The Millimeter Wave Radiation of a Traveling Wave Sinusoidal Wire Antenna

In this paper, investigation of radiation properties of the traveling-wave sinusoidal wire antennas is extended to the millimeter-wave frequencies (Ka-band) for the antennas whose geometrical dimensions vary in a wide range. Far-field patterns and S-parameters of composed three antenna sets were measured. A mathematical model was constructed for the structure and a MATLAB code based on this theoretical approach was written to calculate patterns, phase and attenuation constants of all investigated antennas. Frequency characteristics and the relation of antenna dimensions with wave parameters were investigated. Measured and calculated patterns were also compared with the constructed far-field patterns obtained by MoM (method of moments) and the MoM current distributions were used to explain the loss mechanisms of antennas. A directive, undistorted and smooth radiation can be achieved only choosing small undulated antennas whose peak-to-peak amplitude to period ratio κ is smaller than 0.4 (κ?<?0.4). It is shown that wavelength of broadside radiation is not equal to antenna period for all antennas, except for very small undulated antennas (κ?<?0.2). This antenna type can be used as a frequency-scan antenna for millimeter wave radars.     

On the transient radiation of energy from simple currentdistributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method     

Electromagnetic Subsurace Radar Using the Transient Field Radiated by a Wire Antenna

A radar based on the propagation of a short pulse of current along a wire parallel to the ground surface is described. The wire acts both as a traveling-wave transmitting antenna and as a receiving antenna. For applications to civil engineering, the depth of exploration may be less than 1 m, the purpose being, for example, to detect discontinuities situated near the ground surface, to measure the thickness of a thin layer (10-50 cm), or to characterize a change in its reflection coefficient. Therefore, the width of the pulse of current is about 1 ns. The theoretical approach shows that the wire must be situated at about 10 cm above ground and that there is an optimum position for the current probe that detects the induced current due to a reflecting layer. Several experiments will also be described showing the feasibility of the method.     

基于遗传算法的栅格天线阵旁瓣电平优化方法

在推导出栅格天线阵列方向图的基础上,提出一种基于遗传算法的栅格天线阵旁瓣电平优化的方法。此方法首先简化栅格天线阵物理模型,给出等效模型下的阵列方向图,然后以栅格天线阵的短边电流幅度为优化参量,以阵列天线方位面的副瓣电平为适应度函数,利用遗传算法的最优化搜索得到满足副瓣要求的电流幅度,再通过电流幅度计算辐射单元阻抗,最终设计出满足要求的低旁瓣栅格天线阵。为了验证该方法的有效性,对一种频扫微带栅格天线进行了优化,在电磁仿真软件中对优化后的天线进行了仿真,根据设计结果加工制作了原型天线并进行了测试,测试结果显示优化后天线阵的副瓣电平降低了5dB,优化效果明显。     

Radiation of a travelling-wave antenna in plasma in the whistler frequency band

The far field, the radiation pattern, and the radiation impedance of a traveling-wave antenna oriented in plasma along the external magnetic field in the resonance region of the whistler frequency band are studied. It is shown how the main characteristics of the antenna depend on its length and the current propagation constant along the antenna wire.     

Electromagnetic excitation of a thin wire: a traveling-waveapproach

An approximate representation for the current along a perfectly conducting straight thin wire is presented. The current is approximated in terms of pulsed waves that travel along the wire with the velocity of the exterior medium. At the ends of the wire, these pulses are partially reflected, with a constant reflection coefficient and delay time. Subsequently, the traveling-wave representation for the current is used to derive an approximate expression for the electric field outside the wire that is caused by this current. For voltage excitation, this expression contains only closed-form contributions. For plane-wave excitation, the expression contains a single integral over the initial pulse that must be computed numerically. Although the expression obtained is essentially a far-field approximation, it turns out to be valid from distances of the order of a single wire length. Results for a representative choice of wire dimensions and pulse lengths are presented and discussed     

The wave antenna for transmission and reception

The traveling-wave horizontal-wire antenna over the earth is analyzed in its original form with vertical ground connections (the Beverage antenna) and with these replaced by horizontal terminations. For transmission, the electromagnetic field of an antenna with optimum length is determined, both along the surface of the earth in air and in the earth, in an accurate but simple form that takes full account of the proximity of the earth (lake, sea) on the distribution of current. For reception, the induced currents in the antenna and in the load are determined for a field incident along the surface of the earth. The two types of termination are compared and their contributions shown to be small when the horizontal wire has a length near the optimum.     

