In this paper, a method based on clonal selection algorithm (CLONALG) is proposed for null steering of linear antenna arrays by controlling only the positions of selected elements. The CLONALG is a relatively novel population-based evolutionary algorithm inspired by the clonal selection principle of the human immune system. In order to illustrate the accuracy and flexibility of the proposed algorithm, several numerical examples of Chebyshev pattern with the single and double nulls imposed at the directions of interference are given.

This work reports some recent advances in diffraction theory by canonical shapes like wedges or cones with impedancetype boundary conditions. Our basic aim in the present paper is to demonstrate that functional difference equations of the second order deliver a very natural and efficient tool to study such a kind of problems (For a thorough and up-to-date overview of the scattering and diffraction in general the readers are referred to a special section of the journal ``Radio Science'' edited by Uslenghi [1].). To this end we consider two problems: diffraction of a normally incident plane electromagnetic wave by an impedance wedge whose exterior is divided into two parts by a semi-infinite impedance sheet and diffraction of a plane acoustic wave by a right-circular impedance cone. In both cases the problems can be formulated in a traditional fashion as boundary-value problems of the scattering theory. For the first problem the Sommerfeld-Malyuzhinets technique enables one to reduce it to a problem for a vectorial system of functional Malyuzhinets equations. Then the system is transformed to uncoupled second-order functional difference-equations (SOFDE) for each of the unknown spectra. In the second problem the incomplete separation of variables leads directly to a functional difference-equation of the second order. Hence, it is remarkable that in both cases the key mathematical tool is an SOFDE which is an analog of a second-order differential equation with variable coefficients. The latter is reducible to an integral equation which is known to be the most traditional tool for its solution. It has recently been recognised that reducing SOFDEs to integral equations is also one of the most efficient approaches for their study. The integral equations which are developed for the problems at hand are both of the second kind and obey Fredholm property. In the problem of diffraction by a wedge the generalised Malyuzhinets function is exploited on the preliminary step then ``inversion'' of a simple difference operator with constant coefficients leads to an integral equation of the second kind. The corresponding integral operator is represented as a sum of the identical operator and a compact one [2]. However, in the second problem the situation is slightly different: the integral operator can be represented by a sum of the boundedlyinvertible (Dixon's operator) and compact operators. This situation was earlier considered by Bernard in his study of diffraction by an impedance cone, and important advances have been made (see [3-6]). The Fredholm property is crucial for the elaboration of different numerical schemes. In our cases we exploited direct numerical approaches based on the quadrature formulae and computed the farfield asymptotics for the problems at hand. Various numerical results are demonstrated and discussed.

A new CPW-fed antenna with triple-band is presented for simultaneously satisfying wireless local area network (WLAN) and world interoperability for microwave access (WiMAX) applications. The investigated antenna consists of a T-shaped monopole with a trapeziform ground plane and two parasitic elements to generate triple-band. The design methodology is outlined and the overall size is 32×15×1 mm³. This antenna was numerically designed using Ansoft HFSS simulation software package. The measured 10 dB bandwidth for return loss is from 2.35 to 2.71 GHz and 3.35 to 3.72 GHz and 4.9 to 6.1 GHz, covering all the 2.4/5.2/5.8 GHz WLAN bands and 2.5/3.5/5.5 GHz WiMAX bands.

This paper presents an effective modeling methodology for Ultra-wideband (UWB) antennas. The methodology is based on augmenting an existing narrow-band model with a macro-model while simultaneously perturbing component values of the narrow-band model. The narrow-band model is an empirical-based circuit and the macro-model described by rational functions is determined using data fitting approaches. The perturbation of component values of the narrow-band model is achieved by adjustments in SPICE. This method is demonstrated on the example of a 2.5 cm dipole antenna and a circular disc monopole antenna for UWB systems. Simulation results show that this methodology is effective over a wide bandwidth and suitable for modeling most UWB antennas.

This paper's aim is to classify cylindrical targets from their ultrawide-band radar returns. To calculate the radar returns, image technique formulation is used to obtain the Electric Field Integral Equations (EFIEs). Then, the EFIEs are solved numerically by Method of Moment (MoM). Because of wide frequency range of the ultrawide-band radar signal, the database to be used for target classification becomes very large. To deal with this problem and to provide robustness, wavelet transform is utilized. Application of wavelet transform significantly reduces the size of the database. The coefficients obtained by wavelet transform are used as the inputs of the artificial neural networks (ANNs). Then, the actual performances of the networks are investigated by Receiver Operating Characteristic (ROC) analysis.

Two different types of Vivaldi antenna arrays have been designed for UWB see through wall applications. The first is a 16×1 antipodal Vivaldi antenna covering 8-12GHz, and the second is an 8×1 tapered slot antenna for 2-4GHz frequency range. The array elements are optimized to have a compact size and almost constant gain with frequency. Wilkinson power dividers were designed and fabricated to compose the feed network for the Vivaldi antenna arrays. Measured results of the manufactured antipodal and tapered slot Vivaldi antenna arrays are in excellent agreement with the simulated ones, with a gain of more than 13dBi and 12dBi respectively within their respective operating band. The first array is geared towards see through dry wall with high resolution, while the second is designed at lower frequencies to allow see through concrete wall applications. Full arrays were manufactured and connected to multi-throw switches and have been utilized as part of synthetic aperture radar.

