared to other shapes. EFS of spiral antenna is larger than fish tail. However, ratio of EFS to patch area indicates larger volumetric power deposition for fish tailed bow tie. From simulation results, it can be concluded that an array of fish tailed bow tie and/or spiral patch antennas would provide adjustable heating profile with high power deposition.

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.

An active cold noise source with low return loss and stable operating temperature of 288 K is presented for a microwave radiometer designed for medical application (288 K to 310 K). The noise source is based on Si-Ge Heterojunction Bipolar Transistor (HBT) and is proposed for calibration of our L-band radiometer designed for detection of thermal anomalies in the human body. The noise source is centered at 1.3 GHz with 190 MHz bandwidth for return loss less than -20 dB, 3.01 dB noise figure and variable noise temperature source for the low temperature reference (288 K - 298 K).