A moment method analysis of a broad wall slot coupled crossed rectangular waveguide junction has been presented in this paper. The coupling slot is longitudinal/transverse and offset from the centre lines of the guides. The integral equations, governing the characteristics of the device, have been obtained using Multiple Cavity Modeling Technique, taking into account of the finite wall thickness and the TE00 mode at the slot apertures and have been solved using Moment Method to obtain the aperture distribution. The normalized resonant lengths/complex S-parameters have been calculated from this field distribution for different guide height/slot offsets/slot widths/slot thickness of the longitudinal/transverse slot over the frequency band. Numerical data, thus obtained, have been compared with measured/literature available/CST Microwave Studio Simulated data. The theory has been validated by the reasonable agreement obtained between them. It has been shown that, neglecting the TE₀₀ mode at the slot apertures can adversely effect the estimation of the resonant frequency and equivalent network parameters.

In many cases it is desired to have both high capacity and small antennas in wireless communication systems. Unfortunately, the antenna performance deteriorates when the antennas get electrically small. In this paper, fundamental limitations from antenna theory and broadband matchingare used to analyze fundamental limitations on the spectral efficiency of an arbitrary antenna inserted inside a sphere.

Cellular communications using high altitude platforms will predominate the existing conventional terrestrial or satellite systems. Radio coverage of HAPs is an important issue that affect the cellular system design, therefore a deep analysis of the HAP cell footprint will be presented in two coverage models and the cell parameters are also deduced for subsequent cellular design. In the design stage, a cell generating algorithm is introduced to define their locations more accurate. When deploying the cellular system with such algorithm, the cells have proper overlap and interspacing.

Cellular communications using high altitude platforms (HAPs) will predominate the existing conventional terrestrial or satellite systems but requires some optimization especially in the cellular radio coverage design. The design of HAPs cellular system suffers from increasing cell area when directing the same beam to cover lower elevation regions and therefore requires some modification in its location. In this paper either spot-beam antennas or antenna phased arrays are used in the radio coverage which are optimized in directing their beams to satisfy mostly uniform cellular layout with minimal coverage problems such as coverage gaps between cells or excessive cells overlap. The cells locations determination is done through proposed algorithm that takes into consideration the cell area increase at lower elevation angles and provides mostly uniform radio coverage especially at the cell edges.

Smart Antenna system analysis presented with multipath and null constraint for reducing interference and efficient use of spectrum with the help of LMS algorithm for GPS (Global Positioning System) System. A new simple DOA (Direction of Arrival) estimation method by rotation of antenna plane proposed. Simulated Result obtained using MATLABTM.

Stacked microstrip antennas deserve special attention due to their advantageous properties like dual frequency operation and wide bandwidth. In the present communication a theoretical model for single stacked microstrip disc antenna is proposed using extended cavity model. The method of analysis by this model is easier and intuitive than the full wave analysis. Single stacked microstrip disc antenna with co-axial feed locations at different radial positions is analyzed with this model taking different ratios of patch sizes. Antenna properties like return loss, input impedance, gain and radiation efficiency are calculated with the proposed model different cases and compared with the simulated and experimental results. The results are in fairly good agreement.

In this paper we propose a new method to enhance the gain of rectangular waveguide antenna arrays using the double negative medium (DNG) structure composed of strip wires (SW) and split ring resonators (SRR). The electromagnetic parameters of the DNG structure are retrieved and the rectangular waveguide antennas with and without the DNG structure are studied using numerical simulation method. The simulation results show that the DNG structure can congregate the radiation energy when the index of refraction approximates zero,since that the gain of the antenna arrays is enhanced and the radiation performance of the antenna arrays is effectively improved. Far-field radiation patterns are measured, which indicate that this method is effective to enhance the gain.

In this paper an edge coupled microstrip coupler with defected ground structure is presented. A normally 7 dB coupler designed on Alumina substrate is converted into a 3 dB coupler by cutting single rectangular slot in the ground plane encompassing the two transmission lines. Other properties of backward wave coupler remain the same, except for a tighter coupling. With this method of design optimization, it will be possible to fabricate a 3 dB coupler in compact form without strain in fabrication process. The structure is analyzed considering magnetic and electric coupling between the two transmission lines. Simulation based studies show reasonable agreement between analytical results and corresponding simulation results.

