On input impedance of an arbitrarily oriented small loop antenna in a cold collisionless magnetoplasma

Closed-form expressions are derived for the input impedanceZof a small loop antenna with arbitrary orientation in a cold collisionless uniform multicomponent magnetoplasma. The closed-forms results ofZare compared with numerical results from a full-wave formal solution. It is found that for small loops these approximate formulas adequately represent the loop input impedance.     

Input impedance of an electric dipole arbitrarily oriented in a corner reflector

A technique for calculating the input impedance ? of an electric dipole is developed. The dipole with half-length l is oriented arbitrarily in a corner reflector with infinite dimensions and flare angle π/N (where N is an integer). The technique is based on the method of images and induced electromotive forces. The results of calculating the active and reactive parts of impedance ? are presented for the case of a half-wave dipole oriented in different ways for different dipole-to-corner distances.     

New canonic lossy inductor using a single CDBA and its application

A new lossy inductor (parallel R-L type) using single current differencing buffered amplifier (CDBA) is presented, which employs only two resistors and a grounded capacitor. The proposed lossy inductor is shown to be useful in realizing an oscillator circuit, which provides quadrature voltage outputs using only two CDBAs, three resistors and two grounded capacitors and offers independent control on condition of oscillation and frequency of oscillation up to a frequency of 61 MHz. Detailed non-ideal analysis including single-pole model for voltage and current gain has also been carried out. SPICE simulation results and experimental results based upon the CDBA constructed from commercially available IC AD844 have been included which confirm the practical workability of the proposed lossy inductor and quadrature oscillator circuit.     

Vertical linear antennas in the presence of a lossy half-space: An improved approximate model

The unknown current distribution (UCD) and the input impedance/admittance of the vertical dipole antenna (VDA) in the presence of the homogeneous and isotropic lossy half-space are determined in this paper. The moment method (MoM) with an entire domain polynomial current approximation for numerical approach to the system of integral equations of Hallen's type (SIE-H) is applied for solving of this problem. The influence of the finite ground conductivity, expressed by the Sommerfeld's integral kernel (SIK), is modelled in a new simple way without limitations for the refraction index. This model gives output numerical results of enviable accuracy.     

Bees algorithm for interference suppression of linear antenna arrays by controlling the positions of selected elements

An efficient method based on bees algorithm (BA) for the pattern synthesis of linear antenna arrays with the prescribed nulls is presented. Nulling of the pattern is achieved by controlling only the positions of selected elements. The BA is an optimization algorithm inspired by the natural foraging behavior of honey bees to find the optimal solution. Simulation results for Chebyshev patterns with the imposed single and multiple nulls are given to show the effectiveness of the proposed method.     

Approximating the surface impedance of a homogeneous lossy half-space: an example of "dialable" accuracy

We present an approximation by exponentials of the time-domain surface impedance of a lossy half space. Gauss-Chebyshev quadrature of order N-1 is employed to approximate an integral representation of the modified Bessel functions comprising the time-domain impedance kernel. An explicit error estimate is obtained in terms of the physical parameters, the computation time and the number of quadrature points N. We show that our approximation is as accurate as other approaches which do not come with such an error estimate. The paper investigates the conditions under which the derived error estimate also applies to the approximation of J.A. Roden and S.D. Gedney (see Trans. Microwave Theory Tech., vol.47, p.1954, 1999).     

