Kouroch Mahdjoubi

A new accurate design method for millimeter-wave homogeneous dielectric substrate lens antennas of arbitrary shape

The synthesis and the optimization of three-dimensional (3-D) lens antennas, consisting of homogeneous dielectric lenses of arbitrary shape and fed by printed sources, are studied theoretically and experimentally at millimeter(mm)-wave frequencies. The aim of the synthesis procedure is to find a lens profile that transforms the radiation pattern of the primary feed into a desired amplitude shaped output pattern. This synthesis problem has been previously applied for dielectric lenses and reflectors. As far as we know, we propose, for the first time, to adapt and implement it for the design of substrate lens antennas. The inverse scattering problem is solved in two steps. In the first one, the geometry of the 3-D lens is rigorously derived using geometrical optics (GO) principles. The resulting second-order partial-differential equation is strongly nonlinear and is of the Monge-Ampe/spl grave/re (M.A) type. The iterative algorithm implemented to solve it is described in detail. Then, a surface optimization of the lens profile combined with an analysis kernel based on physical optics (PO) is performed in order to comply with the prescribed pattern. Our algorithms are successfully validated with the design of a lens antenna radiating an asymmetric Gaussian pattern at 58.5 GHz whose half-power beamwidth equals 10/spl deg/ in H plane and 30/spl deg/ in E plane. The lens is illuminated by a microstrip 2/spl times/2 patch antenna array. Two lens prototypes have been manufactured in Teflon. Before optimization, the measured radiation patterns are in very good agreement with the predicted ones; nevertheless, the -12 dB side lobes and oscillations appearing in the main lobe evidence a strong difference between the desired and measured patterns. This discrepancy is significantly reduced using the optimized lens.

Analysis and design of a cylindrical EBG-based directive antenna

In this paper, a cylindrical electromagnetic bandgap (CEBG) structure composed of infinite metallic wires is analyzed, designed and used as a model to develop a new reconfigurable directive antenna. This structure is circularly and radially periodic, and it is excited at its center using an omnidirectional source. The analysis is based on calculating the transmission and reflection coefficients of a single cylindrical frequency selective surface (FSS) and then, considering only the fundamental mode interaction, deducing the frequency response of the CEBG structure composed of multiple cylindrical FSSs. For this structure, new analytical formulas are derived, and their accuracy is assessed compared to those obtained by the finite-difference time-domain method. As in rectangularly periodic structure case, the frequency response of the CEBG structure exhibits pass-bands and bandgaps, and it is possible to obtain directive beams by introducing defects in the periodic structure. Using this concept, a new antenna was developed to obtain a controllable directive beam. An antenna prototype, without control, was designed, fabricated, and tested. An excellent agreement was obtained between theory and experiment for both return loss and radiation patterns.

FOCUSING CHARACTERISTICS OF A METALLIC CYLINDRICAL ELECTROMAGNETIC BAND GAP STRUCTURE WITH DEFECTS

The focusing characteristics of 2D-Cylindrical Electromagnetic Band Gap (CEBG) structures constituted of metallic wires and with defects are analyzed numerically for directive antennas application. The introduction of defects into the periodic structures consists of removing one or multiple wires. The simulations were carried out with a Finite Difference Time Domain (FDTD) code, where the excitation is a line source and the CEBG structure is considered infinite in the vertical direction. Numerical results showing the effects of the number of cylindrical layers and of the number of defects are presented and discussed. These results allow to determine the structures giving best focusing performance and to obtain the frequency band for directive radiation.

Compact Expressions for Efficiency and Bandwidth of Patch Antennas Over Lossy Magneto-Dielectric Materials

Closed-form formulas for calculating the radiation efficiency and bandwidth of patch antennas over lossy magneto-dielectric materials are proposed by using the cavity model. These expressions agree well with simulations. It is well known that the bandwidth of microstrip antennas is enhanced when the dielectric substrate is replaced by a magnetic one. Here, we confirm more generally that not only the bandwidth, but also the radiation efficiency, can be improved when the permeability (μr) is greater than the permittivity (εr).

Dual-band CPW-fed G-antenna using an EBG structure

In this paper, a configuration is proposed to improve the performance of CPW antennas placed over an EBG material for dual band on body antenna applications. The aim was to improve the performance of such type of antennas by varying the position of the EBG with respect to the antenna. Simulation results show that the gain of the antenna is improved over the required frequency bands.

Highly tunable microwave stub resonator on ferroelectric KTa0.5Nb0.5O3 thin film

A coplanar waveguide (CPW) stub resonator has been fabricated on a pulsed-laser deposited KTa(0.5)Nb(0.5)O(3) (KTN) thin film (600 nm-thick) onto a r-plane sapphire substrate. It was designed to operate at 10 GHz when the applied bias voltage is zero. We show experimentally that the resonance frequency is shifted by 44% under a 70 kV/cm DC applied electric field. In addition, the dielectric characteristics of the KTN film have been assessed through post-processed measurements of CPW 50-Omega transmission lines using the conformal mapping method.

Improvement of the on-body performance of a dual-band textile antenna using an EBG structure

A new dual-band wearable textile antenna designed for body-centric communications is introduced. The antenna is fully characterized in free space and on the body model, with and without an electromagnetic band gap (EBG) substrate. The band gap array consists of 3 × 3 elements and is used to reduce the interaction with human tissues. With the EBG back reflector, the radiation into the body is reduced by more than 15 dB. Increases of 5.2 dB and 3 dB gain are noticed at 2.45 GHz and 5.5 GHz, respectively. The results are presented for the return loss, radiation pattern, efficiency, and specific absorption rate (SAR).

Analysis of radius-periodic cylindrical structures

The purpose of this communication is to present an analysis method for cylindrical structure which are periodic along their radii and are excited by a line source placed in the center. At first. we derive analytical formulas for the transmitted wave and the wave inside the structure of a single layer cylindrical structure, which allow to extract the characteristics of the cylindrical layer. Then we obtain the dispersion equation and the transmission coefficient of multiple layer periodic structures.

Bandwidth Enlargement of Planar EBG Antennas

A new structure based on a combination of two PRS (partially reflective surface) is introduced and studied analytically to improve the bandwidth of the FP and EBG antennas. The original procedure developed here leads to a design method for broadening the bandwidth of planar EBG antennas.

Multi-Layer Crystals of Metallic Wires: Analysis of the Transmission Coefficient for Outside and Inside Excitation

This paper proposes a new analysis of the transmission coefficient at normal incidence for 2-D periodic crystals (also called Electromagnetic Band Gap (EBG) structures), which are finite in the direction of wave-propagation and are composed of metallic wires. The crystal is considered as a set of parallel Partially Reflecting Surfaces (PRSs), whose transmission and reflection characteristics are obtained rigorously using the Finite Difference Time Domain (FDTD) method. The transmission coefficient of the EBG structure is then obtained by using a plane-wave cascading approach considering single mode interactions between PRSs. The accuracy of the results given by the hybrid method is assessed compared to those obtained directly by the Finite Difference Time Domain (FDTD) method. The minima and maxima envelops and the resonance frequencies of the transmission coefficient are studied, with analytical expressions, for both, excitation from outside and excitation from inside. A discussion is also presented concerning the strength of the coefficient greater than one obtained when the plane-wave source is inside the EBG structure. In addition, by using a transmission line model, a normalized version for this coefficient is proposed, which considers the available power by the source.