加载法向模螺旋天线的研究

采用矩量法分析了法向模螺旋天线加载后的电特性，探讨用加载来实现法向模螺旋天线宽带化小型化的可行性。计算时采用螺旋线段对天线进行划分，然后采用三角形基函数脉冲基检验的矩量法来快速精确计算天线上的电流分布，计算了一些加载螺旋天线的实例。并与直线段划分的矩量法的结果进行比较。最后借助遗传算法优化设计了一付工作在220-400MHz的法向模螺旋天线。     

Truncated spherical helical antennas

The truncated spherical helix is presented as a compact circularly polarised antenna with relatively high gain and moderate bandwidth. A numerical investigation into the radiation properties of this antenna is also presented. The antenna provides a directivity of >9 dB and an axial ratio of <3 dB over a 10% bandwidth. The radiation pattern has a broad main beam and better than 20 dB front-to-back ratio     

Broadband quasi-taper helical antennas

A unique approach is described for widening the bandwidth of a helical antenna with improved gain, pattern, and axial ratio characteristics. The antenna may be described as a nonuniform or quasi-taper helix, which consists of a combination of uniform and tapered helix sections. Measured patterns, gain, axial ratio, and VSWR for various helical antenna configurations are presented and compared. It is shown that a nonuniform quasi-taper helix can provide an operating bandwidth twice that of a conventional uniform-diameter helix.     

Wave propagation on helical antennas

The current distribution, the near field, and the far field of helical antennas are examined. It is found that the application of the helix as an endfire radiator is dependent on two higher Order modes of the helix wave functions. This differs from the application of the helix as a slow wave structure in a traveling wave tube where the lowest order mode of the helix wave functions is utilized.     

带异形反射腔和寄生螺旋的均匀升角轴向模螺旋天线

为了改善轴向模螺旋天线的辐射特性,在带有寄生螺旋的均匀升角轴向模螺旋天线基础上,设计了一种带有曲反射面背腔的螺旋天线,并用HFSS软件对天线的辐射特性进行了仿真分析。通过对比几种不同形式的螺旋天线的仿真结果,证明了该种螺旋天线可以有效地提高轴向模螺旋天线增益系数,圆极化一致性良好,是一种提高轴向模螺旋天线性能的有效方法。     

A new moment method for the fast and accurate analysis of normal mode helical antennas

In this letter, a new moment method using helical segments is presented to model Normal Mode Helical Antenna (NMHA). Using this method, the NMHA can be modeled by a few segments. The current distributions and radiation patterns of some NMHAs are calculated. A comparison is made between results obtained using this helical segment algorithm and a linear segment algorithm, and the results of the two algorithms agree fairly well. When calculating the impedance matrix [Z], all the elements of the matrix can be obtained by only calculating a few elements with the application of the symmetric and periodic characteristics of the NMHA. Therefore, the CPU time and the memory storage are significantly reduced, with the accuracy and speed enhanced.     

The near field of omnidirectional helical antennas

The close near field of helical antennas, radiators widely used in connection with two-way portable communication equipment, is investigated theoretically and experimentally. The investigation has been motivated by safety related considerations. A simplified mathematical model for the radiation from helical antennas with a large number of turns is derived. The near E-field intensity obtained from the theoretical model is compared to the values measured using an accurate E-field probe. The agreement between experimental and theoretical values is excellent. The results show that there is a substantial buildup of static-type electric energy in the close vicinity of helical antennas. The intensity of these electric fields in the vicinity of a helical radiator depends essentially on the Q factor of the antenna. For one experimental helix the far-field power density equivalent (|E|²/377) of the electric field at 1-cm distance from the radiator exceeds some proposed safety standards for less than 250-µW radiated power. These values are in complete agreement with the results of previous studies which showed that helical radiators are very ineffective in depositing electromagnetic energy into simulated muscle tissue located in the close vicinity of the antenna. If safety standards of independent or government agencies do not take into account the peculiar nature of the electromagnetic energy in the close vicinity of some radiating devices, it is conceivable that the power of portable two-way communication equipment might be forced down to useless levels.     

Conical beam control of quadrifilar helical antennas

The conical beam control of quadrifilar helical antennas with a fixed helical arm length is investigated. By adjusting a portion of the antenna height to vary the pitch angle, the elevation angle of the beam direction can be controlled with an almost constant gain. It is found that the elevation angle of the beam direction varies in proportion to the pitch angle     

Main-beam polarization properties of modified helical antennas

The polarization properties of modified helical antennas are investigated for a wide range of directions using a two-dimensional polarization diagram. The current distribution on the antenna and the radiated field are calculated for several variations of the antenna shape to show the influences of the helix design on the main-beam polarization characteristics. This extended knowledge about typical polarization diagrams yields conditions for the antenna design. Special emphasis is placed on the choice of the geometrical parameters for the open end and the feed region of the antenna to meet different system requirements. The calculated results are confirmed with measured polarization characteristics and radiation patterns for experimental helices     

THE MOMENT METHOD ON RESONANT QUADRIFILAR HELICAL ANTENNAS

An efficient measure is taken to rearrange Nakano's kernels of integral equationsfor an antenna system composed of arbitrarily bent wires.By means of the moment method,great efforts are made to analyze and compute the circularly polarized patterns,directivity,axialratio,front-to-back ratio and beam-shaping characteristics of resonant quadrifilar helical antennasas well as the feeding technique and the effect of an electrically large conducting body on theperformance of the antennas.     

