Ahad Tavakoli

Amirkabir University of Technology

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Parisa Dehkhoda

Amirkabir University of Technology

Mojtaba Dehmollaian

Safe Engineering Services & Technologies (Canada)

R. Moini

Safe Engineering Services & Technologies (Canada)

Abdolali Abdipour

Amirkabir University of Technology

Babak Honarbakhsh

Shahid Beheshti University

Farzad Tavakkol Hamedani

Semnan University

Gevork B. Gharehpetian

Amirkabir University of Technology

Behbod Ghalamkari

Islamic Azad University, Science and Research Branch

Ershad Mohammadi

École Polytechnique Fédérale de Lausanne

M. J. Sanjari

Griffith University

Cooperative Institutions

Amirkabir University of Technology

107

University of Tehran

Sharif University of Technology

Islamic Azad University, Tehran

Shahid Beheshti University

Iran University of Science and Technology

Chinese Academy of Sciences

K.N.Toosi University of Technology

University of Waterloo

École Polytechnique Fédérale de Lausanne

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An Iterative Modified Diffraction Tomography Method for Reconstruction of a High-Contrast Buried Object

IEEE Transactions on Geoscience and Remote Sensing (2018)

Maryam Hajebi Ahad Tavakoli Mojtaba Dehmollaian Parisa Dehkhoda

An iterative subsurface inverse scattering algorithm has been proposed to profile high-contrast 2-D dielectric objects buried under a lossy ground. The proposed iterative modified diffraction tomography (IMDT) method is based on the combination of the traditional iterative Born method and DT technique which is well known for its simplicity and robustness. In effect, IMDT is an iterative Born algorithm that utilizes the spectral domain concept of DT for solving the inversion problem. The proposed iterative approach results in removing the Born approximation's limitation of DT technique in dealing with high-contrast scatterers. To this end, the total field inside the reconstruction domain is renewed and then expanded into upgoing and downgoing plane waves in each iteration. Consequently, by exponential expansion of the fields and deriving a modified DT formulation, high-contrast targets are also efficiently reconstructed. To assess the proposed IMDT method, various high-contrast objects and noise conditions are studied. It is shown that the IMDT algorithm significantly outperforms the DT technique and is capable of reconstructing high-contrast objects efficiently and accurately even in noisy environments.

Diffraction tomography

Robustness

Lossy compression

10.1109/tgrs.2018.2827123

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Citations (26)

Improved modal method of moments technique to compensate the effect of wall dimension in shielding effectiveness evaluation

IET Science Measurement & Technology (2013)

Marzieh Nobakhti Parisa Dehkhoda Ahad Tavakoli

In the conventional modal method of moments (MoM) method, when shielding effectiveness (SE) of a rectangular enclosure is the goal, it is assumed that the incident wave is reflected from an infinite wall. This assumption makes the results inaccurate specifically at low frequencies where the wall size is not large compared to the illumination wavelength. In this study, the mentioned reflected wave is corrected by considering the reflection from a box without any aperture. To obtain the reflected wave from the box, an induced electric current on the metallic surface is assumed. Using Rao Wilton Glisson basis functions along with free space Green's function, the unknown electric currents and consequently the reflected wave are calculated. The results are compared with full wave numerical methods. A very good agreement is observed between the results and it is shown that for lower frequencies the proposed method has more accuracy rather than the modal MoM itself specifically for an enclosure with a very large aperture.

Enclosure

Aperture (computer memory)

Reflection

10.1049/iet-smt.2012.0103

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Citations (15)

Nonlinear and fully distributed field effect transistor modelling procedure using time-domain method

IET Microwaves Antennas & Propagation (2008)

Kambiz Afrooz Abdolali Abdipour Ahad Tavakoli Masoud Movahhedi

An accurate and efficient modelling approach for field effect transistors (FET) as nonlinear active transmission lines is presented. The nonlinear active multiconductor transmission line (NAMTL) equations are obtained by considering the transistor as three active coupled lines operating in a nonlinear regime. This modelling procedure accurately spots the effect of wave propagation along the device electrodes. This modelling approach is applied to an FET by solving the NAMTL equations using a finite-difference time-domain technique. The results of this model are compared with the semi-distributed (slice) model. This method produces more accurate results than the slice model, especially at high frequencies.

Distributed element model

10.1049/iet-map:20080246

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Citations (28)

Analysis of dual-arm logarithmic spiral microstrip patch antennas

Ahad Tavakoli A. Fard R. Moini

A dual-arm logarithmic spiral patch antenna is analyzed using the segmentation method and multiport network theory. Applying this approach, the antenna characteristics such as bandwidth, input impedance, scattering parameters, and radiation patterns are calculated. The theoretical and experimental results are in very good agreement with each other and show that the antenna is circularly polarized over a wide frequency band. The model along with an optimization routine can be used to design dual-arm spiral patch antennas.

