In this paper, an accurate modeling procedure for GaAs MESFET as active coupled transmission line is presented. This model can consider the effect of wave propagation along the device electrodes. In this modeling technique the active multiconductor transmission line (AMTL) equations are obtained, which satisfy the TEM wave propagation along the GaAs MESFET electrodes. This modeling procedure is applied to a GaAs MESFETs by solving the AMTL equations using Finite-Difference Time-Domain (FDTD) technique. The scattering parameters are computed from time domain results over a frequency range of 20-220 GHz. This model investigates the effect of wave propagation along the transistor more accurate than the slice model, especially at high frequencies.

A rigorous semi-analytical solution is presented for electromagnetic scattering from an array of parallel-coated circular cylinders of arbitrary radii and positions due to an obliquely incident TM_z plane wave excitation. In order to check the validity of this technique, the radar cross-section of a single coated cylinder, a linear array of cylinders, and an arbitrary position array of cylinders are calculated and compared with available data in the literature. Furthermore, the near field is calculated to prove the validity of the boundary conditions on the surface of any cylinder with obliquely incidence wave. As an application, circular metamaterial cylinders are used to show the effect of metamaterial characteristics in altering the forward and backward scattering and in focusing the near field around the objects.

Impedance-Matching technique is in common use for antennas to broaden their bandwidth. Its application in hightemperature superconducting microstrip antennas is studied theoretically in this paper. It is found that employing an impedance-matching network directly to HTS microstrip antennas to broaden their bandwidth is of little significance.

A novel and compact ultra wideband (UWB) bandpass filter (BPF) with two transmission zeros near both passband edges of lower and higher frequency is proposed by using a new structure of fork-formresonators. The fork-formresonator generates a attenuation pole at the higher passband edge, lower insertion loss, wider bandwidth and compacter dimension, as compared with the traditional parallel unilateral-coupled resonator. A microstrip bandpass filter cascaded by two stages fork-formresonators with a 3-dB fractional bandwidth of 128% (from1.0 GHz to 4.6 GHz) is designed, fabricated, and tested. The measured characteristics of the filter agree with the theoretical simulations, and the measured results show good specifications which are very low insertion loss 0.5±0.3 dB within the passband and good return loss less than −15 dB from1.5 GHz to 4.0 GHz, respectively.

A method is presented to calculate the monostatic Radar Cross Section (RCS) of an electrically large perfect conducting cylinder vertically located over a dielectric half-space using Physical Optics (PO) technique. The four-path modal method is used to approximate the influence of the half-space to the scattering mechanism. The comparison between the results calculated by this expression and that by Moment Method (MOM) show that the expression is effective and efficient.

In this paper, the linear and nonlinear applications including optical filtering and switching of superimposed Bragg grating are presented. For realization of superimposed Bragg grating electrooptic effect is used. The introduced system acts as an optical chip. The induced superimposed index of refractions due to sampled electric potentials applied through metallic strips on electro-optically active core-cladding are investigated analytically and simulated numerically using the Transfer Matrix Method (TMM). It is shown that the applied electric field induces superimposed refractive index grating, which can be controlled using amplitudes and frequency contents of potential samples as well as optical waveguide parameters. Our proposed structure is analog programmable device for realization of many interesting optical signal conditioners such as optical filters, optical beam splitters, and many other special transfer functions in linear case. The proposed device is tunable and can be controlled using the applied potential parameters (samples) and easily satisfy dense wavelength division multiplexing (DWDM) system demand specifications. The electro-optic Pockels effect for generation of the superimposed gratings in this building block will be used. Then we propose an optical chip for performing the introduced functions. In practical cases, for realization of DWDM demands, we need very large number of potential samples approximately 3 to 4 orders of magnitudes. So, this type of block as optical controllable chip really from practical point of views is impossible and illegal. In this paper, we will present a simple approach for decreasing the number of efficient control samples from outside for managing the proposed tasks. Our calculations in this paper shows that with less than approximately 200 control pins, we can realize all of proposed practical ideas with acceptable precision. Also, with 3 samples per period, our design will cover 215 individual DWDM channels theoretically from 1.55um towards lower wavelengths and 325 channels for 4 samples per period case, which is infinity from practical point of views. All of transfer functions corresponding to these channels can be manipulated using applied potential samples. Also, as nonlinear applications of the superimposed Bragg grating multi-wavelength optical switching is presented. For this purpose the switching operation is illustrated first and then switching thresholds in the case of three predefined wavelengths are shown. Thus we illustrate numerical results for demonstration of the ability of the proposed structure. At the same time, we investigate effects of the parameters of the proposed structure such as the nonlinear refractive index and the grating length (number of layers) on switching performance including threshold intensity and slope of transition function. The proposed structure can be used as multi-wavelength switching applicable to DWDM and multi wavelength communication systems.

