Amitabha Bhattacharya

Shape Reconstruction of Mixed Boundary Objects by Linear Sampling Method

Geometric reconstruction of dielectric objects in the presence of a perfect electric conducting (PEC) object is investigated with the application of linear sampling method (LSM). The scattered fields from the cylindrical objects of circular cross sections are represented as a multipole expansion, and a numerical analysis is carried out to explore the behavior and limitations of LSM. By considering various numerical examples, the imaging capabilities of LSM for mixed cases (both dielectric and PEC objects) are studied in terms of frequency of operation and permittivity of dielectric objects. To overcome the drawbacks of standard LSM indicator, a modified LSM indicator based on a multifrequency approach is introduced. Apart from the synthetic data, the analysis is also validated by the experimental data provided by the Fresnel Institute.

Improved ultra-wide bandwidth bow-tie antenna with metamaterial lens for GPR applications

A new resistively loaded bow-tie antenna which can operate from 300 MHz to more than 3 GHz bandwidth is proposed. The proposed antenna employs a metamaterial lens to improve the forward gain and front-to-back ratio. The antenna radiates pulses with minimal late-time ringing and high peak amplitude. A stable radiation pattern obtained from 300 MHz to 2 GHz, makes it suitable for time domain as well as stepped frequency continuous wave (SFCW) GPR applications. The antenna is characterized by measurements of return loss, transmission scattering parameter, radiation pattern, gain, front-to-back ratio, and GPR B-scan with object buried under soil. The measured results are in good agreement with the simulation results.

A stub tapped compact hybrid coupler with broad-band harmonic rejection

This paper presents a modified, stub tapped hybrid ring coupler for broadband harmonic suppression. In the new design, a tapped stub is used to realize 900 transmission line sections of the conventional coupler. The slow wave effect and the stopband produced by shunt open stub are utilized to achieve the size reduction and harmonic suppression respectively. Explicit design equations are derived using the ABCD-matrix. A prototype coupler operating at 1.0 GHz is designed, fabricated and measured for demonstration. The circuit area of the proposed coupler is only 45.6% of that of the conventional one. In addition, the proposed coupler has spurious pass-band suppression of up to at least nine harmonics.

A simple method for measuring the dielectric constant of solids

This paper presents a simple method for determining the dielectric constant of a solid material. The method consists of measuring the return loss due to a slab of such material inserted into a rectangular waveguide. The dielectric slab presents a discontinuity inside the waveguide, and the electric field at each interface of the slab is described as a summation of weighted sinusoidal basis functions. The scattered magnetic field on either side of the slab is determined using the modal expansion approach. These weights are solved by using the method of moments on the boundary conditions of continuity of the tangential magnetic field at the interfaces. The component of the electric field (scattered) for the dominant TE/sub 10/ mode can then be determined in terms of these weights, and from this result the reflection coefficient can be theoretically evaluated. A comparison between the experimentally obtained reflection/transmission coefficient and the theoretical values provide a figure for the dielectric constant of the material.

A Modified Plane Wave Model for Fast and Accurate Characterization of Layered Media

The accuracy and computational speed of ground penetrating radar (GPR) largely rely on the inverse model implemented. In most cases, both accuracy and processing speed cannot be achieved together because of the inherent limitation of modeling GPR signal in complex media. Full wave models (FWMs) are most promising approaches to characterize multilayered media. However, they are inefficient due to the requirement of significant time for integration over singularity. In this paper, a modified plane wave model (MPWM) is proposed to achieve accuracy and better computational speed. The model is derived based on the analytical solution of an FWM. It is versatile for finding response due to multilayered media. The rigorous analysis has shown the similarity between the proposed model and FWMs with cross-correlation value close to unity across a broad frequency spectrum and large ranges of media parameters. Model inversion is achieved by a novel layer stripping technique followed by a gradient-based method. Testing of layered media in laboratory environment demonstrates that the proposed MPWM has significant advantage over FWMs.

