Transmitter-to-Antenna Power Transfer Under Unmatched Conditions

One phase of the electromagnetic compatibility program is the determianation of the electromagnetic energy actually radiated into space by an antenna. Methods have long been established, or proposed, for measuring the power output of a transmitter, the antenna impedance, and the antenna-radiation pattern. However, very little definitive work has been published regarding the methods for computing the transmitter energy coupled to the antenna and radiated at harmonic and spurious frequencies. It is the purpose of this paper to reduce to a simple form the equations for determining the power absorbed by an antenna connected through a coaxial transmission line to a transmitter. Most discussions of transmission-line-power transfer assume the matched conditions so universally desired at the operating frequency. When harmonic and spurious frequencies are considered, both the transmitter and the antenna are likely to be poorly matched to the transmission line and to each other. This paper points out what measurements need to be made and gives the functional relations necessary to compute the power radiated under unmatched conditions. Since the actual power coupled to the antenna in a given instance may be critically dependent upon the exact length of transmission line, some means must be available to take this into account. The expressions derived give the maximum and minimum power that will be absorbed, and also give the probability that any specified intermediate power will be exceeded if a random choice of transmissionline length is used.     

电小线圈近场分析

天线的近场分析在近场通信(NFC)、电磁兼容(EMC)、阵列天线设计等领域越来越受到关注。准确高效的近场分析方法对于近场的分布控制和引导机制等有着重要的作用。本文针对典型天线磁偶极子(电小线圈)的近场进行了初步的理论研究,主要分析了磁偶极子的近场平均能量密度分布、有功功率和无功功率的关系和分布。并通过磁偶极子的场分量计算平均坡印廷矢量,从而定性分析了线圈"储能"和"辐射能量"的关系,同时根据平均坡印廷矢量给出了电小线圈的近场"储能"中能量流动的物理图像。本文还计算了磁偶极子的平均电场能量密度和平均磁场能量密度,并进一步分析了二者的分布以及比较了二者的大小关系。     

A 915-MHz antenna for microwave thermal ablation treatment: physical design, computer modeling and experimental measurement

A 915-MHz antenna design that produces specific absorption rate distributions with preferential power deposition in tissues surrounding and including the distal end of the catheter antenna is described. The design features minimal reflected microwave current from the antenna flowing up the transmission line. This cap-choke antenna consists of an annular cap and a coaxial choke which matches the antenna to the coaxial transmission line. The design minimizes heating of the coaxial cable and its performance is not affected by the depth of insertion of the antenna into tissue. The paper provides a comparison of results obtained from computer modeling and experimental measurements made in tissue equivalent phantom materials. There is excellent agreement between numerical modeling and experimental measurement. The cap-choke, matched-dipole type antenna is suitable for intracavitary microwave thermal ablation therapy.     

Measurement of equivalent power density and RF energy deposition inthe immediate vicinity of a 24-GHz traffic radar antenna

In response to reports of alleged health effects associated with the use of hand-held traffic radars, e.g., testicular cancer, a study was undertaken to quantify: (1) the distribution of the electric field (E) in the immediate vicinity of the antenna aperture of a typical 24-GHz hand-held traffic radar; and (2) the relative match and the corresponding depth of penetration of the absorbed energy when the antenna aperture is positioned in contact with material having properties similar to human tissue. The former measurements are important for assessing a large number of measurements reported by others using techniques that are not expected to be reliable indicators of power density at distances small compared with the size of the radiator; the latter are important for verifying predictions that energy from the antenna at 24-GHz will be predominantly absorbed in superficial tissue, i.e., the skin. The results of the first part of the study indicate that the equivalent-plane-wave power density in the plane of the aperture of the radar antenna is approximately one-half of the corresponding values reported in the literature; the results of the second part of the study indicate that the depth of penetration into material simulating human skin is approximately 0.5 mm     

A low-loss unidirectional dielectric radiator (UDR) for antenna andspace power combining circuits

A low-loss unidirectional dielectric radiator (UDR) is proposed for applications to antenna and space power combining circuits. The operating principle of this novel radiator is explained in detail. Experimental prototypes using polystyrene are fabricated and measured around 9.5 GHz. A coaxial open-ended probe is used to excite this resonant radiator. A detailed study based on experimental results demonstrates interesting characteristics of a broad-beam with gain of up to 10 dBi and low cross-polarization. This type of radiator may find millimeter-wave applications in mobile and satellite communication systems     

Power deposition of a microstrip applicator radiating into alayered biological structure

The power deposited by a microstrip antenna into a layered biological structure is presented. The solution is based on an integral equation for the surface current density on the antenna and on an electric Green's dyadic for the fields inside a planar stratified medium. The integral equation is solved using the method of moments in conjunction with the point-matching technique. The modeling of the surface current takes the edge conditions into account. Special attention is devoted to a correct modeling of the excitation of the antenna by a coaxial feed. The numerical results focus on the power deposition as a function of depth     

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.     

