Ali El-Hajj's research while affiliated with American University of Beirut and other places

Publications (167)

Preprint
Full-text available
In this paper, a waveguide-based dielectric sensing technique for the measurement of the relative permittivity of planar low-loss materials is presented. The material under test (MUT), assumed to be thicker than one half of wavelength (λg/2), is sandwiched between the waveguide's aperture and a matched load. The reflected voltage magnitude is measu...
Article
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As virtual reality (VR) tools continue to improve, more fields are finding ways of implementing the technology to take advantage of training opportunities that reduce costs, alleviate logistical challenges , and more. Where humanitarian deminers must prepare for dangerous work, VR facilitates training that minimizes the danger while giving trainers...
Article
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This article presents a new method for the calculation of the dielectric constant of low-loss materials having a thickness of more than half a wavelength in the sample. The proposed method relies solely on the frequency values at which the reflection coefficient $S_{11}$ resonance occurs and does not require precalibration or shifting of the refe...
Article
Improving the ethical thinking and analysis of university students is becoming an essential goal of modern curricula and program designers in various disciplines. To support ethics learning via practical applications and decision-making scenarios, ethics instructors are adding innovative methods to their multidisciplinary courses by following creat...
Article
The current landmine clearance methods mostly rely on the manual use of metal detectors (MDs) and on the deminer’s experience in differentiating between the sounds emitted due to the presence of a landmine or of harmless clutter. This process suffers from high false-alarm rates, which renders the demining effort slow and costly. In this paper, we r...
Article
In modern and well-structured universities, ethics centers are playing a key role in hosting, organizing, and managing activities to enrich and guide students’ ethical thinking and analysis. This paper presents a comprehensive survey of the goals, activities, and administration of ethics centers, as well as their role in promoting ethical thinking...
Conference Paper
An antipodal Vivaldi antenna is designed for use in the ground penetrating radar (GPR) detection of landmines. Two such antennas, one transmitting and one receiving, are connected to an ultra-wideband GPR to record the responses of exact models of certain types of landmines. Later, a periodic structure converter is used to transform the antennas' l...
Conference Paper
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A simple method for the design of rectangular slotted waveguide antennas (SWAs) with specified sidelobe levels is briefly described, and is used to design an example SWA with 7 broadwall elliptical slots and sidelobes lower than −20 dB. A special corrugation geometry is then added to the SWA, where the uniform height of the corrugations controls th...
Article
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The Wireless Access in Vehicular Environments (WAVE) protocol stack has been recently defined to enable vehicular communication on the Dedicated Short Range Communication (DSRC) frequencies. Recent studies have demonstrated that the Control Channel (CCH) of the DSRC protocol on which all vehicular safety messages are sent might not provide sufficie...
Article
The smart behavior of cognitive receivers is based on an efficient spectrum sensing, leading to an opportunistic use of vacant resources. Therefore, an error-free detection of spectrum white spaces is required to enable all the adaptive functionalities of cognitive radios. Practically, the RF tunable frontend is a source of various RF impairments t...
Conference Paper
In this paper, a circularly polarized antenna operating at 1.575GHz, for GPS applications is designed. The antenna has a square patch, with cross slots to ensure Right Hand Circular Polarization (RHCP) over the operating frequency range. Electronic Band Gap (EBG) structures are incorporated between the patch and the ground plane to reduce the back...
Article
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http://www.hindawi.com/journals/ijap/2014/231027/
Article
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This paper presents multiband circularly polarized (CP) antennas for radio frequency identification (RFID). A coax-fed and a microstrip-line-fed antennas having optimized cross-slots in their patches are first designed for dual-band CP operation. The microstrip-line-fed design is then modified, by incorporating a U-shaped slot in its partial ground...
Article
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A cognitive transceiver is required to opportunistically use vacant spectrum resources licensed to primary users. Thus, it relies on a complete adaptive behavior composed of: reconfigurable radio frequency (RF) parts, enhanced spectrum sensing algorithms, and sophisticated machine learning techniques. In this paper, we present a review of the recen...