A hybrid technique for wire antennas in a cavity

Two numerical techniques for solving the current distribution on wire antennas, namely point matching and reaction matching, have been combined in a particular fashion to yield a new technique termed the "hybrid method." The hybrid method is well suited for a wire antenna mounted in a cavity. Over the antenna, piecewise sinusoidal basis functions with reaction matching are used, and on the support structure, point matching is used with pulse basis functions. The hybrid method has been applied to calculate the input impedance and the radiation efficiency of the three-turn loop antenna in an open circular cavity. Calculated input resistance of the antenna is found to agree quite well with measured data. Effects of varying the extent of recession of the antenna and varying the radius and the material of the antenna wire on the bandwidth, the efficiency, the antiresonance frequencies, and the general input reactance level of the antenna have been examined for a specific antenna geometry. The results are considered to be indicative of the general characteristics of a multiturn loop antenna with nearby support structure. The computer program that has been developed may be applied to a systematic parametric study of this type of antenna in order to optimize it for a particular application.     

A near-field to far-field transformation for spheroidal geometry utilizing an eigenfunction expansion

This work presents a near-field to far-field (NF-FF) transformation for antenna and scatterer radiation evaluation. The transformation allows practical computation by making use of a sampling surface in the near-field that is spheroidal in shape: namely a prolate or oblate spheroid. The resulting vector wave equation does not support orthogonal vector solutions in spheroidal coordinates and instead rectangular field components are solved for using the scalar wave equation in spheroidal coordinates. The new transformation only requires knowledge of the completely-specified near-field electric field along the spheroidal transformation surface and does not need any information associated with the corresponding magnetic field. The benefit of using a spheroidal near-field geometry is its ability to closely conform to both linear and planar radiating structures while still permitting evaluation of the full far-field radiation pattern. Our approach makes use of an eigenfunction expansion of spheroidal wave-harmonics to develop two distinct, yet closely related, NF-FF transformation algorithms for each type of spheroidal surface. The spheroidal NF-FF transformation is validated and performance assessed using a well-characterized radiation structure. By applying the prolate and oblate algorithms to a radiating structure with known analytical near- and far-field electric fields, viz., a filament dipole with sinusoidal current distribution, we are able to setup and conduct multiple numerical tests that serve as a proof-of-concept for the spheroidal NF-FF transformation.     

Some effects of asymmetric excitations on dipole radiators

The recent model due to Shen, Wu, and King for the current distribution along the asymmetrically-driven thin cylindrical antenna is used in the calculation of current and far-field values for several cases of interest. The graphs show how the current distribution approaches the limiting sinusoidal assumption as the generator location is moved toward a free end. The antenna conductor thickness is shown to cause decaying sinusoidal variations on the radiation fields from the half-and full-wave dipoles having large excitation asymmetries. The model was found to yield accurate current amplitudes for dipoles at least0.30lambdain total lengths; however, the current phase is inaccurate for points closer than about0.025lambdafrom a free end. This latter value is also the restriction on how close the generator may be from the free end.     

A Miniaturized Ground Edge Current Choke—Design, Measurement, and Applications

We propose a miniaturized microwave current choke for blocking the current flowing along the edge of a substrate's ground plane. The proposed current choke is composed of a printed inductor and a printed capacitor, which possesses a size much smaller than a conventional quarter-wavelength current choke. By introducing the choke at one side of the ground plane, an effective electrical open circuit is performed for reflecting the ground edge current. The size of the proposed ground edge current choke (GECC) is as small as around 0.06 wavelength in free space. Two applications of the GECC are presented in this paper. The first is the radiation pattern regulation of a printed monopole antenna with long ground plane. The GECC in this application reflects the induced traveling-wave current along the ground plane edge and changes it to a standing-wave one, thus regulating the tilted radiation pattern due to the traveling-wave current to a broadside pattern. The other application is the decoupling of two nearby monopole antennas. By placing the proposed compact GECC in between the antennas, it is found that the isolation between the antenna ports can be enhanced from 8 dB to 32 dB. The experimental results agree well with the simulation, which demonstrate the feasibility of the proposed GECC.      

The moment method solution for printed wire antennas of arbitraryconfiguration

A printed wire antenna of arbitrary configuration is analyzed. The electric field tangential to the wire is derived using the current expanded by piecewise sinusoidal functions. These functions are also used to form the impedance matrix elements. Use of the stationary phase method leads to a simple expression for the radiation field. Numerical analyses based on the present formulation yield radiation characteristics of a zigzag dipole antenna, a loop antenna, and a round spiral antenna     

Printed figure-of-eight wire antenna for circular polarization

A new printed wire antenna with circular polarization properties is presented. The geometry of the printed wire which takes the form of a figure-of-eight has a total length of 1.3/spl lambda//sub o/ and serves as a nonresonant traveling-wave antenna. It is shown that a 3-dB axial ratio bandwidth of 15% can be achieved. The half-power and 3-dB axial ratio beamwidth is approximately /spl plusmn/45/spl deg/, while its gain is of the order of 6.5 dB.     