In this paper a simple model has been introduced to simulate the propagation of signal in a so called edged microstrip transmission line (EMTL). EMTL is a transmission line in which the signal strip is laid on the edge of the structure (Fig. 1). First a simple structure of EMTL is modeled with an ordinary MTL with improved per unit length inductances and capacitances, and an additional resistance to represent the radiation from the edges. This method is then applied to model a multilayer cross orthogonal EMTL structure as shown in Fig. 2. The model is finally validated using full wave analysis simulator, HFSS. The S-parameters of our model show good agreement with the results of the full wave analysis (HFSS) up to some GHz.

This paper presents an effective medium approach to calculate the attenuation and phase constants of modes in a 3D connected wire medium both below and above the plasma frequency. Physical and nonphysical modes in the structure are identified for all the important lattice directions. According to this, the medium behaves as an isotropic material in the vicinity of the plasma frequency. These results were compared with the numerical simulation and it was observed that the wave spreads below the plasma frequency along all the important lattice directions with the same attenuation constant. This implies isotropic behavior of the 3D wire lattice below the plasma frequency, and thus this medium can be considered as an isotropic negative permittivity medium.

Standard spectral-domain method (SDM) is one of the popular approaches to analyze frequency selective surfaces (FSS). However, it is inherently incapable of handling normal incidence because of its dubious definition of excitation fields, reflection and transmission coefficients using z-component of vector potentials. Moreover, as far as the author knows, it has never been applied to analyze FSS with gangbuster arrays. In this paper, an improved SDM, the vector spectral-domain method, is presented. By proving the equivalence of the spectra of unit cell current and element current, the scattered field from FSS structures is formulated in terms of spectraldomain element current instead of spectral-domain unit cell current. Galerkin's method is applied to obtain the unknown induced surface current. Well-established definition of excitation fields, reflection and transmission coefficients is adopted. Extensive experimental validation has been conducted.

Concentric circular antenna array (CCAA) has interesting features over other array configurations.A uniform arrangement of elements is assumed where the interelement spacing is kept almost half of the wavelength and the array parameters such as the steering matrix and gain are determined.The array performance such as beam power pattern, sidelobe level and beamwidth are discussed in two cases of central element feeding.The two cases are compared showing the reduction in the sidelobe level to more than 20 dB in the case of central element feeding without extra signal processing especially for smallsized arrays that have smaller number of elements and rings.

A rigorous semi-analytical solution is presented for electromagnetic scattering from an array of circular cylinders due to an obliquely incident plane wave. The cylinders are illuminated by either TMz or TEz incident plane wave. The solution is based on the application of the boundary conditions on the surface of each cylinder in terms of the local coordinate system of each individual cylinder. The principle of equal volume model is used to represent cylindrical cross-sections by an array of circular cylinders for both dielectric and conductor cases in order to proof the validity of the presented technique.

This paper presents a mode matching method to analyze axisymmetric coaxial discontinuity structures, commonly used in the permeability and/or permittivity measurement.By performing the mode matching procedures at all discontinuity interfaces, a set of general simultaneous equations are derived, which can be easily solved.The s parameters and field distribution in the structures are readily obtained from the solution to the simultaneous equations. As a preliminary preparation for the mode matching method, the propagation constants of all the sections in the structure have to be solved.A one-dimensional frequency domain finite difference method is presented in this paper to efficiently solve the propagation constants for the multi-layered axisymmetric structures. Numerical examples show that the results obtained from the method in this paper are in good agreement with those from other methods in the published literature papers, and the method presented here has much higher efficiency.