直线阵列天线的设计与性能分析

基于工作频率在1.79 GHz的矩形微带天线,利用FDTD进行建模和仿真,设计出了几种直线阵列天线。阵列天线单元数分别为2n(n=0,1,2,3),分别计算了它们的远场辐射方向图。讨论了阵列天线单元数目的影响,分析了阵列天线的主要性能参数,并得出了结论。     

Diffraction tomographic algorithm for the detection ofthree-dimensional objects buried in a lossy half-space

A diffraction tomographic (DT) algorithm has been proposed for detecting three-dimensional (3-D) dielectric objects buried in a lossy ground, using electric dipoles or magnetic dipoles as transmitter and receiver, where the air-earth interface has been taken into account and the background is lossy. To derive closed-form reconstruction formulas, an approximate generalized Fourier transform is introduced. Using this algorithm, the locations, shapes, and dielectric properties of buried objects can be well reconstructed under the low-contrast condition, and the objects can be well detected even when the contrast is high. Due to the use of fast Fourier transforms to implement the problem, the proposed algorithm is fast and quite tolerant to the error of measurement data, making it possible to solve realistic problems. Reconstruction examples are given to show the validity of the algorithm     

A galerkin moment method for the analysis of insulated wires above a lossy half-space

In this paper, an efficient method is proposed for the analysis of insulated wires above a lossy half-space. The reaction concept is used to formulate the electromagnetic boundary value problem. The solution is a Galerkin moment method solution with piecewise sinusoids as basis and testing functions. The influence of lossy half-space is taken into account via equivalent images, and the currents of images are related to the source current. The dielectric coating is modeled by equivalent volume polarization currents, which are simply related to the conduction current distribution. In this way, the dielectric-coating and lossy half-space cause no new unknowns to the problem solution, the size of the impedance matrix for insulated wires above lossy ground is the same as that of bare wires in free space. The insulation is accounted for entirely through a modification of the symmetric impedance matrix. The results obtained exhibit excellent agreement with those by rigorous method or experiments.     

双交换遗传算法及其在非周期天线阵方向图综合中的应用

提出了一种双交换遗传出法,它拓展了传统遗传算法的基本思想,通过引入双交换繁殖操作和参数化染色体,摆脱了传统遗传算法收敛对初始群体的依赖性,避免了传统遗传算法繁琐的编解码过程,因而,它比传统遗传算法更有效和更实用,为了说明双交换遗传算法的有效性和实用性,将双交换遗传算法应用于非周期圆形天线阵扫描方向图综合,应用表明双交换遗传算法可以成为一种通用的阵列天线辐射方向图综合工具。     

An efficient recursive procedure for evaluating the impedance matrix of linear and planar fractal arrays

The self-similar geometrical properties of fractal arrays are exploited in this paper to develop fast recursive algorithms for efficient evaluation of the associated impedance matrices as well as driving point impedances. The methodology is demonstrated by considering two types of uniformly excited fractal arrays consisting of side-by-side half-wave dipole antenna elements. These examples include a triadic Cantor linear fractal array and a Sierpinski carpet planar fractal array. This class of self-similar antenna arrays become significantly large at higher order stages of growth and utilization of fractal analysis allows the impedance matrix, and hence the driving point impedances, to be obtained much more efficiently than would be possible using conventional analysis techniques.     

On the realizability of the impedance matrix for lossy dielectricposts in a rectangular waveguide

The equivalent T-network for some lossy dielectric posts in a rectangular waveguide is found to have a negative resistance in the parallel arm although the realizability conditions for the impedance matrix are strictly satisfied. Furthermore, the reactive part of the same impedance is found to be a monotonically decreasing function of frequency. These difficulties are overcome in the case of symmetrical post structures by using lattice networks. A simplified lattice network of lumped elements is developed to approximately realize the impedance matrix for resonant lossy post structures in the bandwidth     

Asymptotic theory for dipole radiation in the presence of a lossy slab lying on a conducting half-space

The basic theory for dipole radiation in the presence of a two-layer half-space is outlined with special reference to its use as a model for studying radio propagation through and over heavily vegetated terrain. The source dipole may be located above or below the top surface of the slab. The dipole orientation is either vertical or horizontal. The asymptotic derivations for the field expressions are carried out without making the usual assumption that the refractive index of the uppermost layer is large compared with unity. The final results exhibit the expected inverse square dependence of the field on horizontal range.     