Article
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Spectrum sensing has been identified as the key step of the cognition cycle and the most important function for the establishment of cognitive radio. In this paper, a blind cyclostationary feature detector, which is based on the symmetry property of cyclic autocorrelation function (SP-CAF), is implemented and tested using universal software radio p...
Article
In this chapter, the concepts of Cognitive Radio (CR) and multi-dimensional spectrum sensing are introduced. Spectrum sensing methodologies, energy efficiency consideration, resources scheduling, and self-management and learning mechanisms in cognitive radio networks are also discussed. The entailed challenges of CR RF front-end architectures are l...
Article
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In this paper, two types of reconfigurable Dielectric Resonator Antennas (DRAs) are presented. The designs are based on rotating a Dielectric Resonator (DR), placed on the patch of the antenna, using a DC stepper motor connected to the DR to reconfigure the notch frequency, in the first, and the resonance frequency in the second design. The attaine...
Conference Paper
In this paper, a metamaterial unit cell with low effective permeability attained over a specific frequency band is used to form a planar single-layer metamaterial structure, which is utilized as a lens for broadside gain enhancement of a patch antenna. The proposed metamaterial lens consists of an 8-by-8 matrix of Split-Ring Resonators (SRRs), and...
Conference Paper
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This paper presents a Vlasov antenna with optimized reflector position and angle suitable for high power microwave applications. With the proposed configuration, the reflector is directly attached to the waveguide, which is an advantage and makes it simpler to radiate in the direction of the axis of the waveguide. Bevel-cut and Step-cut Vlasov ante...
Conference Paper
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This paper presents a novel design of a reconfigurable miniaturized planar spiral monopole antenna suitable for TVWS applications. The proposed antenna is designed on a FR4 epoxy substrate fed by a microstrip line over a partial ground plane. This antenna, designed for the high frequency of the UHF band (600-800 MHz), operates with 1dB transmission...
Conference Paper
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This paper presents an improved Vlasov antenna with a novel cut shape suitable for high power microwave applications. The curved shape of the proposed cut totally eliminates the sharp edges and angles present in Vlasov antennas with Step and Bevel cuts. A Vlasov antenna, designed for operation at 3 GHz, is used to compare the three cut types. In ad...
Chapter
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An increasing demand for radio spectrum has resulted from the emergence of feature-rich and high-data-rate wireless applications. The spectrum is scarce, and the current radio spectrum regulations make its use inefficient. This necessitates the development of new dynamic spectrum allocation policies to better exploit the existing spectrum. Accordi...
Article
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Slotted waveguide antenna arrays offer clear advantages in terms of their design, weight, volume, power handling, directivity and efficiency. Slots with rounded corners are more robust for high power applications. This paper presents a slotted waveguide antenna with elliptical slots made to one broadwall of an S-band rectangular waveguide. The ante...
Conference Paper
In this paper, a compact quad-band antenna based on Open Loop Resonators (OLRs) is presented. The antenna is microstrip line-fed, and is printed on a 40 mm × 35 mm × 1.6 mm FR4-Epoxy substrate over a partial ground plane. The incorporation of each OLR contributes a resonance that depends on the OLR's design frequency and electrical length. Ansoft H...
Conference Paper
In this paper, a simple method to estimate the relative permittivity εr and the tangential loss tan(δ) of wearable textiles is presented. Quarter-wavelength stub-line band-stop filters, based on jeans and cordura textiles, are designed assuming reasonable initial values of εr and tan(δ), fabricated and experimentally tested. The filter designs are...
Conference Paper
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The design of an ultra-wideband (UWB) dielectric resonator antenna with reconfigurable band rejection is presented in this paper. The induced band notch helps to limit interference to a narrowband service operated inside the UWB frequency range. The proposed antenna is based on a cylindrical dielectric resonator (DR) with a relative permittivity of...
Conference Paper
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The design of a simple small-size multi-band antenna for wireless local area net-work (WLAN) and worldwide interoperability for microwave access (WiMAX) application is presented in this paper. The antenna covers the 2.4/5.2/5.8-GHz WLAN operating bands and the 2.5/3.5/5.5-GHz WiMAX bands. The proposed printed-type antenna is based on a 1.6 mm-thick...