Two-port higher mode circular microstrip antennas

For circular microstrip antennas, single-mode design techniques work well for the dominant mode (n=1, where n is the azimuthal dependence of the fields in the antenna), but this is not the case for zero-order (n=0) and higher order (n>1) modes, where a modal expansion is necessary. The design of higher order dominant mode circular microstrip antennas is addressed, with an example and measurements for the n=3 case. Measurements for the n =3 modes show reasonable agreement with theory in the forms of the impedance loci, but the frequency dependence is in error by the order of the bandwidth of the antenna. A design example is given for a roof-mounted circular patch for vehicular communications. The example indicates that an n=3 dominant mode patch can be used as a two-port antenna for diversity applications or with a 90° phase shifter and adder as a single-port antenna for omnidirectional coverage with (essentially) circular polarization     

Input impedance of arc antennas and short helical radiators

The problem of the input impedance of curved wire antennas is generally formulated in terms of an integral equation. A stationary formula is used in evaluating the input impedances of arc antennas and short helical radiators of which the cylindrical antenna is a special case. The computational results are presented in graphical form. The impedance characteristics of these curved antennas are discussed.     

A 50-ohm input impedance for helical beam antennas

A helical beam antenna with uniform conductor size has a nominal impedance of 140 ohms. This may not be convenient for some applications. The purpose of this note is to call attention to the fact that the impedance can be adjusted to 50 ohms by increasing the conductor size close to the feed point at the ground plane. This lowers the characteristic impedance of the conductor-ground plane combination (acting as a transmission line) and transforms the helix impedance to a lower value over a substantial bandwidth     

椭球面螺旋天线特性分析

采用螺旋基函数和检验函数的矩量法对椭球面螺旋天线进行了分析,给出了天线轴向尺寸变化对天线的驻波、增益、极化及方向图等特性的影响,为不同要求下天线形式的选取提供了良好的依据。     

Backfire/endfire radiation performance of quadrifilar helical antennas

The relations between the radiation pattern and feeding method in quadrifilar helical antennas is discussed. As a result, it is found that by changing the phase of each wire the helical antenna can achieve two radiation modes; both backfire and endfire radiation. These patterns do not have a difference except in radiation direction. Antenna polarisation of quadrifilar helical antenna depends only on the helical winding direction.<>     

Axial cross polarization in reflector antennas with surface imperfections

Expressions for the probability density function and mean of boresight cross polarization field intensity in terms of the rms surface errorepsilonare derived. It is shown that for small errors the mean on-axis cross polarization field intensity is directly proportional to the rms surface error. For relatively large errors (epsilon > 0.04 lambda) forward cross polarization is proportional to the square root of (1 - exp (-2sigma^{2})) wheresigmais the rms phase error. It is also shown that forward cross polarization is proportional to the correlation diameter and is a function of the polarization efficiency. Experimental measurements are revealed and results are compared with theoretical findings.     

Cross polarization from dual mode horn antennas

The cross polarization from dual mode horn antennas supporting the TE11and TM11-modes has been analyzed both theoretically and experimentally. The cross polarization has been shown to be strongly dependent on the length and flare angle of the horn. When the flare angle for a given horn length becomes larger than a certain threshold angle, the cross-polar sidelobes increase rapidly. Simple formulas for this threshold angle are presented. This cross-polar degradation will not occur if a plane phase front is provided over the horn aperture, either by profiling the horn or by applying a lens in the aperture.     

Heating characteristics of thin helical antennas with conductingcores in a lossy medium. I. Noninsulated antennas

We report a combined theoretical and experimental study of the heating characteristics of helical antennas in lossy dielectric media. Proposed biomedical application of such antennas include angioplasty, hyperthermia, and catheter ablation of tissue. The study focuses on helical antennas, operated in the normal mode (wavelength greater than antenna diameter but comparable to antenna length), that are terminated at one end by a short circuit and at the other by a coaxial feedpoint. The analytical model is based on the helical sheath approximation, extended to the case of lossy media. In addition, experimental studies were performed on helical antennas immersed in aqueous electrolyte of various conductivity. The antennas show two distinct modes of propagation: a slow mode similar to that observed in helical antennas in loss-free media, and a faster mode. The analytical/numerical results are in good agreement with experimental data, thus demonstrating the validity of the model     

Coaxial-feed axial mode hemispherical helical antenna

A coaxial-feed axial mode hemispherical helical antenna is studied experimentally and theoretically. Results for a 3-turn hemispherical helix indicate that circular polarisation can be obtained over a broad beamwidth. The gain bandwidth is ~33% and the peak gain is 7.6 dB. The input impedance shows that it is a standing wave antenna