Helical antenna and planar inverted-F antenna (PIFA) are two commonly used handset antennas. This paper presents a comprehensive study on the performance of a dual band PIFA and a dual band helical antenna designed for operating in GSM900 and DCS1800 frequency bands. Radiation patterns and VSWR of these antennas are computed in free space as well as in the presence of head and hand. The specific absorption rate (SAR) of the helical antenna is calculated and compared with that of the PIFA handset antenna. The peak average SAR in the head is compared with SAR limits in the safety standards and so the maximum radiation power of each antenna is determined. In addition, radiation efficiencies of these handset antennas are computed in the presence of head and hand. All numerical simulations are performed using the Ansoft HFSS software. Numerical simulations results are in good agreement with published measurement results.

Backscattering Radar Cross Section (RCS) of electrically large targets is analyzed using Physical Optics (PO) approximation. The targets are located in a dielectric half-space,and modeled with Nonuniform Rational B-spline (NURBS) surfaces. The influence of the half-space is considered by the "four-path" model approximation. Results show the validity of the method.

An analytical model of a composite dielectric presented in this paper is the extension of Maxwell Garnett formulation. It takes into account the simultaneous statistical (Gaussian) distribution of conductivity and aspect ratio of inclusions. The inclusions are randomly oriented elongated conducting spheroids at concentrations below the percolation threshold. The formulation presented herein is limited to microwave frequencies. However, taking subtle frequencydependent effects that play important part at optical frequencies into account is straightforward. Some results of computations of microwave complex effective permittivity of composites with different input parameters have been obtained using analytical and numerical integration in Maple 10 software. It is shown how the parameters of the distribution laws - mean values and standard deviations of aspect ratio and conductivity - affect the resultant complex effective permittivity. The results of computations demonstrate that the most important factors affecting frequency characteristics of microwave effective permittivity are the mean values of the aspect ratio and conductivity. As for the standard deviations of aspect ratio and conductivity, their effects are the most noticeable in the transition between the static and optical limits of the Debye characteristic for the effective permittivity. There is almost no effect in the static and "optic" regions of the Debye curves.

A new hybrid technique for optimization of a multivariable function is proposed. This method is applied to the problem of complex time Green's function of multilayer media. This technique combines Particle Swarm search algorithm with the gradient based quasi-Newton method. Superiority of the method is demonstrated by comparing its results with other optimization techniques.

An approximate perturbative technique for the analysis of electromagnetic scattering from dielectric bodies of arbitrary shape containing dielectric inclusions, illuminated by an arbitrarily polarized incident plane wave, is investigated. The perturbative approach here presented allows for the efficient computation of the scattering properties of a given body as the inclusions vary, with a formulation solving only for the inclusion bound field component.

In this paper a novel method for ultra-high-resolution, compact and tunable optical displacement sensor using ring resonator and electromagnetically induced transparency (EIT) is proposed. The introduced technique uses ring resonator as an interesting element including high quality factor. We show that the proposed sensor can easily detect well below nanometer ranges. It is shown that the proposed idea of using EIT the resolution of the sensor is so high for displacement below mm range and for ranges larger than mm the sensitivity of the proposed sensor in both EIT and traditional cases is same approximately. Also, the proposed sensor is optically tunable. So, depends on required resolution optical control field can be used to tune the sensitivity of the proposed device.