Conference Paper
A simple frequency-tunable antenna for cognitive radio is proposed. It features a narrow bandpass filter, rendered tunable with a single varactor, and embedded in the feed line of an ultrawideband antenna. A modified version capable of both ultrawideband and tunable narrowband operations is also presented. Due to the topology of the antenna, the re...
Conference Paper
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Remote Laboratory Experimentation is a technique used in modern engineering laboratories to help academic researchers and students perform laboratory experiments remotely through the internet. Remote control of experiments is gaining more importance in training and education. However, remote real-time training on instruments programming still have...
Article
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Spectrum scarcity has emerged as a primary problem in the communications technology. The combination of cognitive radio (CR) and ultra-wideband impulse radio (UWB-IR) has been proposed to solve the shortage problem by allowing smart and adaptive spectrum management, leading to UWB-CR. In a UWB-CR scheme, secondary users are supposed to ensure inter...
Conference Paper
In this paper, a frequency-tunable microstrip antenna, for cognitive radio applications, is proposed. The approach, presented herein, is based on electronically adjusting the resonant frequency of a band-pass filter, which is integrated into a wideband antenna, to achieve a tunable narrowband frequency operation for the antenna. For this sake, the...
Article
The paper presents the design and the implementation of a reduced-size antipodal Vivaldi antenna with a reconfigurable band notch. The antenna could be mainly used for UWB applications since it covers a very wide frequency band. It is also suitable for Cognitive Radio (CR) systems because it is capable to sense the spectrum to determine the bands u...
Article
This paper presents a frequency reconfigurable printed antenna obtained from the modification of a simple ultrawideband (UWB) design. The initial UWB version is based on a patch with round corners, for smoother current flow, and a microstrip-line feed. The ground plane is partial and rectangular in shape. The used substrate is a Rogers RO3203 board...
Article
This paper presents a simple technique for determining the permittivity of a textile material, and the use of this material in the design of a wearable antenna based on Electromagnetic Band Gap (EBG) structures. Finding the permittivity of some types of textile is usually a challenge. Knowing the exact permittivity value is very important to preven...
Article
Full-text available
The design of filter antennas with reconfigurable band stops is proposed. They are meant for employment in ultrawideband cognitive radio (UWB-CR) systems, where unlicensed users communicate using adaptive pulses that have nulls in the bands used by licensed users. Neural networks or circuits implementing the Parks-McClellan algorithm can generate s...
Article
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Abstract-This paper presents a frequency reconfigurable cedar-shaped fractal antenna. The special shape of the patch makes it simpler to integrate RF switches to connect consecutive branches. The proper activation/deactivation of the switches alters the current flow and changes the resonance frequency. Simulated and measured results show the charac...
Article
In this paper, a polarization reconfigurable microstrip antenna, for WLAN applications, is proposed. The antenna structure is based on a reconfigurable feed network, which drives a circular patch. The approach, presented herein, is based on electronically reconfiguring the phase difference, between the antenna’s feeding ports, to achieve polarizati...
Conference Paper
In this paper, a modified Finite Volume Control (FVC) method is introduced and used to solve for the fields in a 3D rectangular waveguide. The FVC method finds its main applications in Computational Fluid Dynamics (CFD) problems and has been rarely used in Computational Electromagnetics (CEM). The 3D rectangular waveguide under consideration is cou...
Conference Paper
This paper presents a planar antenna for employment in overlay and underlay ultra-wideband cognitive radio (UWB-CR). The antenna can be operated as ultra-wideband, for underlay CR and for sensing in the overlay CR. When used in the overlay CR mode, the proposed antenna can selectively have one, two or three notches in the WLAN, WiMAX, and U-NII ban...
Article
This article describes a spreadsheet tool for the design of major antenna elements and antenna arrays. The antenna elements included are the dipole, circular loop, long wire, rectangular and circular patch antenna. Linear, circular, planar, and cylindrical array types can also be simulated. It is also possible to study an antenna behavior for given...