The noise effect is very challenging in radar target recognition. It usually degrades the accuracy of target recognition and then makes the recognition unreliable. In this study, we present a noise-reduction technique to improve the accuracy of radar target recognition. Our noise-reduction technique is based on the SVD (singular value decomposition). The PCA (principal components analysis) based radar recognition algorithm is utilized to verify our noise-reduction scheme. In our treatment, the received signals are arranged into a Hankel-form matrix. This Hankel-form matrix is decomposed into two subspaces, i.e., the noise-related subspace and clean-signal subspace. The noise reduction is obtained by suppressing the noise-related subspace and retaining the clean-signal space only. Simulation results show that the accuracy of target recognition is greatly improved as the received signals are first processed by the SVD noise-reduction technique. With the use of proposed noise-reduction scheme, the radar target recognition can tolerate more noises and then becomes more reliable. The noise-reduction technique in this study can also be applied to many other problems in radar engineering.

We present a new scheme for all optical multi-wavelengths switching and filtering using photorefractive materials to route optical signals without converting to electronic state. For this purpose the photorefractive effect which is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index is used. When a photorefractive material is illuminated by patterned command light of intensity I(x), a dynamic superimposed Bragg grating can be obtained which is used for optical multi-wavelength switching and filtering purposes.

In this article, a design for wideband dual-frequency folded dual monopole antenna is presented. By the proper choice of the dimensions of the Y-shape patch and the gap distance for the feeding structure, an additional resonant mode and wide impedance matching can be realized. Both simulated and experimental results, such as antenna impedance bandwidth, antenna radiation characteristic, and antenna gain have been presented and discussed. The lower impedance bandwidth covers from 8.78 GHz to 9.05 GHz, and the upper impedance bandwidth covers from 11.64 GHz to 12.01 GHz.

Optical and electrical properties of Er-doped Sinanocrystal (Si-NC) fiber amplifier are studied. Presence of Si-NC near Er³⁺ ions in silica matrix induces strong coupling mechanism and improves the efficiency of Er³⁺ excitation but in this process the size of Si-NC is important. We investigate effect of radius variation of Si-NC in the range of 2-6nm by studying the steady state and time resolved luminescence signals at 1.54 μm. We conclude that by limiting the range of Si-NC sizes the amplifier gain is improved. On the other hand Si-NCs may introduce optical loss mechanism, such as confined carrier absorption loss that affects the possibility of obtaining positive net gain. But this detrimental event can be affected by change in the radius of Si-NC as reported in this article.

Multiple diffraction propagation path loss due to the successively located different shaped building roofs are important for mobile communication. In the literature, building roofs are considered as wedge shaped structures. In this paper, building roof is modelled with a more realistic structure and the propagation path loss between the transmitting antenna and the receiving antenna is calculated.

In this paper six novel Dielectric Resonator Antennas (DRAs) providing Circular Polarization (CP) using single probe feeds are proposed. By splitting the fundamental mode of conventional rectangular or cylindrical DRAin to two near-degenerate orthogonal resonant modes, CP is obtained. The proposed antennas are numerically investigated using Finite Element Method (FEM). Parametric study on all antennas is carried out. The results show that the impedance bandwidth (S₁₁ < -10 dB) of all reported antennas is in the range of 112-140 MHz. Also, the Axial Ratio bandwidth (AR < 3 dB) range of presented antennas is 28-33 MHz. The investigation shows radiation patterns of all proposed antennas are remaining broadside throughout the bandwidth.

With the recent growth in the use of satellites for an increasing range of devices, services, applications and users, there is a need to optimize the signal received for availability, reliability and tolerance to interference. A lot of prediction models have been used in recent times to estimate intersystem interference due to hydrometeor scattering on vertically polarized microwave signals into the receiver of earth-space communications systems operating at the same frequency. The horizontal polarization is usually not investigated because coupling between the transmitting and receiving systems is much less than in vertical polarization. Much as this is true in the temperate regions, the nature and characteristics of tropical rainfall which are quite distinct from the temperate rainfall, means that the horizontal polarization when transmitted should be investigated for hydrometeor induced interference in tropical regions. This present work computes transmission loss on horizontally polarized signals based on two models (Awaka and Capsoni). The results obtained are compared and it is observed that there is only a difference of 1 dB in the transmission loss between the models. However, at higher frequency (>20 GHz), the Capsoni model does not produce values for the transmission loss L, while Awaka model predict a low interference at various antenna gain for percentages of time >0.1%.

In this letter,an application of energy balance method is applied to solve the nonlinear oscillators with uⁿ force. Comparison is made between the modification of harmonic balance method and energy balance method. The results reveal that the energy balance method is very effective and simple. Energy balance method is very effective and convenient and quite accurate to both linear and nonlinear physics and engineering problems.

In this paper, we design the zero-dispersion wavelength shifted fiber based on the WII-type triple clad single mode optical fiber and consider the transmission parameters fluctuations owing to environmental conditions such as temperature variations on dispersion behavior of fiber. In order to estimate the thermal coefficients, the model introduced by Ghosh [1] is applied. Our calculation show that the thermal coefficient extracted for the chromatic dispersion, its slope, and the zero dispersion wavelength swing are -1.21×10^-3 ps/km/nm/^oC, +2.96×10^-3 ps/km/nm²/^oC, and +3.33× 10^-2 nm/^oC at 1.55 μm respectively. It is shown that in optical fiber design especially for dense wavelength division multiplexing (DWDM) systems, effect of temperature on channel displacement is critical and should be considered carefully.

In this paper we introduce a rigorous numerical analysis to investigate the characteristics of double carrier multiplication homojunction avalanche photodiodes (APDs) considering the nonlocal nature of the ionization process in the wide range of multiplication region width. Also in our calculations the effects of dead space has been considered. Our analyses based on the history dependent multiplication theory (HDMT) and width independent ionization coefficient.