The performance properties of electrically small resonant multiple-arm folded wire antennas

In this tutorial article, the resonant radiation properties of electrically small, multiple-arm, folded wire antennas are considered and compared as a function of the antenna's height, the cylindrical diameter occupied, the geometry, and the number of folded arms within the antenna structure. The radiation properties considered include resonant resistance, efficiency, radiation patterns, and the operating bandwidth, which is characterized using the antenna's quality factor (Q). It is shown that electrically small, multiple-arm, folded wire antenna designs offer significant performance improvements relative to simple open-ended wire antennas, in terms of increased resonant resistance, bandwidth, and efficiency. However, when multiple-arm, folded wire antennas of the same height and cylindrical diameter, but having significantly different geometries, are made to be self-resonant at the same frequency, they exhibit similar resonant performance properties. This illustrates that the resonant performance properties of these antennas are primarily established by their height and the physical volume occupied relative to the resonant wavelength. Various design parameters are considered and described for achieving self-resonance and a reasonable impedance match with an electrically small, multiple-arm folded wire antenna. Finally, the performance properties of the multiple-arm folded wire configurations are compared with those of an impedance-matched, non-folded, open-ended wire configuration. It is shown that the multiple-arm folded wire configurations exhibit a lower Q than the impedance-matched, non-folded wire configuration.     

Radiation characteristics of a travelling-wave multi-turn† circular loop antenna in compressible electron plasma

The paper is on extension of the authors1 previous work on travelling-wave circular loop and helical antennas immersed in an isotropic compressible electron plasma. In the present, paper the expressions for the electromagnetic and electroacoustic components of the radiation field and of radiation resistance in the case of on electrically largo multi-turn circular loop antenna in plasma having a travelling-wave current distribution are derived, finally, treating the multi-turn loop in vacuum as a particular case of the general results for the same in a plasma medium, expressions for electromagnetic field and radiation resistance have been obtained far different loop sizes and the number of turns in it. It is shown that its field and the radiation resistance, both in plasma and in vacuum, are seriously affected by the number of turns in the loop and under certain conditions much higher values of radiation resistance can be obtained.     

Deembedding of lossy Foster networks

Data-processing procedures are described, which should be useful for measurement of the radiation Q factors of small antennas such as microstrip patch antennas or dielectric-resonator antennas. Typical values of radiation Q factors are quite low, of the order of ten or even smaller. Two equivalent Foster-type networks are proposed, modified for the presence of losses: one for the single resonance behavior, and another for the dual resonance behavior. The individual network elements are determined by optimization. A least squares procedure is described that can provide a good estimate for the starting values of the optimization.     

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.     

A discussion on the properties of electrically small self-resonant wire antennas

The performance properties of several electrically small, self-resonant wire antennas are compared as a function of their total wire length, geometry, and effective volume. The radiation properties considered include resonant frequency, radiation resistance, and quality factor (Q). It is shown that the resonant properties of these antennas are directly a function of the antenna's effective height and effective volume, which are established by both total wire length and geometry. When the total wire length and geometry of these antennas are configured such that the antennas exhibit the same effective height and volume, their resonant properties are essentially identical, independent of any differences in their total wire length and geometry. Both computed and measured data are presented to support the comparison of the antennas' resonant properties.     

Small but long Koch fractal monopole

A small but long wire fractal antenna based on the Koch curve is presented. Experimental and numerical results show that the antenna improves the features of a common linear monopole. The radiation resistance is increased and the Q is reduced at each fractal iteration, approaching the fundamental limit on small antennas     

Theory and analysis of differentially-driven microstrip antennas

This paper studies differentially-driven microstrip antennas. The theory of microstrip antennas based on the improved cavity model is expanded to analyze the input impedance and radiation characteristics of the differentially-driven microstrip antennas. The differentially-driven microstrip antennas were fabricated. Their performances were experimentally verified. Results show that the occurrence of resonance for the differentially-driven microstrip antennas also depends on the ratio of the separation /spl xi/ of the dual feeds to the free-space wavelength /spl lambda//sub o/. When the dual feeds are located far from each other /spl xi///spl lambda//sub o/>0.1, the resonance occurs, and the input resistance at resonance is rather large. However, when the dual feeds are located near to each other /spl xi///spl lambda//sub o/<0.1, the resonance does not occur, the input resistance is quite small, and the input impedance is inductive. Compared with single-ended microstrip antennas, the differentially-driven microstrip antennas have larger resonant resistance, similar co-polar radiation patterns, and lower cross-polar radiation component.     

