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.     

Uncertainties in 3-m radiated emission measurements due to the use of different types of receive antennas

This work describes the results of an investigation into uncertainties in radiated emission measurement results due to the use of different receive antennas. These antennas are used during electromagnetic-compatibility tests of products which must comply with regulatory radiated emission requirements. In the past few years, the standardization International Special Committee for Radio Protection (CISPR) Subcommittee A (SCA) has tried to improve the radiated emission measurements by quantifying most of the uncertainty sources. We deal with the uncertainty due to the free space antenna factor's inability to fully specify the antenna properties at frequencies up to 1000 MHz. We have simulated and measured the radiated emission of an equipment-under-test (EUT) using various types of receive antennas. The simulated radiated emission measurement results using a biconical antenna give a maximum difference of 1.2 dB in relation to the use of a tuned dipole. The use of bow-tie antennas leads to a radiated emission difference of maximal 2.5 dB in relation to the tuned dipole. Finally, the use of a log-periodical antenna leads to a maximum deviation of 2.8 dB in relation to the tuned dipole. This means that the uncertainty caused by using different types of receive antennas in 3-m radiated emission measurements may be significant, so the presented uncertainty budgets in CISPR 16-4-2 should therefore be reconsidered and expanded.     

基于电驻极体运动的小型化低频发射天线

低频电磁波具有较低的传输损耗,但是传统的谐振式天线往往尺寸较大,工程应用的难度很大。运流电流可以产生时变的辐射电磁场,并且工作频率不再受传统谐振天线的尺寸限制,这为小型化的低频发射天线提供了一种有价值的思路。通过理论计算,研究了振动电驻极体的电磁场和等效模型,通过仿真进行验证,并对基于电驻极体机械运动的小型化的低频天线进行了有益的探索。研究表明,振动的电驻极体可实现类似于微小电流元的辐射特性。     

利用FDTD方法比较手提电话天线的方向图,输入阻抗及人头部的比吸收率(SAR)(英文)

本文给出了利用FDTD电磁模拟方法对常用的五种个人通信手提电话的天线和人头部组织相互作用的比较研究。在研究中考虑的蜂窝通信手提电话常用的天线种类为单极子、偶极子和倒F型（IFA）天线。不均匀的实际人头模型被用于预测天线特性（包括输入阻抗，方向图和辐射效率等）和对操作作者耦合的电磁效应。天线在人头部的潜在危险影响是用1W辐射功率在人组织产生的比吸收率（SAR）来评估。     

The input impedance of the dielectric resonator antenna

Dielectric cylinders of very high permittivity have been used in the past as resonant cavities, but since the structure is not enclosed by metallic walls, electromagnetic fields do exist beyond the geometrical boundaries of the structure and part of the power is radiated. Through the proper choice of geometry and permittivity this radiation can become the dominant feature of the structure and become an efficient antenna for use at millimeter wave frequencies. Both experimental and theoretical investigations of a variety of these dielectric resonator antennas have been undertaken. In particular, the input impedance of a probe-fed cylindrical structure was examined in detail and a comparison of theoretical and experimental results was made.     

Propagation of EM pulses excited by an electric dipole in aconducting medium

Exact solutions are given for the transient electromagnetic (EM) fields excited by an electric dipole antenna with an impulsive current in a conducting medium. There exists an optimum waveform for the antenna current which can generate an EM pulse with a maximum intensity at a particular distance from the antenna. It is found that the EM fields of an EM pulse excited by an antenna in a conducting medium can be divided into two parts. The first part is an impulse wave which propagates with the speed of light (1/√(μϵ)) and decays exponentially. The second part builds up gradually and propagates slowly, and more importantly this part attenuates as an inverse power of distance which is a much slower rate than an exponential decay     

超高频射频识别小间隔双标签天线增益特性研究

该文针对超高频射频识别(UHF RFID)标签自身参数和标签间互相耦合对标签天线增益的影响,推导了相应的理论模型。首先从辐射场的角度将标签等效为带集总负载的对称振子天线,然后基于对称振子天线阵列的理论建立了小间隔双标签天线增益的简化模型,并对密集多标签应用场合进行了简单拓展,仿真研究验证了所建模型的有效性。最后对增益的方向性和辐射效率进行了研究,研究结果对密集标签的性能研究具有一定的指导意义。     

Coupling between small thin-wire antennas and a biological body

Coupling between the electromagnetic (EM) field of a small thin-wire antenna and an adjacent finite biological body is investigated analytically. Proximity effects of the body on the impedance and radiated power of the antenna are studied; the induced field and dissipated power in the biological body are determined. Radiation fields of the antenna-body system are evaluated.     

