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。     

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.     

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.     

Study of the coupling between human head and cellular phone helical antennas

The interaction between normal-mode helical antennas and human head models is analyzed, using both a novel accurate semi-analytical method and finite-difference time-domain (FDTD) simulations. The semi-analytical method is based on the combination of Green's functions theory with the method of moments (Green/MoM) and is able to model arbitrarily shaped wire antennas radiating in the close proximity of layered lossy dielectric spheres representing simplified models of the human head. The purpose of the development of the Green/MoM technique is to provide a reliable tool for preliminary (worst case) estimation of human head exposure to the field generated by different antenna configurations with emphasis on the helical antenna, representing the most diffused antenna type used in modern cellular handsets. Furthermore, the accurate semi-analytical character of the Green/MoM technique permits the accuracy assessment of purely numerical techniques, such as the FDTD, which is currently the most widely used computational method in mobile communication dosimetric problems, since it allows modeling of anatomically based head models. After appropriate benchmarking, FDTD simulations are used to study the interaction between a heterogeneous anatomically correct model of the human head exposed to a normal-mode helix monopole operating at 1710 MHz mounted on the top of a metal box representing a realistic mobile communication terminal. The study of both canonical and realistic exposure problems includes computations of specific absorption rates (SARs) inside the human head, total power absorbed by the head and assessment of antenna performance. Emphasis is placed on the comparative dosimetric assessment between adults and children head models.     

Wire antennas over a lossy half-space

A recently developed technique for approximate but accurate evaluation of the various vector potential components associated with a current element radiating over a lossy ground is used to study the problem of antennas radiating over a lossy ground. A general integral equation for an arbitrarily shaped thin-wire antenna over a lossy half-space is derived, and the method of moments is employed to process this equation numerically. Illustrative numerical results are presented to demonstrate the effect of the lossy half-space on a number of antenna configurations.     

Radiation and scattering features of patch antennas with bianisotropic substrates

The analysis of patch antennas residing in cavities filled by general inhomogeneous and lossy bianisotropic substrates is presented. The theoretical study is based on a variational formulation associated with the boundary value problem under analysis, and a hybrid finite element-boundary integral method is employed to solve the electromagnetic field inside and outside cavities numerically. Initially, the general formulation presented is applied to some particular cases known in the literature. Then, the main scattering and radiation features of cavity backed patch antennas with chiral, anisotropic, and bianisotropic materials are presented. Particularly, it is shown that complex media allow for frequency control and for a reduced antenna size at a given operating frequency.     

Physical limitations of antennas in a lossy medium

The dissipated power and the directivity of antennas in a homogeneous, lossy medium are systematically analyzed in this paper. The antennas are ideal and located inside a lossless sphere. In the lossy space outside the sphere, the electromagnetic fields are expanded in a complete set of vector wave functions. The radiation efficiency, the directivity, and the power gain are defined for antennas in a lossy medium, and the optimal values of these quantities are derived. Simple relations between the maximal number of ports, or channels, an antenna can use and the optimal directivity and gain of the antenna are presented.     

近地短波鱼骨天线矩量法建模及应用

结合等效电路理论和离散复镜像方法,对架设在有耗地面上方的水平鱼骨天线进行矩量法建模,计算了其短波段内的输入阻抗、方向图、效率和增益.在分析双鱼骨天线的基础上,从抗极化衰落的角度提出并分析了一种正交鱼骨天线.仿真结果表明,所用方法准确地考虑了有耗地面的影响以及各振子间的互相耦合,具有很高的精度.     

Impedance, bandwidth, and Q of antennas

To address the need for fundamental universally valid definitions of exact bandwidth and quality factor (Q) of tuned antennas, as well as the need for efficient accurate approximate formulas for computing this bandwidth and Q, exact and approximate expressions are found for the bandwidth and Q of a general single-feed (one-port) lossy or lossless linear antenna tuned to resonance or antiresonance. The approximate expression derived for the exact bandwidth of a tuned antenna differs from previous approximate expressions in that it is inversely proportional to the magnitude |Z'/sub 0/(/spl omega//sub 0/)| of the frequency derivative of the input impedance and, for not too large a bandwidth, it is nearly equal to the exact bandwidth of the tuned antenna at every frequency /spl omega//sub 0/, that is, throughout antiresonant as well as resonant frequency bands. It is also shown that an appropriately defined exact Q of a tuned lossy or lossless antenna is approximately proportional to |Z'/sub 0/(/spl omega//sub 0/)| and thus this Q is approximately inversely proportional to the bandwidth (for not too large a bandwidth) of a simply tuned antenna at all frequencies. The exact Q of a tuned antenna is defined in terms of average internal energies that emerge naturally from Maxwell's equations applied to the tuned antenna. These internal energies, which are similar but not identical to previously defined quality-factor energies, and the associated Q are proven to increase without bound as the size of an antenna is decreased. Numerical solutions to thin straight-wire and wire-loop lossy and lossless antennas, as well as to a Yagi antenna and a straight-wire antenna embedded in a lossy dispersive dielectric, confirm the accuracy of the approximate expressions and the inverse relationship between the defined bandwidth and the defined Q over frequency ranges that cover several resonant and antiresonant frequency bands.     