Inductively coupled small self resonant coil (SSRC) reader antennas for HF RFID applications

A new model of inductively coupled high frequency radio frequency identification (HF RFID) reader antennas is presented in this paper based on the idea of using the self resonance frequency (SRF) of a small multi turn coil. The introduced multi turn small self resonant coil (MT SSRC) antenna is mathematically analyzed in terms of SRF, number of turns, dimensions and dielectric characteristics of the insulation, where present. Based on the analysis, a compact planar version of MT SSRC antennas having two turns, the two turn planar SSRC (TTP SSRC), is investigated and the dependency of the SRF to the antenna dimension is observed. A TTP SSRC antenna operating at 13.56 MHz is fabricated and is compared with an old model of HF RFID antennas; an optimized Q factor and a more uniform near field pattern is obtained for the new antenna. The benefits of the obtained optimized Q factor and uniform near pattern is explained for smart shelf application. Also, a number of TTP SSRC antennas operating at a distinct frequency, 13.56 MHz here, are fabricated on different substrates and it is shown that the Q factor and dimension of the TTP SSRC antenna could be controlled and adjusted based on the dielectric characteristics of the substrate. The new antenna prototype has a beneficial application to smart shelf applications in HF RFID.     

Investigation of properties of electrical-small spherical ceramicantennas

The fundamental properties of electrically small material loaded antennas are investigated by analysis of a spherical model. Data for antenna Q, resonance and efficiency demonstrate the new concept of compatible boundary conditions and its implication for practical design, particularly for mobile phones     

Low Q electrically small linear and elliptical polarized spherical dipole antennas

Electrically small antennas are generally presumed to exhibit high impedance mismatch (high VSWR), low efficiency, high quality factor (Q); and, therefore, narrow operating bandwidth. For an electric or magnetic dipole antenna, there is a fundamental lower bound for the quality factor that is determined as a function of the antenna's occupied physical volume. In this paper, the quality factor of a resonant, electrically small electric dipole is minimized by allowing the antenna geometry to utilize the occupied spherical volume to the greatest extent possible. A self-resonant, electrically small electric dipole antenna is presented that exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and a quality factor that is within 1.5 times the fundamental lower bound at a value of ka less than 0.27. Through an arrangement of the antenna's wire geometry, the electrically small dipole's polarization is converted from linear to elliptical (with an axial ratio of 3 dB), resulting in a further reduction in the quality factor. The elliptically polarized, electrically small antenna exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and it has an omnidirectional, figure-eight radiation pattern.     

Antennas for VHF/UHF personal radio: A theoretical and experimental study of characteristics and performance

A theoretical analysis of the characteristics of small personal radio antennas for the 68- to 470-MHz frequency range is given. Representing the human body by a simplified lossy dielectric structure, the influence of the body on the performance of the antenna is investigated in detail, and it is shown how antenna impedance, gain, and radiation patterns can be calculated taking the presence of the body into account. For very short antennas the results indicate that radiation from the body may dominate over the radiation contributed by the antenna itself, and that the presence of the body can increase the antenna efficiency considerably, indicating that even very short antennas may provide acceptable radiation efficiencies. The results of the theoretical work are supported by measurements on practical antennas. Quarter-wave and short antennas of the helical type are compared with respect to efficiency and radiation patterns at 80, 160, and 450 MHz, and it is demonstrated how the physical length of the antenna affects the antenna performance. The design of a very short and compact personal radio antenna is described.     

On the radiation Q and the gain of crossed electric and magnetic dipole moments

Exact expressions for the radiation quality factor Q and the gain pattern function of the corresponding antenna are derived and their properties are analyzed for crossed electric and magnetic dipole moment configuration. Both omni-directional and directional antennas with linear, elliptical, or circular polarization can achieve the same minimum radiation Q. The maximum gain of the antenna ranges from 3/2 to 3. The maximum gain of three can be achieved only for linear polarization when the source vectors make a right angle and the amplitudes of the TM and the TE modes are equal. The increase in the maximum gain from 3/2 for omni-directional antennas to 3 for directional antennas does not require any increase in the radiation Q, which suggests that practical directional antennas with higher gain may be realized with smaller size compared with small omni-directional antennas assuming similar operation bandwidths.     

