Low Profile Miniaturized Planar Antenna With Omnidirectional Vertically Polarized Radiation

A novel miniaturized cavity-backed composite slot loop antenna (CBCSLA) with low profile and omnidirectional radiation similar to a short dipole is presented. The antenna displays vertical polarization while having less than lambda/100 dimension in height. The geometry of the antenna is inspired from a small magnetic loop which is realized using a slot configuration on metallic ground. Since a small slot loop antenna cannot be impedance matched, a composite geometry is devised that acts like as a small slot loop and can easily be impedance matched using this configuration. The diameter of the CBCSLA can be made as small as lambda/10 or smaller. The feeding mechanism is designed in a manner to create a circumferentially-constant current distribution around the composite loop while effectively matching to a 50 Omega coaxial feed from the antenna center. Additional methods are discussed to further enhance the bandwidth and achieve a dual-band response. The antennas are designed, fabricated, and measured. It is shown that despite their small lateral dimension and extremely small height, the miniaturized cavity-backed slot loop features extremely well input impedance match, uniform radiation pattern, low cross-polarization levels and expected gain.     

Segmented loop antenna for UHF near-field RFID applications

A segmented loop antenna is presented for ultra-high frequency (UHF) near-field radio frequency identification (RFID) applications. The proposed segmented configuration makes the current along the loop remain in-phase even though the perimeter of the loop is comparable to the operating wavelength, so that a strong and uniform magnetic field is generated in the region surrounding the antenna. The antenna printed on a FR4 printed circuit board (PCB) with an overall size of 160 x 180 x 0.5 mm achieves good impedance matching and uniform magnetic field distribution over an operating bandwidth of 800?1040 MHz, which is desirable for UHF near-field RFID reader applications.     

On the excitation of a circular loop antenna by travelling-and standing-wave current distributions

Presented here is an exact formulation of the electric and magnetic (EM) fields radiated by a circular loop antenna, assuming both travelling- and standing-wave current distributions. By using a differential current element positioned in an azimuthal direction as the starting point, the paper systematically develops the EM fields radiated by the circular loop via a vector potential theory. This approach leads to a general integral representation for the radiation characteristics of the loop antenna where we completely evaluate the resulting expressions, when the excitations of the loop assume travelling- and standing-wave distribution forms. In addition, this paper briefly examines a method to generate approximately such current distributions by coupling the loop to a two- or four-wire transmission line. Furthermore, the paper discusses a graphical representation of the current distribution plotted as a function of frequency or loop size. From the field expressions determined, we derive generalized closed-form results for some important design parameters for the loop antenna. These parameters include the radial component of the Poynting vector, the total power radiated by the loop, the directivity, the radiation resistance, and the effective area and height of the antenna. When we specialize to the important case of uniform current excitation, the exact values of the parameters deduced from the general expressions, are summarized and exhibited in a comprehensive table. This table facilitates the computations of these important physical parameters. Further analysis involving small argument and asymptotic approximations in the residts for the travelling-wave current excitation leads to closed-form expressions in terms of tabulated functions. Numerical results presented, include the fields radiated by the loop when the standing-wave current excitation admits a Fourier series representation. The present approach via potential theory reveals that the fields can be calculated in any arbitrary direction : this is consistent with a previous observation of Knudsen (1951, 1953) who employed a different approach.     

共模环路闭合与开路的天线模型及其计算分析

应用矩量法计算了折线电天线、方形环天线和加载电容方形环天线的电流分布,计算结果表明:当加载电容的方形环天线所加载的电容很大或很小时,其电流分布分别与闭合环天线和开路折线电天线的电流分布相吻合;并在此基础上进一步计算和分析了不同频率下环路闭合与开路的天线辐射场,研究结果对分析电磁兼容共模辐射问题有一定的参考价值.     

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.     

A Source of E and H Fields for Antenna-Factor Calibration

The loop cell used for calibration is fabricated using two intersecting metal sheets joined at the intersection and forming a 360 angle. A section of a loop is mounted between two coaxial panel jacks, one on each sheet located at a distance equal to the loop radius from the intersection. A known current through this section of electrically small loop produces calculable E and H fields between the sheets in the plane of the loop. These known fields may be used to determine the antenna factor of small E and H antennas placed in the field if the mutual impedance due to the antenna images in the sheets is negligible and the antenna is not close to the open edges of the cell. Measured and calculated antenna factors agree within ± 2 dB between 0.25 and 1000 MHz.     

Near-Field Loop Antenna for the UHF RFID Reader

A loop antenna for near field readers is proposed.Through periodic interdigital capacitors,the phase of the current on the loop is compensated and kept in phase.Hence,a loop with a perimeter of one wavelength at 900 MHz achieves a uniform near magnetic field distribution inside the loop.A novel method is proposed to evaluate the performance of the coplanar waveguide (CPW) to coplanar stripline (CPS) transition,which is used as a balun for the feeding network in this paper.This loop antenna has a 70 MHz operating bandwidth and 12 cm maximum reading range when the output power is 24 dBm,which is suitable for most near field radio frequency identification (RFID) applications.     