Numerical simulations of emission for two-dimensional randomly rough surfaces with an inhomogeneous layered medium are presented. The inhomogeneous layered medium is modeled by a generalized n-layered stratified media. The numerical method was adopted from the physics-based two-grid method (PBTG). To ensure the strict accuracy requirement while to relief the memory and CPU resources, the PBTG in conjunction with the sparse-matrix canonical grid method (SMCG) was used in this paper. In so doing the reflected terms of the dyadic Green's function that accounts for layered media were added into the integral equations governing the surface tangential fields. Since the reflected part of the dyadic Green's function does not contain any singularity, the normal components of the fields remain the same as in the case of homogeneous surfaces. It was found that the elements of Green's tensor are only important to the near-field since they decay very fast as spatial distance goes beyond a few wavelengths. The resulting integral equations are then solved by the Method of Moment (MoM). Comparisons between the inhomogeneous and the homogeneous rough surfaces suggest that the presence of the inhomogeneous layered medium has non-negligible contributions to emission, depending on the dielectric gradient and is polarization dependent.

A microstrip square-ring slot antenna (MSRSA) for UWB (Ultra Wideband) antenna applications is proposed and improved by compaction. This structure is fed by a single microstrip line with a fork like-tuning stub. By splitting the square-ring slot antenna (SRSA) and optimization ofthe feeding network, the required impedance bandwidth is achieved over the UWB frequency range (3.1 to 10.6 GHz). The experimental and simulation results exhibit good agreement together. Parametric study is applied to compaction of structure. This compaction provides a good radiation pattern and a relatively constant gain over the entire band off requency.

Wildfires are uncontrolled exothermic oxidation of vegetation. Flame combustion temperatures could be in excess of 1600 K. Under the high temperature environment, plants' organic structure crumbles to release omnipresent alkali nutrients into the combustion zone. The alkali based compounds thermally decomposed to constituent atoms which ultimately ionised to give ions and electrons. The presence of electrons in the flame lowers its refractive index, thereby creating a medium of spatially varying refractive index. In the medium, incident radio waves change speed and are consequently deflected from their original path. The refraction has an effect of decreasing signal intensity at a targeted receiver which is at the same height as a collimated beam transmitter which is at a considerable distance away from the former. A numerical experiment was set to investigate the sub refractive behaviour of a very high intensity eucalyptus wildfire (90 MWm^−1) plume using two dimensional (2D) ray tracing scheme. The scheme traces radio rays as they traverse the plume. The ratio of number rays in a collimated beam reaching the targeted receiver to number of rays leaving the transmitter is used to calculate signal intensity loss in decibels (dB) at the receiver. Assuming an average natural plant alkali (potassium) content of 0.5%, attenuation (dB) was observed to be factor of both propagation frequency and temperature at the seat of the fire plume; and only of temperature at cooler parts of the plume. The 2D ray tracing scheme predicted a maximum attenuations of 14.84 and 5.47 dB for 3000 and 150 MHz respectively at 0.8 m above canopy-flame interface over propagation path of 48.25 m. An attenuation of 0.85 dB was predicted for frequencies from 150-3000 MHz over the same propagation distance at plume height of 52.8 m above ground.

Radiation from a superstrate loaded cylindrical-rectangular microstrip patch antenna with an air gap between the substrate and the superstrate, is analyzed using the full-wave approach and the electric surface current model. Results are presented in the form of normalized radiation patterns for various thicknesses of the air gap and also for superstrates made of lossy dielectric material, to show the effects of these on the radiation from the antenna. Both axialand azimuthal current elements are considered.

The scattering of a plane electromagnetic wave by a perfectly conducting prolate or oblate spheroid is considered analytically by a shape perturbation method. The electromagnetic field is expressed in terms of spherical eigenvectors only, while the equation of the spheroidal boundary is given in spherical coordinates. There is no need for using any spheroidal eigenvectors in our solution. Analytical expressions are obtained for the scattered field and the scattering cross-sections, when the solution is specialized to small values of the eccentricity h = d/(2a), (h<<1), where d is the interfocal distance of the spheroid and 2a the length of its rotation axis. In this case exact, closed-form expressions, valid for each small h, are obtained for the expansion coefficients g⁽²⁾ and g⁽⁴⁾ in the relation S(h) = S(0)[1 + g⁽²⁾h² + g⁽⁴⁾h⁴ + O(h⁶)] expressing the scattering cross-sections. Numerical results are given for various values of the parameters.