This paper presents a novel modified Printed Tapered Monopole Antenna (PTMA)for ultra wideband (UWB)wireless communication applications. The proposed antenna consists of a truncated ground plane and two-tapered radiating patch separated by a slot (air gap)of different slopes, which provides a wideband behavior and relatively good matching. Moreover, the effects of a modified T-shaped slot inserted in the first tapered patch, on the impedance matching is investigated. The antenna has a small area of 23 × 26.5mm² and offers an impedance bandwidth as high as 100% at a centre frequency of 7.2 GHz for S₁₁ < −10 dB, which has an area reduction of 15% and a frequency bandwidth increment of 72% with respect to the previous similar antenna. The presented antenna covers the 5.2/5.8 GHz WLAN and 5.5 GHz WIMAX operating bands. Numerical analysis using Ansoft HFSS and measurement results is also presented in the paper.

One of the most important functions of radar remote sensing is to retrieve the soil moisture and surface parameters where surface parameters generally includes soil surface roughness and texture of soil (i.e.,% of coarse sand,silt and clay). Variation of soil moisture and surface parameters changes the soil permittivity, and affects the observation of the radar wave scattering (σ0). How to invert the moisture and surface parameters from radar data has been one of the most interesting problems to be resolved. Still,v ery few reported work is available to retrieve the soil textures with radar data. Therefore,in present paper an attempt has been made to retrieve the soil textures with soil moisture and surface roughness from Synthetic Aperture Radar (SAR) data. In this case number of variables are more and it is difficult to invert and retrieve the various parameters. To overcome this difficulty,an approach based on Genetic Algorithm (GA) with inclusion of empirical modeling has been proposed to retrieve the soil moisture,roughness and soil texture with ERS-2 (European Remote Sensing) SAR (Synthetic Aperture Radar) data of Haridwar region of India. The retrieved surface parameters and moisture content with proposed approach show quite good agreement with observed values of soil moisture and surface parameters. This study infers that modeling with GA has great potential to retrieve several variables simultaneously with good results.

One of most difficult challenges in simulating hybrid RF systems by finite-difference time-domain (FDTD) method is to construct the models for active and lumped elements. In this article, by combining the circuit equations with an integral transform, an enhanced FDTD method is proposed to model the hybrid lumped network which consists of nonlinear and high order linear elements with arbitrary connections. Based on this model, an active radiation system is analyzed. And it shows the application extension of the traditional FDTD method.

A triple-band omni-directional antenna which comprises three pairs of dipoles placed back to back and printed on a dielectric substrate is presented. A prototype is constructed and tested. The experimental results show that the 10 dB return loss bandwidth (VSWR < 2.0) in 2.4 GHz, 5.2 GHz and 5.8 GHz reaches as much as 130 MHz, 500MHz and 200 MHz, respectively. Moreover, the radiation patterns are almost omni-directional in the azimuthal plane. Peak antenna gain is 1.63 dBi, 2.36 dBi and 1.54 dBi, which indicate that the proposed antenna can be used as a triple-band antenna for the WLAN application.

Recently, a marching-on in degree finite difference method (MOD-FDM) was employed in the finite-difference time-domain (FDTD) formulation to obtain unconditionally stable transient responses. The objective of this work is to implement a plane wave excitation in the MOD-FDM formulation for scattering problems for an open region. This formulation has volume electric and magnetic current densities related to the incident field in Maxwell's equations explicitly. Numerical results computed by the proposed formulation are presented and compared with the solutions of the conventional FDTD method.

Besides the return loss and radiation pattern, dispersion characteristic of the antenna is one of the most important factors which should be considered in ultra wideband applications. In this paper, dispersion behavior of two specific Vivaldi antennas has been fully investigated in both frequency and time domains. All simulations are carried out by CST MS software. Moreover, by fabricating the antennas, the simulation results are verified by experimental data.

The transmission line techniques are generalized to measure the permittivity function of one-dimensional inhomogeneous dielectrics. A rectangular waveguide is used and the dielectric slab under test is placed at the input flange of it. Then the measured scattering parameters or the short-circuited reflection coefficient are used to extract the permittivity function of dielectric. To solve the problem an optimization-based procedure is used. The usefulness of the proposed method is verified using a comprehensive example.

A new dual-mode microstrip bandpass filter with wide stop-band is presented using the square loop resonator with treeshaped patches attached to the four inner corners of the loop. The mode splitting is realized by introducing a small cut locating at a 45^◦ offset from its two orthogonal modes. It is shown that the dual-mode filter has a wide stop-band including the first spurious resonance frequency. The center frequency can be tuned. Moreover, the proposed filter has a smaller size compared with conventional dualmode bandpass filters at the same central frequency.