Design of electrically small Yagi antenna

A two-element electrically small Yagi antenna is presented. The antenna is composed of a spiral-shaped driver and a director. To step up the low radiation resistance due to the small element size and close element spacing, multiple folded arms are used on the driver element. The design has been optimised, built and measured. The total volume of the antenna is 0.065lambdatimes0.095lambdatimes0.095lambda and the measured realised gain toward the director direction is 8.81 dB     

A Reevaluation of the Radiation Q Bounds for Loop Antennas

A new realistic lower bound for the radiation Q of loop antennas is presented. It includes the contribution of energy within a circumscribed sphere, and is at least three times the often-referenced Q based only on external energy. The new bound is compared with actual Q values over a range of electrical diameters and aspect ratios, using design equations and electromagnetic simulations.     

Approximations to the radiation resistance and directivity ofcircular-loop antennas

The purpose of this article is to derive approximate formulas for the radiation resistance (R), and the directivity (D) of circular loop antennas. It is shown that simple approximations to the Bessel functions can be employed, to accurately model the oscillatory behavior of the Bessel function integral for both small and intermediate-sized loop antennas. Furthermore, when these approximations are combined with the usual asymptotic contributions to the integral in the case of a large input parameter ka (a=loop radius, a=2π/λ) accurate and relatively simple results for R and D can be secured for all loop sizes. Numerical results can, if necessary, be obtained using a simple pocket calculator     

A singular integral representation of the electromagnetic field: Physical regularization of the ill-posedness of the near-field zones of antennas

A classification of ill-posedness in the antenna theory is presented in the case when the basic principles of electromagnetics are not fulfilled, which is observed in the near-field radiation zones. The Tikhonov regularization of Fredholm integral equations of the first kind is compared with the physical regularization (a self-consistent method). The ill-posedness arising in the analysis of the electromagnetic field (EMF) of an electric dipole, an equidistant array on its basis, frame antennas, and a cylindrical helical antenna is considered. In particular, for the EMF of an electric dipole, the gap between the field and surface current density is removed and the limits of applicability of the method of induced electromotive forces of coupled dipoles are established. It is shown that unidirectional radiation in the azimuthal plane is typical of a ring frame antenna with a certain dimension of the ring. For a cylindrical helical antenna, the calculation of the surface current density is reduced to a hypersingular integral equation with a quadratic Cauchy singularity. The analysis of the behavior of the current flowing along the helix made it possible to explain the peculiarities of the antenna pattern for various helical turns.     

印刷环天线的谐振特性

本文基于环形几何上的超二次曲线表示,分析一组印刷环天线的谐振特性,首先给出求解天线电流分布和辐射场的统一数值计算公式,然后通过改变超二次曲线的开头参数,分析印刷在不同介质基片上环形天线的形状与其谐振电阻和谐振环周长的关系。结果表明,可以通过调整环形天线的几何形状获得与馈线相匹配的谐振电阻。     

Axial mode helical antennas

The radiation characteristics of helical antennas operating in the axial mode are evaluated on the basis of the theoretical current distributions and are verified by experimental work. A comparison between monofilar helix and bifilar helix antennas is made, and a helical antenna with a parasitic helix (HAP) is proposed in order to enhance the power gain. It is found that the gain of the HAP in which the parasitic helix is wound from a point diametrically opposite to that of one and one-half turns of the driven helix is about 1 dB higher than that of the monofilar helix. The ratio of the frequency band in which the HAP radiates a circularly polarized wave within an axial ratio of 3 dB is calculated to be 1:1.8.     

Electric surface current model for the analysis of microstrip antennas with application to rectangular elements

An approach to the analysis of microstrip antennas which is applicable also to relatively thick substrates using the relevant Green's function is presented. The Green's function is derived and closed form expressions for various antenna characteristics which explicitly take into account the presence of the dielectric material are obtained in terms of the electric surface current density. For rectangular microstrip elements near resonance the current distribution is approximated using lossless transmission line analysis, thus enabling the complete evaluation of the characteristics of the element near resonance. The results obtained in this approach for the radiation resistance, surface wave resistance, radiation pattern, directivity, and bandwidth are presented in a detailed set of graphs for a representative set of parameters.     

加载小型天线的谐振腔特性分析及应用研究

提出了一种测量小型天线的方法,以适应对数量极为庞大的终端天线产品特性的逐一检测.理论分析和实验验证均表明:谐振腔内放入双频天线时,会出现三个谐振频率,其一为加载天线后的腔体的谐振频率,另外两个是受外加激励由天线产生的辐射场.前者携带有天线几何结构及其构成材料的信息,后者含有天线辐射特性方面的信息.据此,建立了一套可对批量天线产品性能进行检测的谐振腔系统,该系统还可对其它小体积样品的物理特性进行测量,同时使谐振腔的应用由对一般材料特性的测量扩展到了对有辐射特性器件的测试.