GPR Modeling for Rapid Characterization of Layered Media

The success of a ground penetrating radar (GPR) signal modeling scheme largely depends on its accuracy and computational efficiency. Most of the modeling schemes suffer from inaccuracy because of unrealistic assumptions of complex GPR environment. In this respect full wave model (FWM) of GPR signal is a promising approach for accurate characterization of multi-layered media. However, large computation time of FWM compared to other simplified models makes the approach inefficient for real time application. In this work an FWM scheme is developed based on electric field equivalent magnetic current density at antenna phase center. The compact analytical expression of Greens function representing response due to layered media is derived. Then a plane wave model (PWM) is proposed by introducing a spreading factor based on simplified expression of the FWM. The model inversion is successfuly carried out by a gradient based algorithm. A stepped frequency continuous wave GPR in off-ground monostatic configuration is implemented in laboratory environment to verify performances of the models. Experimental analysis proves that the proposed PWM is as accurate as FWM, and its computation efficiency is enormous to detect layered media parameters.

Modeling GPR signal for fast and accurate characterization of layered media

Accuracy and efficiency are two important objectives of modeling ground penetrating radar (GPR) signal. Achieving both together is a driving factor for advanced research in the state of art for GPR community. Full wave model (FWM) promises great accuracy to detect layered media. However time efficiency of FWM is poor compared to other simplified models. This work presents a new plane wave model (PWM) based on simplified expression of an FWM. The proposed model is capable of detecting layered media in far field condition with great accuracy and efficiency. Based on this scheme a monostatic frequency domain GPR system is realized in laboratory environment. The model is validated by testing water layer and comparing accuracy and efficiency with an existing FWM scheme.

Selection of Ideal Feed profile for Asymptotic Conical Dipole Fed Impulse Radiating Antenna

Equivalent charge method is used to design ultra wide band asymptotic conical dipole (ACD) antenna. Combination of linear charge density and point charges is used to generate difierent proflles for ACD antennas. Two difierent proflles of ACD antenna are used as a feed for re∞ector based impulse-radiating antennas (IRAs). This paper focuses on the selection of ideal ACD proflle as well as the requisite charge distribution for ACD antenna as a feed to design 100› input impedance re∞ector IRA. An ideal Conflguration of ACD feeding structure for re∞ector IRA is chosen based on FDTD analysis results. To validate the utility of the proposed new feed an ACD-fed half IRA is realized with input impedance of nearly 50›. Measurements are carried out using single-ended instrumentation without any impedance adaptor as commonly done with Conventional IRAs.

System level realization and analysis of MEMS integrated voltage controlled oscillator

This paper presents the system level realization of a MEMS varactor based voltage controlled oscillator (VCO). The MEMS varactor has been optimally designed with Finite Element analysis in Coventorware platform. The corresponding lumped element version of the varactor has been exported to Saber Schematic platform. Circuit of the VCO has also been implemented in Saber platform, integrating the MEMS varactors in its tank circuit. Performance of the VCO has been analyzed with the MEMS capacitor of 439 fF and at a frequency of 2.4 GHz. The varactor structure has been fabricated and experimentally characterized for mechanical and electrical performance.

Miniaturized Planar 90° Hybrid Coupler with Unchanged Bandwidth Using Single Characteristic Impedance Line

A planar branch-line coupler which is entirely composed of transmission line of single characteristic impedance equal to the system reference impedance is presented. The low impedance series arms of the conventional coupler are replaced by their equivalent structures with 50Omega lines. A transmission-line model is derived to obtain the design curves. The main advantage of the present design is that the bandwidth of the coupler remains unchanged. In addition, the area of present coupler is only 50% and also the second harmonic is suppressed by at least 45 dB as compared to the conventional design. Full-wave EM simulation is used to confirm the design approach for the prototype coupler operating at 1.2 GHz.