Patch antenna

Multi-band device

Logarithmic spiral

Spiral antenna

10.1109/aps.1996.549783

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Performance Amelioration of an Active Integrated Spiral Antenna System for UWB Applications

International workshop on Antenna Technology: Small and Smart Antennas Metamaterials and Applications (2007)

Mahdi Jalali Abdolali Abdipour Ahad Tavakoli Gholamreza Moradi

This paper describes the implementation of an active integrated spiral antenna for UWB (ultra-wideband) applications. By integrating a low-noise amplifier with antenna, the voltage standing wave ratio (VSWR) can be kept low for a large bandwidth and this antenna indicate a 13 dB improvement in gain compared with one using a passive antenna with the same design, resulting in an improved spiral antenna performance for UWB. For the designed circuit in the UWB frequency range (3.1-10.6 GHz), the gain of the LNA is better than 13 dB, its noise figure is less than 3 dB, S ₁₁ and S ₂₂ are less than -13 dB, respectively

Spiral antenna

10.1109/iwat.2007.370140

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Breast tumour identification based on inverse scattering approach

IET Microwaves Antennas & Propagation (2013)

Nariman Firoozy Ahad Tavakoli

This study introduces an iterative evolutionary method in detection of breast tumour, identification of its size and location. At primary measurement step, a set of antennas capable of transmitting wide‐band Gaussian pulses, generated through the chirping technique, is placed around the breast and total wave received by each directional antenna is recorded in a bistatic manner for every cross section of the breast corresponding to a specific height for a full three‐dimensional scan. Each set of data for each cross section is then analysed separately. Measured fields are used in an inverse‐scattering problem, where the unknown is the tumour. To solve this problem, the finite‐difference time‐domain technique produces the total field resulted from the same incident wave as in measurement while illuminating an electromagnetic model of the breast, each time having a new tumour specification as a part of an evolutionary algorithm scheme. A comparison between various generated and measured reference data is repeated to reach a minimum cost function that corresponds to a minimum error in identification. Particle swarm optimisation and differential evolution are the two optimisations, each using all field elements of both TE z and TM z modes to reach convergence. The final results show an accurate identification of the tumour by this method.

Identification

10.1049/iet-map.2012.0618

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Citations (3)

Analysis of finite-microstrip structures using surface equivalence principle and multiple network theory (SEMN)

IEEE Transactions on Antennas and Propagation (2002)

Farzad Tavakkol Hamedani Ahad Tavakoli L. Shafai

Two techniques are developed for the analysis of finite microstrip structures. They are based on electric and magnetic field integral equation (EFIE and MFIE) formulations of the surface equivalent principle and multiple network theory (SEMN) method. Using pulse surface basis functions, the surface of each homogeneous dielectric body is modeled by small flat segments of arbitrary geometry and constant electromagnetic field. Using the surface equivalence principle and the Green's functions of the homogeneous space, the admittance and impedance matrices are computed. Then, the boundary conditions and multiple network theory are applied to determine the overall characteristic of the entire space. Several radiation and scattering examples are numerically analyzed and their calculated near and far fields are presented. The numerical results of the two formulations are then compared with measurement and those of HFSS and ENSEMBLE.

Admittance

Basis function

HFSS

10.1109/tap.2002.802268

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Citations (10)

A single-feed cross-shaped multi-band dual-polarized zeroth-order resonator microstrip antenna

Babak Honarbakhsh Ahad Tavakoli

Multi-band device

Patch antenna

10.1109/aps.2007.4395622

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Accessible Superchiral Near-Fields Driven by Tailored Electric and Magnetic Resonances in All-Dielectric Nanostructures

ACS Photonics (2019)

Ershad Mohammadi Ahad Tavakoli Parisa Dehkhoda Yasaman Jahani Kosmas L. Tsakmakidis

Detection and differentiation of enantiomers in small quantities are crucially important in many scientific fields, including biology, chemistry, and pharmacy. Chiral molecules manifest their handedness in their interaction with the chiral state of light (e.g., circularly polarized light), which is commonly leveraged in circular dichroism (CD) spectroscopy. However, compared to the linear refractive index molecular chirality is extremely weak, resulting in low detection efficiencies. Recently, it has been shown that these weak chiroptical signals can be enhanced by increasing the optical chirality of the electromagnetic fields interacting with chiral samples. Here, we show numerically and analytically that dielectric structures can provide an optimum chiral sensing platform by offering uniform superchiral near-fields. To illustrate this, we first study a simple dielectric dimer and show that circularly polarized light can induce parallel and out of phase electric and magnetic fields, which are spectrally and spatially overlapped, and therefore produce superchiral fields at the midpoint of the dimer. This behavior is in contrast to, for example, a plasmonic dimer, where the optical chirality is limited by the electric dipolar field, which is not completely out of phase with the incident magnetic field. With the insights gained from this analysis, next we develop an approach for overlapping electric and magnetic fields in a single particle, based on Kerker effect. In particular, we introduce a Kerker-inspired metasurface consisting of holey dielectric disks, which offers uniform and accessible superchiral near-fields with CD signal enhancements of nearly 24 times.

Chirality

10.1021/acsphotonics.8b01767

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Citations (117)

Improving power sharing and reduction circulating current in parallel inverters of isolated microgrids

A. A. Khodadoost Arani Ahad Tavakoli Gevork B. Gharehpetian Mohammad Seydali Seyf Abad

A new control scheme to improve in dynamic response of parallel inverters is proposed in this paper. The conventional droop method is a good choice for managing microgrid in a decentralized manner. The proposed control technique, based on the droop control method, uses current feedback signal and Power Angle of VSIs. This method is usually applied to achieve good active and reactive power sharing and decreasing circulating current in transient. In this work, a great improvement in transient response is achieved by introducing this new droop for voltage and frequency. Simulation results that validate the proposed controller are provided.

Microgrid

Transient (computer programming)

10.1109/sgc.2013.6733802

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Citations (11)