Radiation Characteristics of Bent-Wire Antennas

A mathematical model, which predicts the far-field power pattern of a wire antenna bent in one plane, is developed, and the classical approximation of a sinusoidal current distribution along the arc length is employed, neglecting all end and mutual coupling effects. General expressions for the complex Poynting vector, electric- and magnetic-field intensities are also derived in terms of the retarded vector potential. Although the theory for parabolically-bent antennas compares favorably with experiment, it is shown that the agreement is more favorable when the current distribution is computed by the method of moments. In contrast to the figure-eight ?-plane pattern of the conventional straight dipole, the circularly and parabolically bent antennas operating at the first resonance are shown to have radiation patterns approaching omnidirection as the bending curvature is optimized, thus lending themselves to use as replacements for many common dipole applications for gain standards, field probing, reflector feeds, as well as mobile and broadcast antennas.     

自由空间导线天线辐射模式分析

采用分段正弦基矩量法计算线天线电流,借助分段正弦电流辐射场表达式,将各分段电流产生的辐射场进行线性叠加,进而获得天线总的辐射场。该辐射场利用球面波展开法进行展开,同时采用数值积分方法求取辐射场球面波展开式中各模式的系数值。以3种基本单元天线:偶极子天线、圆环天线以及螺旋天线为例,定量分析了3种基本单元天线的辐射模式,验证了这一方法的正确性。     

The time-domain characteristics of a traveling-wave linear antenna with linear and nonlinear parallel loads

The time-domain characteristics of a traveling-wave linear antenna with linear and nonlinear parallel loads are discussed. The fast Fourier transform (FFT) is used to analyze the antenna with a linear parallel load. A numerical time-stepping finite-difference equation method is used to analyze the antenna with a nonlinear parallel load. The nonlinear effect is treated by the Newton-Raphson iteration technique. The effects of various linear and nonlinear parallel loads are examined. Physical insight into the nonlinear parallel loading of the antenna is also given in terms of detected time-domain sinusoidal electromagnetic (EM) waves.     

The traveling-wave linear antenna with nondissipative loading

An approximate theory for a doubly impedance loaded, traveling-wave linear antenna is presented. The distribution of current on the thin, cylindrical antenna is determined approximately in terms of its dimensions and the impedance and position of the lumped impedance loading. From this result, the optimum loading required to excite an outward traveling current wave along most of the cylinder is determined. Significantly, it is indicated that a purely reactive loading may be utilized if its location is properly chosen. Expressions for the input impedance and radiation fields of the traveling-wave antenna are presented. An experimental study is made of a traveling-wave cylindrical antenna utilizing a purely nondissipative loading. Numerical results from the theory are compared with experimental data, and an excellent agreement is noted. The technique described may be utilized to realize a high-efficiency traveling-wave antenna.     

Millimeter-Wave Antenna of a Travelling-Wave Long Sinusoidal Slot in the Narrow Face of a Rectangular Waveguide

A travelling-wave long sinusoidal slot in the narrow face of a millimeter-wave (Ka-Band) rectangular waveguide is investigated. To the best of our knowledge, a slot of this shape is applied to the narrow face of a rectangular waveguide for the first time at this frequency range. The analytical study is based on the magnetic current model. Experimentally, far-field and S-parameters are measured and a good match is obtained between analytical and measured patterns. Frequency scan rates and velocity ratios of the structure are calculated. The effect of the sinusoidal slot’s geometrical parameters (amplitude, period and number of periods) on the radiation patterns is also investigated. The drawback of the sinusoidal radiator is its cross-polarized beam that is not convenient for antenna applications. These co and cross-polarized beams scan in two different angle regions. To filter/suppress the cross-polarized beam, a grid polarizer is mounted to the sinusoidal slot and a 15 dB suppression level is reached. S-parameters measurement shows that the antenna efficiency increases remarkably after the application of the grid polarizer to the slot.     

A reciprocity approach for calculating the far-field radiationpatterns of a center-fed helical microstrip antenna mounted on adielectric-coated circular cylinder

This paper presents an efficient approach based on reciprocity for calculating the far-field radiation pattern of a center-fed helical microstrip antenna mounted on a circular cylinder. The reciprocity theorem is used to find a completely analytical solution to the problem assuming that the helical microstrip is sufficiently thin with a sinusoidal current distribution. Two degenerate cases of the helical microstrip antenna, i.e., an axially and an azimuthally fed half-wave cylindrical-rectangular patch, are examined and compared to known results in order to provide validation for the general analytical solution. Further comparisons are made between the analytical results and the results obtained using a numerically rigorous method of moments (MoM) computer code     

Cylindrical transmitting antenna: Tapered resistivity and multiple impedance loadings

A general formulation is presented which treats with accuracy the perfectly conducting antenna, the uniformly resistive antenna, the tapered resistive antenna, and the multiple impedance loaded antenna. The current distribution is obtained in analytic form that is easily used to obtain the effective length and radiated field. The presented theory has application in traveling-wave antenna design. Traveling-wave antennas are of interest for broadband and directional communication. Extensive numerical results are obtained, and comparisons are made with experimental data. The results indicate the feasibility of cylindrical traveling-wave antennas.