标签天线弯曲对射频识别系统性能影响的研究

探讨了工作在微波段的无源射频识别(Radio frequencyIdentification, RFID)系统中标签天线的弧形弯曲对RFID系统性能的影响。分析比较了几种典型的半波振子标签线天线弧形弯曲的情况。通过理论分析确立标签天线的方向系数与输入阻抗为RFID系统性能的主要影响因子。弯曲振子天线特性参数由线天线分析软件NEC WinBasic数值模拟得到。分析结果表明,标签天线的弧形弯曲使得无源RFID系统的工作距离明显变短。因此,为避免此类问题有必要在RFID系统中考虑对于弯曲具有低敏感性的天线结构。     

Energy deposition in simulated human operators of 800-MHz portable transmitters

The measured values of energy deposited in simulated human tissue exposed for one minute in the immediate vicinity of 800 MHz portable radio transmitters are presented. The deposited RF energy was evaluated by temperature measurements. The portable radio used in the tests had a 6-W experimental transmitter operating at 840 MHz. Two different antennas were tested for energy deposition: a sleeve dipole and a resonant whip. The two antennas have given substantially different results indicating different field structures near the two radiators. The experiments with flat slabs have shown that the sleeve dipole deposits higher levels of power density than the resonant whip in the near field although the length of the latter radiator is about half the size of the former. The temperature profiles generated by both antennas inside the head of the simulated operator indicate the presence of a "hot spot" about 1 in below the surface of the temporal bone. This phenomenon was not detected previously at lower frequencies. The short antenna exposes the eye of the operator to more intense power deposition than the sleeve dipole. The temperature increases measured during the investigation are so small that no thermal damage to tissue should be caused by normal use of the portable radio.     

Electromagnetic absorption in the human head from experimental6-GHz handheld transceivers

We have used a new millimeter-resolution MRI-based model of the human body to calculate the electromagnetic absorption in the head and neck for three experimental Yagi antennas suggested for handheld transceivers of a proposed 6-GHz personal communication network (PCN) system. The SAR distributions are obtained with a resolution of 1.974×1.974×1.5 mm for transceivers that are held against the ears and tilted forward by 33°. The finite-difference time-domain technique is used to calculate the EM fields and SARs for the transceiver, antenna, and head and neck coupled region that is divided into 158×84×188 or nearly 2.5 million cells. The highlights of the numerical calculations are verified by means of a head-shaped experimental model made of tissue-equivalent materials simulating the electrical properties (ϵ_r,σ) of the skull, brain, muscle, eyes, and ears developed for use at 6 GHz. Because of the proximity to the antenna, the highest SARs are obtained for the upper part of the ear. For a planned radiated power of 0.6 W, the peak SARs averaged over any 1 g of tissue defined as a tissue volume in the shape of a cube are on the order of 0.5-1.0 W/kg for two of the proposed antennas and considerably higher (2.06 W/kg) for the third antenna using a narrower off-axis reflector. Low SARs for the first two antennas are likely due to the shielding provided by the relatively wider strip reflector plates used for these antennas     

Exact solutions of electromagnetic fields in both near and farzones radiated by thin circular-loop antennas: a general representation

This paper presents an alternative vector analysis of the electromagnetic (EM) fields radiated from thin circular-loop antennas of arbitrary radius a. This method, which employs the dyadic Green's function in the derivation of the EM radiated fields, makes the analysis more general, compact, and straightforward than those two methods published recently by Werner (1996) and Overfelt (1996). Both near and far zones are considered so that the EM radiated fields are expressed in terms of the vector-wave eigenfunctions. Not only the exact solution of the EM fields in the near and far zones outside the region (where r>a) is derived by the use of the spherical Hankel function of the first kind, but also the closed-series form of the EM fields radiated in the near zone inside the region 0⩽r   

A transient array to increase the peak power delivered to a localized region in space .I. Theory and modeling

It is shown that an array of pulsed antennas (a transient array) can be used for providing a high concentration of electromagnetic energy into small regions. By controlling the timing of the pulses radiated from the individual elements, selected regions of space will see a coherent superposition of the radiated pulses forming "hot spots." The formation of "hot spots" is demonstrated by theoretical analysis. Using a highly damped sinusoid for the radiated pulse and dipole antenna elements, a contour plot of the peak electric field is determined for a linear array of six equally spaced sources. The impact of pulse-timing errors and the factors determining the size of the hot spot are also considered.     