The problem of determining the Green's function of an electric line source located in a permeable, anisotropic (Ëœμ_xx, Ëœμ_xy, Ëœμ_yx, Ëœμ_yy and Ëœε_zz nonzero interacting parameters) half-space above a Perfect Magnetic Conductor (PMC) ground plane (called the TM^z case herein) for the case where image theory cannot be applied to find the Green's function of the PMC ground plane system has been studied. Monzon [2, 3] studied the Green's function TE^z problem dual to the present one for two cases; (1) when the system was unbounded, anisotropic space where Ëœε_xx, Ëœε_xy, Ëœε_yx, Ëœε_yy and Ëœμ_zz were the nonzero interacting parameters; and (2) when the scattering system was an anisotropic half-space located above a Perfect Electric Conductor (PEC) ground plane and where Ëœε_xx, Ëœε_yy and Ëœμ_zz were the nonzero interacting parameters and Ëœε_xy = Ëœε_yx = 0. Monzon [2] referred to the latter ground plane case as the case where "usual image" theory could be used to find the Green's function of the system. The Green's function for the TM^z-PMC case studied herein was derived by introducing and using a novel, linear coordinate transformation, namely Ëœx = (σ_P /Ï„)Ëœx + σ_MËœy, Ëœy = Ëœy, (Eqs. (6c,d), herein). This transformation, a modification of that used by [2, 3], reduced Maxwell's anisotropic equations of the system to a non-homogeneous, Helmholtz wave equation from which the Green's function, G, meeting boundary conditions, could be determined. The coordinate transformation introduced was useful for the present PMC ground plane problem because it left the position of the PMC ground plane and all lines parallel to it, unchanged in position from the original coordinate system, thus facilitating imposition of EM boundary conditions at the PMC ground plane. In transformed (or primed) coordinates, for the TMz-PMC and TE^z-PEC ground plane problems, respectively, the boundary conditions for the Green's functions G_TM ≡ E_z and G_TE ≡ H_z were shown to be [α*(∂G/∂˜x) + (∂G/∂˜y) |_Ëœy=0 = 0, where G = G_TM or G_TE, α = α_TM or α_TE, where α_TM and α_TE are complex constants (dually related to each other by Ëœμ ↔ Ëœε), and where Ëœy' = 0 is the position of the ground plane. An interesting result of the analysis was that the constant αTE (as far as the author knows) coincidently turned out to be the same as the first of two important constants, namely S₁(dl_v/dl) and S2(dl_v/dl), which were used by Monzon [2, 3] to formulate integral equations (based on Green's second theorem) from which EM scattering from anisotropic objects could be studied. Spatial Fourier transform (k-space) techniques were used to determine the Green's function of the Helmholtz wave equation expressed in transformed coordinates which satisfied the mixed-partial derivative boundary condition of the system. The Green's function G was expressed as a sum of a "free space" Green's function gf (proportional to a Hankel function H⁽²⁾₀ and assumed excited by the line source in unbounded space) and a homogeneous Green's function g whose spectral amplitude was chosen such that, when g was added to g_f , the sum G = g_f + g, satisfied boundary conditions. The k-space, Sommerfeld integrals making up g turned out to converge slowly, and so, using contour integration in the complex plane (Appendix B), g was expressed in a rapidly convergent form, and thus one leading to its practical numerical evaluation. Extensive numerical testing of how well the Green's functions G = g_f + g satisfied boundary conditions and how well the homogeneous Green's function g satisfied Green's second theorem was performed. Excellent verification of the numerical and analytical procedures were found and displayed in the error tables of Tables 1-4 of the paper. Plots illustrating the novel coordinate transformation introduced were presented and several numerical plots of the Green's function developed herein were presented. The application of the present work to find the Green's function for the case when a negative-index, anisotropic metamaterial is adjacent to a conducting ground plane is discussed. Application of the Green's function theory developed herein to multi-layer anisotropic systems is discussed.