Path loss model for wireless narrowband communication above flat phantom

A new empirical path loss model for wireless communication at 2.4 GHz above a flat, lossy medium, representing human tissue, is presented. The model is valid for dipole antennas for heights up to 5 cm above the phantom and for distances up to 40 cm, and was applied to muscle and brain simulating media. For antennas placed close to the lossy medium, it was found that antenna height has a major influence on path loss. The model has been validated by measurements and simulations, which show excellent agreement     

Microstrip antennas

Microstrip antennas have been one of the most innovative topics in antenna theory and design in recent years, and are increasingly finding application in a wide range of modern microwave systems. This paper begins with a brief overview of the basic characteristics of microstrip antennas, and then concentrates on the most significant developments in microstrip antenna technology that have been made in the last several years. Emphasis is on new antenna configurations for improved electrical performance and manufacturability and on advances in the analytical modeling of microstrip antennas and arrays     

Asymmetry phenomenon of log-periodic dipole antennas

The existence of asymmetric resonant modes on standard log-periodic dipole antennas is established experimentally. These modes are characterized by sharply-resonant side radiation sometimes accompanied by reduction in front-lobe gain. The resonances occur at frequencies whose relationship is approximately log-periodic. In a single antenna the resonant modes are excited by any structural asymmetry, while in anE-plane array each individual antenna excites the others asymmetrically. The resonances can be eliminated by the addition of lossy material to appropriate parts of the antenna. Qualitatively the phenomenon is explained in terms of transmission-line resonances along the two-wire lines formed by adjacent dipoles. Automated swept-frequency far-field measurement techniques were employed throughout, and their worth is clearly established for broadband antenna research and development.     

Signal correlation between two normal-mode helical antennas for diversity reception in a multipath environment

The spatial correlation coefficient of signals received by two normal-mode helical antennas in a multipath environment characterized by Clarke's model is rigorously obtained. A new formula relating the signal correlation coefficient to the mutual impedance is derived. The mutual impedance is defined and calculated in a new manner. Our findings show that: 1) the envelope correlation coefficient of two helical antennas is greater than that of two monopole antennas; 2) the envelope correlation coefficient is dependent on the antenna terminal load and the mutual impedance and an optimal antenna load can be determined to produce the smallest correlation coefficient; and 3) the frequency response of the envelope correlation coefficient resembles that of the voltage standing-wave ratio.     

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     

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.     

A rigorous derivation of the relation between the effective areaand the directive gain and its extension to lossy antennas

It is well known that the effective area of a receiving antenna is linearly related to the directive gain of the same antenna in the transmitting mode. Conventionally, this relation is derived by constructing a two-port network involving the transfer impedance between a pair of antennas. Next, by use of the Thevenin theorem, the received power is related with the open-circuit voltage, which in turn is proportional to the transfer impedance. Alternatively, from field analysis, the received power is related to the effective area and the directive gain. Then, the reciprocity theorem is used to equate the transfer impedances between a pair of antennas. Based on these, the area-directivity relation is derived. However, it is indicated that the circuit model with the open-circuit voltage is not rigorous. In this investigation, the reciprocity theorem is used in a different way to directly derive the power absorbed at the load in terms of the radiated electric field in the transmitting mode. Thereby, the area-directivity relation is derived in a straightforward and rigorous manner. In the derivation of the relation, it is seen that the current in the terminal gap should be uniform. Thus, one limitation of this well-known relation is found. The effect of ohmic loss due to finite conductivity of the antenna material can be taken into consideration in the derivation. Thus, the area-directivity relation is extended such that it is applicable to lossy antennas     

Characteristics of body-mounted antennas for personal radio sets

The development of a multiturn loop antenna for a police officer's personal radio set is described. The antenna is small, low profile, lightweight, with dimensions of2.7 times 2.7 times 0.7in high (1.06 times 1.06 times 0.28cm), and is tunable over the 150- to 170-MHz band. VSWR and radiation patterns are presented with the antenna mounted on a man's shoulder. The shoulder-mounted antenna characteristics are described with conditions where the man is in standing, stooping, and prone positions, with various head and arm movements, and with light and heavy clothing. Radiation patterns are referenced to a 6-in helical whip commonly used by police officers. Radiation patterns also indicate body absorption loss for loop and whip antennas mounted at waist level as compared with shoulder levels. The instantaneous bandwidth is 1.4 MHz for a VSWRleq 3:1. A dual-frequency model for use in a two-frequency push-to-talk mode, is also shown to be practical.     

Conical-reflector antennas

The mechanical advantages of a singly curved conical reflector are demonstrated by the experimental test of a furlable 1.83 m conical-Gregorian antenna at 16.33 GHz. The measured gain of 47.5 dB corresponds to a net efficiency of over 57 percent. A ray-optics analysis of conical-reflector antennas is presented, and data useful in the design of conical antennas is given. The conical-Gregorian antenna, in which a subreflector is used in conjunction with a conventional horn feed, is considered in detail. A physical-optics analysis of the conical-Gregorian antenna is used to investigate diffraction and other effects, and to analytically confirm the high performance of the antenna. It is concluded that conical antennas are a valuable addition to available antenna-design concepts.     

Passive intermodulation on large reflector antennas

In this work, an analytical model for the study of passive intermodulation (PIM) on large reflector antennas is presented. Passive intermodulation, in the scattered field, arises when the scatterer is nonlinear, or when it presents junctions connecting linear materials. Its presence causes a degradation of some antenna parameters and, especially, the cross-polar level, which may rise by several tens of dB. It would then be useful to develop a technique to predict its influence, in order to take appropriate steps during antenna design. A heuristic model for the junction problem has previously been derived and validated with measurements in a time-domain physical optics (TD-PO) framework. These results are applied here to a TD-PO analysis of reflector antennas, in particular, for a satellite-communication antenna and for a radio-astronomy antenna.     

Improved theory for microstrip antennas

An improvement to a recently reported theory for the analysis of the pattern and impedance loci of microstrip antennas is developed. It yields a theory that is simple and inexpensive to apply. This is achieved by lumping all antenna losses into an effective dielectric loss tangent and then analysing the microstrip antenna as a lossy cavity. It is found that the resulting expression for impedance of the microstrip antenna is in good agreement with measured results for all modes and feed locations.