Evolutionary optimization for pattern synthesis and reduction of mutual coupling of linear antenna arrays

Optimal design of antenna arrays to minimize the mutual coupling effects in the geometrical arrangements of the linear antenna array (LAA) and circular antenna array (CAA) is dealt with in this work. Two different cases are considered to reduce the effect of LAA and CAA: Case‐1 in which the current excitations of the antenna array are considered to get the optimal radiation pattern of two geometry called LAA and CAA and Case‐2 in which inter‐element spacing and current excitations are both optimized for LAA geometry. A cost function that involves the mutual coupling factor as an optimization factor is developed to reduce the side lobe level (SLL), which takes mutual coupling effects into consideration. Excitation values and inter‐elemental spacing are optimized using particle swarm optimization (PSO). In LAA, for 8‐, 12‐, 16‐element arrays, SLLs are reduced by ?15.52, ?16.71, and ?17.78 dB in Case‐1. For the same sets of element arrays, SLLs were reduced by ?17.35, ?19.71, and ?20.26 dB in Case‐2. In CAA, the current excitations of the antenna array are optimized. For 8‐, 12‐, and 16‐ element arrays, SLLs are reduced to ?7.405, ?10.52, and ?9.43 dB, respectively. The arrays coded with the help of MATLAB based computation and the results obtained by MATLAB are validated by using CST.     

Exact, closed-form representations for the time-domain surface impedances of a homogeneous, lossy half-space

In this paper, we derive exact, closed-form representations for the transverse electric and transverse magnetic time-domain, surface impedances associated with a plane wave that is obliquely incident on a homogeneous, lossy half-space. The time-domain surface impedances are expressed in terms of modified Bessel functions and incomplete Lipschitz-Hankel integrals. The limitations on the validity of a previously published approximate expression are also discussed.     

Resonances of perfectly conducting wires and bodies of revolutionburied in a lossy dispersive half-space

The method of moments (MoM) is utilized to compute the complex resonant frequencies and modal currents of perfectly conducting wires and bodies of revolution buried in a lossy dispersive half space. To make such an analysis tractable computationally, the half-space Green's function is computed via the method of complex images, with appropriate modifications made to account for the complex frequencies characteristic of resonant modes. Results are presented for wires and bodies of revolution buried in lossy soil using frequency-dependent measured parameters for the complex permittivity, and we demonstrate that the resonant frequencies generally vary with target depth. In addition to presenting results, relevant issues are addressed concerning the numerical computation of buried-target resonant frequencies     

Wave propagation and coupling in linear arrays with application to the analysis of large arrays

Waves inside linear arrays are analyzed for the case of single-port excitation. A frequency-domain method is proposed for the extraction of the amplitude decay and phase velocity of the waves, based on simulation results obtained for terminal excitation at one end and near the middle of the array. The waves reflected by the array ends are also extracted, under a single-reflection assumption. This model is then exploited for estimating the port currents when other elements of the array are excited, as well as for larger arrays. Simulation results are shown for port currents and element patterns in arrays of broad-plate dipoles.     

High-accuracy direction finding in multipath environment with short-wave antenna arrays comprising a small number of elements

A method for short-wave multipath direction finding is considered. The method allows high-accuracy determination of the directions of arrival and the amplitudes of a priori unknown arriving signals through the use of an antenna array comprising a small number of independent receiving elements with narrow-bandwidth filters placed at their inputs. An algorithm based on eigenvector decomposition of the correlation matrix of the complex signals arriving at the inputs of the radiating elements in each subband; separation of the eigenvectors into signal vectors and noise vectors; determination of the receiving-antenna eigenpatterns from the obtained vectors; and substantiation of the functional equation that allows, with the antenna eigenpatterns taken into account, the signals’ amplitudes and angles of arrival of the signals to be determined, is proposed. The results of a numerical simulation, which confirm the high accuracy of direction finding by the proposed method, are presented.