Article
No security mechanism has been deemed secure enough. Thus, the research into application security has ventured into the alternative technologies that consider the user aspects of implementation. In this paper, we present an enhanced approach to password security based on Key-Pattern Analysis (KPA). Our approach relies on its two focal pillars, inte...
Article
Full-text available
This paper presents a frequency reconfigurable microstrip antenna for operation in the ISM, WiMAX and U-NII bands. The antenna, which is printed on a 1.6-mm-thick FR4 epoxy substrate, originally resonates at 2.4 GHz (ISM band), where it has good omnidirectional patterns. To obtain resonances in the 3.5 and 5.5 GHz bands, two matching stubs have bee...
Article
A polarization-reconfigurable slot antenna, for WLAN applications, is presented in this paper. The antenna incorporates a reconfigurable Wilkinson power divider-based feed network, which couples electromagnetic energy to a decagonal slot in the ground plane. The approach, presented herein, is based on electronically reconfiguring the phase differen...
Conference Paper
Full-text available
Ultra-wideband cognitive radio (UWB-CR) features the use of UWB transmission as the enabling technology for cognitive radio, in both the underlay and overlay spectrum sharing approaches. In this paper, we propose an antenna that is suitable for underlay and overlay UWB cognitive radio. The antenna can be operated as ultrawideband, for underlay CR a...
Article
Full-text available
In this paper, an ultra-wideband antenna based on an egg-shaped conductor and a co-planar waveguide (CPW) feed is presented. The ground plane incorporates an egg-shaped slot with parametrized center and dimensions. By resizing the slot and moving its center, a slotted or a partial ground can be obtained. Based on this configuration, a parametric st...
Conference Paper
A planar antenna for employment in overlay and underlay ultra-wideband cognitive radio is presented in this paper. The antenna can be operated as ultra-wideband, for underlay CR and for sensing in the overlay CR. When used in the overlay CR mode, the proposed antenna can selectively have one, two or three notches in the WLAN, WiMAX, and U-NII bands...
Conference Paper
Full-text available
This paper presents a simple antenna system design for cognitive radio applications. The system comprises two printed monopoles sharing a common partial ground plane. The first monopole is used for channel sensing over the ultra-wideband (UWB) frequency range. The second one is frequency-reconfigurable and is used to communicate over a selected fre...
Conference Paper
A novel circularly polarized microstrip antenna that incorporates a decagonal slot in the ground plane is presented in this paper. The antenna is based on a 0.762-mm-thick Rogers RO3203 substrate with dimensions 5 cm × 8 cm, and employs a Wilkinson power divider as a feeder. The proposed antenna is designed and simulated using Ansoft HFSS, a Finite...
Conference Paper
In this paper, two designs of an ultra-wideband antenna with controllable band notch are presented. The first design is based on one complementary split-ring resonator (CSRR), whereas the second has two slightly-different conjoined CSRRs. By mounting an electronic switch across the slot of a CSRR, this latter can be activated/deactivated, thus offe...
Conference Paper
Opportunistic beamforming is investigated in the context of multicell OFDMA uplink scheduling. A low complexity scheduling algorithm for utility maximization with opportunistic beamforming is presented. Beams are formed in a pseudorandom fashion thus allowing users to opportunistically benefit from the antenna gain and interference rejection capabi...
Conference Paper
In this paper, a low-cost multiband PCB antenna that employs Koch fractal geometry and reconfigurability is presented. The antenna is fabricated on a 1.6 mm-thick FR4-epoxy substrate with dimensions 4 cm × 4.5 cm, is microstripline fed and has a partial ground plane flushed with the feed line. The proposed antenna is simulated using the Finite-Elem...
Conference Paper
The design of a frequency and pattern reconfigurable monopole-type printed antenna is presented in this paper. The antenna features a circular patch fed using a microstrip line, a shape-optimized partial ground plane, and two PIN diodes mounted over two slots in the ground plane. Three switching cases are considered. The first results in a single-b...
Conference Paper
Full-text available
In this paper, two designs of an ultra-wideband antenna with reconfigurable band notches are presented. The first design is based on several nested complementary split-ring resonators, whereas the second has two identical split-ring resonators. Electronic switches mounted across or along these resonators serve to activate or deactivate their corres...