Backfire antennas with dipole elements

A method is set up for a theoretical investigation of arbitrary backfire antennas based upon dipole structures. The mutual impedance between the dipole elements of the antenna is taken into account, and the field radiated due to a surface wave reflector of finite extent is determined by calculating the surface current distribution on the reflector plate. Numerical results obtained for Yagi backfire antennas and short-backfire antennas using this theory are compared with experimental results.     

Standard antennas for electromagnetic interference measurements andmethods to calibrate them

This tutorial paper discusses various standard antennas for measuring radio-frequency electric and magnetic fields. A theoretical analysis of each antenna's receiving characteristics is summarized and referenced. The antennas described are an electrically short dipole, a resistively loaded short dipole and halfwave dipole, an electrically small loop, a resistively loaded loop, photonic probes, and a single-turn loop designed for simultaneous measurement of the electric and magnetic components of nearfields and other complex electromagnetic environments. Each type demonstrates a different compromise between broadband responses and sensitivity. This paper also discusses the calibration techniques for these probes using standard EM fields established in TEM cells, waveguide cells, anechoic chambers, and open-field sites     

NUMERICAL SIMULATION OF ANNULAR PHASED ARRAY OF DIPOLE ANTENNAS USING THE FD-TD METHOD

The problem for calculating near fields of EM radiation systems by using the finitedifference time domain(FD-TD)method are discussed and the annular phased array of dipoleantennas has been simulated numerically by use of the FD-TD method.For a test run thenear field and current distribution of the single dipole antenna are calculated.The near fieldsof the annular phased array of dipole antennas in central region filled with deionized water arecomputed and the interaction of near fields with an anatomically-based inhomogeneous model ofhuman torso is considered as well.     

左手介质在增强电偶极子辐射功率上的应用

基于严格的电磁场理论,给出了左手介质壳包围电偶极子后的辐射场本征函数级数解.应用该级数解求解出该情况下的辐射功率,并与自由空间中电偶极子辐射场进行了比较,指出了左手介质在增强电偶极子辐射功率上的应用,并运用微波电路理论对该特性进行了分析验证.     

A Coaxial Microwave Applicator for Direct Heating of Liquids Filling Chemical Reactors

A coaxial microwave applicator radiating in a liquid medium contained inside a chemical reactor is described. The applicator consists of an insulated asymmetrical dipole antenna that radiates almost isotropically. Hence, it appears well suited to directly heat the medium that fills the vessel, making it unnecessary to use a microwave oven as currently done in microwave assisted chemistry. The electromagnetic (EM) properties of the insulated dipole antenna emitting in high permittivity lossy media are first reviewed to define the applicator design criteria. A 3-D numerical EM solver is then employed to analyze the radiation of the applicator in the surrounding medium taking into account the whole structure of the reactor. Safe operations are always assured by the proper design of the applicator-vessel mechanical connection that drastically reduces unwanted stray radiation. The agreement between the theoretical analysis and the experiments performed is good, thus confirming that the coaxial applicator compares favorably with the traditional methods of activation, which use a single or multimode resonant EM cavity.     

Standard probes for electromagnetic field measurements

Discusses various standard antennas for measuring radio-frequency electric and magnetic fields. A theoretical analysis of each antenna's receiving characteristics is summarized and referenced. The standard probes described are an electrically short dipole, a resistively-loaded dipole, a half-wave dipole, an electrically small loop, and a resistively-loaded loop. A single-turn loop designed for simultaneous measurement of the electric and magnetic components of near-fields and other complex electromagnetic environments is also described. Each type of antenna demonstrates a different compromise between broadband frequency response and sensitivity     

An implanted spherical head model exposed to electromagnetic fields at a mobile communication frequency

Can cellular phones and personal communication systems base station antennas affect the active or passive implantable medical devices adversely? Concerns over the possible harmful effects of nonionizing irradiaton upon implanted medical devices have been present for many years. Key issues to address are the questions of whether mobile phones have a detrimental effect on implants, and how the interaction of the handset with the body can be minimized in order to both alleviate public fears and improve handset antenna performance and new implant designs. This paper presents a thorough investigation of the scattering of an electromagnetic (EM) wave from a perfectly conducting implant (a cylindrical wire and a very thin cylindrical disk) of electrically small radius (of resonant length), embedded eccentrically into a dielectric spherical head model by a dipole antenna (0.4 wavelength) at 900 MHz. The dyadic Green's function (DGF) for spherical vector wave functions is employed. Analytical expressions for the scattered fields of an implant embedded head model is obtained. Numerical results from analytical expressions are computed for this problem and then compared with the results from the same model using the finite-difference time-domain, EMU-FDTD electromagnetic simulator. Good agreement is observed between the analytical results on the proposed method in comparison with the FDTD method.