Heat Potential Distribution in an Inhomogeneous Spherical Model of a Cranial Structure Exposed to Microwaves Due to Loop or Dipole Antennas

An inhomogeneous spherical model of a 3.3-cm radius cranial structure is assumed to be placed symmetically in the near field of a small loop antenna or an electrical dipole antenna at 3 GHz. The transitions between the layers are taken to be sharp but sinusoidal. Calculations of the heat potential are performed using a spherical wave expansion technique in which linear differential equations are solved for the unknown multipole coefficients. The results are also compared with the plane-wave excitations. It is seen that a more uniform distribution of the heat potential occurs for the dipole antenna excitation which is also similar to the E-plane distribution in the case of plane-wave excitation. For the loop excitation, a significant hot spot occurs near the center of the structure.     

Radiation resistance of a small filamentary loop antenna in a cold multicomponent magnetoplasma

A study is made of the radiation resistanceRof a small filamentary loop antenna immersed in a cold collisionless uniform multicomponent magnetoplasma. Assuming that the current distribution along the loop is uniform and that the loop axis is parallel to the static magnetic field, an integral expression is derived forRwhich is valid for arbitrary values of driving frequency, plasma composition and density, and static magnetic field strength. The mathematical properties of this integral are such thatRis finite for all values of the driving frequency, including the upper and lower hybrid frequencies as well as the multiple-ion hybrid-resonance frequencies. Application of the integral expression is made to the case of the inner magnetospheric plasma and approximate closed-form expressions are developed forRfor the very low-frequency/extremely low-frequency (VLF/ELF) range in the magnetosphere. Numerical results in the VLF/ELF range are also presented. It is found that the inclusion of multiple ions introduces interesting effects: sharp maxima appear at the gyrofrequency and the multipleion hybrid-resonance frequency associated with each ion, while sharp minima occur at the "crossover" frequencies. It is concluded that the presence of these relative extrema in the radiation resistance presents interesting possibilities for the use of a small loop as a diagnostic tool in a multicomponent plasma.     

Impedance of a finite insulated cylindrical antenna in a cold plasma with a longitudinal magnetic field

A variational formulation is developed for the impedance of a finite cylindrical antenna embedded in a dielectric cylinder, which is surrounded by a magnetoionic medium (cold electron plasma) with the static magnetic field impressed in a direction parallel to the antenna axis. Closed form expressions are obtained in the limit of low frequencies, and for short antennas in a uniaxial medium. The impedance of a short antenna is nearly the same as for an assumed triangular current distribution, except that further resonances are observed in the vicinity of the gyro frequency, where the antenna becomes electrically long. These resonances may be shifted to frequencies exceeding the gyro frequency in the presence of an insulating layer around the antenna. For very thin insulating layers the wave number of the variationally approximated current distribution is to the first order equal tosqrt{epsilon_{1}} k_{0}(epsilon_{1}is the leading diagonal element of the permittivity matrix), where the gyro frequency may be both smaller or larger than the plasma frequency. However, this approximation does not apply to current distributions along the insulated antenna. The present calculations are also compared with earlier work on antenna impedances.     

The multiturn half-loop antenna

The multiturn half-loop antenna consists of a coaxial array of N circular, half loops mounted perpendicular to a conducting image plane and interconnected beneath the image plane by a set of N-1 coaxial transmission lines. This antenna is analyzed using a Fourier series expansion for the current in the loops and a new, accurate model for the excitation of the loops by the TEM field in the coaxial apertures at their ends. When this structure is electrically small, the currents in all of the loops are approximately equal; hence, it has the well-know characteristics of an electrically small multiturn loop antenna. For a structure of general electrical size, both the radius of the loops and the lengths of the transmission lines can be adjusted to optimize the performance of the antenna for a particular application. Surprisingly, proper phasing of the excitations can make a multiturn half-loop antenna of moderate electrical size have a simple toroidal pattern, a pattern that can be obtained for a single-turn, half-loop antenna only when it is electrically small. This result is verified experimentally for a two-turn loop. Examples are shown where electrically large loops are phased with the interconnecting transmission lines to produce nearly optimum end-fire and broadside patterns     

Determination of reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution

A simple theory is presented to predict the lumped reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution. The reactive load is located on a circular wire loop of one-wavelength circumference at a position of 45/spl deg/ away from the feed point. To achieve a uniform traveling-wave current distribution, the loading reactance and the input impedances of the loaded and unloaded loop antennas need to satisfy certain conditions. First, the input resistance and the input reactance of the unloaded loop antenna should have the same absolute value. Second, the input impedance of the loaded loop must be purely resistive and its value needs to be two times of the input resistance of the unloaded loop. Third, the loading reactance should be chosen to be two times in value and opposite in sense of the input reactance of the unloaded loop. These conditions can be approximately met when the circular loop is placed above a ground plane. The loading reactance is determined from the input impedance of the unloaded loop and is optimized for an optimal performance of circular polarization. It is found that the reactive load must be capacitive and its value depends on the height of the loop above the ground plane and the thickness of the wire. The characteristics of the circular polarization and the input impedance of the capacitance-loaded circular loop antennas are investigated. An experimental example is presented to verify the theoretical prediction.     

Consideration on the measurement of current distribution on bentwire antennas

As part of the measurement of the current distribution on a bent wire antenna with a shielded loop, the electromotive force (EMF) induced on the loop is theoretically formulated, and the relation between the current distribution and the measured values is investigated. The EMF is expressed as a summation of terms involving the current and the derivative of the current with coefficients depending on the measurement point. The calculated EMF agrees well with the measured value. At points away from the antenna ends and bend, the EMF is proportional to the current, but near the ends and bend the difference between EMF and current increases. It is shown that the current distribution is easily estimated from the measured data. The authors also present the measurement of charge distribution using a small dipole. The expression for the open-circuit voltage of the dipole is formulated and compared with measured values     

基于零阶谐振的高增益共形全向微带天线

为了实现与飞行器的共形,通过对等效磁流模型及负介电常数零阶谐振器的研究,设计了一种零阶谐振的共面磁流环阵列天线。该天线的工作频率为5 GHz,工作于TM01模式,具有全向辐射特性,水平面增益为3.4 dB,不圆度为2 dB,半功率波瓣宽度为50°。该天线具有水平面增益高,方向图不圆度小,剖面低的特点,且易于共形。     

Distributed Lumped Loads and Lossy Transmission Line Model for Wideband Spiral Antenna Miniaturization and Characterization

A lossy transmission-line model that incorporates the radiation resistance of small loop antennas is introduced to characterize the input impedance of a spiral antenna over a large frequency range. This model also leads to a concept of controlling the current velocity and impedance on a spiral antenna by introducing distributed reactive loading along the spiral arms. An important application of the concept is to reduce antenna size and this is demonstrated by numerical simulation examples.     

The shunt-driven circular loop antenna

The distribution of current, admittance, and far field of a shunt-driven circular loop antenna are derived and related to those of an equivalent series-driven loop in parallel with a section of transmission line. The effective length of a shunt-loaded receiving antenna is obtained in terms of the corresponding quantity for the series-loaded loop.     

Input impedance of a small loop of uniform electric current in an anisotropic cold plasma

The impedance of a small loop of uniform electric current immersed in a magnetoionic medium is studied for the case of a steady magnetic field normal to the plane of the loop. The quasistatic input impedance is calculated, and a first-order correction term is obtained. The impedance in a uniaxial medium is also given.     

基于矩量法的分形环八木天线的特性分析

八木天线是一种常用的天线形式.本文将Minkowski分形环应用于八木天线设计中,利用分形结构的空间填充特性来减小天线的横向尺寸,从而实现天线小型化设计.设计了工作于880～960MHz的6元二阶Minkowski分形环八木天线,以矩量法为核心对设计天线进行数值分析,将线天线模拟为细带线模型,天线表面采用三角单元进行剖分,RwG基函数作为电流展开函数.同方环八木天线进行了比较,在驻波特性、辐射特性相似情况下,分形环八木天线的尺寸缩减了29.8%.     

VHF helicopter antennas that incorporate the airframe and reducerotor modulation

Antennas at VHF frequencies mounted to small (with respect to wavelength) helicopter platforms are usually limited to whip and loop antennas. Antenna placement and rotor position can drastically change antenna performance due to induced currents on the platform. This work illustrates deliberate antenna-platform interactions that cause induced currents to reinforce and improve the radiating system performance. Additionally, rotor modulation effects can be simultaneously limited. This is accomplished by using oppositely placed loop antennas at a location on the airframe that is nominally one wavelength in circumference. The resulting broadcast pattern admits low cross-polarization radiation and low copolarized rotor modulation, but is not uniform. The dipole moment of the loops dictates this nonuniformity. Double-loop antennas are used to create a more uniform broadcast pattern without sacrificing the desired low cross-polarized radiation and low copolarized rotor modulation     

Radiation resistance of an elliptical loop antenna with constant current in compressible plasma

The radiation resistance of an elliptical loop antenna with constant current in compressible plasma is found by using a two-fluid magnetohydrodynamic model. The finite but small dimensions of the elliptical wire are taken into consideration in evaluating the contributions from the ion- and electron-plasma modes.