Characterization of the heat loading of Nd-doped YAG, YOS, YLF, andGGG excited at diode pumping wavelengths

The parameter ξ (xi) is proposed as an alternative to the traditional solid-state laser media heating parameter, χ (chi). ξ is the ratio of heat produced to energy absorbed, and χ is the ratio of heat produced to the maximum stored energy in the upper laser level. The parameter ξ is particularly relevant to diode pumped systems. We demonstrate an experimental ξ characterization based on the determination of the steady state cooling rate (hence heating rate) of small sample crystals subjected to pump laser heating. Using measured fluorescent lifetimes of the samples and near zero doping (intrinsic) values, the doping independent (zero doping or zero quenching) parameters χφ and ξφ are determined. The results for all samples are in excellent agreement with calculations based solely on energy defect and nonradiative quenching of the upper level     

A radio access system with distributed antennas

This paper analyzes a radio access system that has multiple antennas spatially distributed throughout each cell, instead of a single antenna at each cell. The same signal is simulcast by each antenna in the cell. The signals at a given cell could be multiplexed and modulated for radio transmission at a single node, then transmitted on a coaxial cable distribution system to and from the distributed antennas in a cell. Frequency translation and radio frequency (RF) amplification may need to be performed at each antenna, except for indoor applications with short coaxial cables. This paper presents calculations of signal-to-interference ratios (SIR) and handoff performance for cells with distributed antennas. It is shown that the use of distributed antennas can substantially increase the SIR above a system with a single antenna per cell, and this increase is quantified. It is also shown that increasing the number of antennas results in better overall handoff performance, with a lower number of handoffs and higher received signal power. Minimizing the number of handoffs will reduce the impact of wireless services on network elements     

一种新型超宽带喇叭天线的研究

提出一种基于H波导的新型超宽带喇叭天线以减小一般的超宽带天线如TEM喇叭天线侧向泄漏较大的问题.先在理论上研究了H波导用作超宽带天线的可行性以及参数选择的依据,然后利用FDTD方法研究了同轴探针激励的H波导的时域传输特性和截断的H波导及喇叭的辐射特性,得出该天线可用于超宽带天线,尤其是该天线具有较小的侧向泄漏,使其可以用作天线阵的单元的结论.     

Tissue dielectric measurement using an interstitial dipole antenna

The purpose of this study was to develop a technique to measure the dielectric properties of biological tissues with an interstitial dipole antenna based upon previous efforts for open-ended coaxial probes. The primary motivation for this technique is to facilitate treatment monitoring during microwave tumor ablation by utilizing the heating antenna without additional intervention or interruption of the treatment. The complex permittivity of a tissue volume surrounding the antenna was calculated from reflection coefficients measured after high-temperature microwave heating by using a rational function model of the antenna's input admittance. Three referencing liquids were needed for measurement calibration. The dielectric measurement technique was validated ex vivo in normal and ablated bovine livers. Relative permittivity and effective conductivity were lower in the ablation zone when compared to normal tissue, consistent with previous results. The dipole technique demonstrated a mean 10% difference of permittivity values when compared to open-ended coaxial cable measurements in the frequency range of 0.5-20 GHz. Variability in measured permittivities could be smoothed by fitting to a Cole-Cole dispersion model. Further development of this technique may facilitate real-time monitoring of microwave ablation treatments through the treatment applicator.     

Open-ended coaxial exposure device for applying RF/microwave fieldsto very small biological preparations

An easily fabricated open-ended coaxial exposure device for applying radiofrequency/microwave energy to very small biological preparations is described. The device utilizes the fringing fields of a coaxial cable opening into a ground plane. Operation of the device is easily integrated into standard laboratory procedures in order to: observe a biological specimen; monitor temperature; regulate temperature, pH, and pO₂; and record cellular membrane potentials. The electromagnetic field configuration of the device leads to elimination of detectable interaction with microelectrodes. Measured patterns of electric field and specific absorption rate are given for a device built with quarter-inch semirigid coaxial cable and operating at 2450 MHz. Comparison is made to previous exposure devices for small biological preparations. The particular advantages of the open-ended coaxial exposure device over previous devices are the easy access to the preparation during exposure and the minimal, if any, coupling RF/microwave fields to microelectrodes used in electrophysiological studies     

A 2 1/4-turn spiral antenna for catheter cardiac ablation

To investigate the delivery of microwave energy by a catheter positioned inside the heart for ablating small abnormal regions producing cardiac arrhythmias, a 2 1/4-turn spiral catheter-based microwave antenna applicator has been developed. The antenna consists of the center conductor with continuous insulating material extending from the coaxial feed cable formed into a spiral antenna. The insulator completely isolates the center conductor from tissue. Phantom experiments were performed on homogeneous tissue equivalent medium. The reflection coefficient of the antenna at different frequencies and for different spiral lengths, the time course and temperature profile of an ablation, and the dosimetry of power versus temperature, all indicate that the high-power heating patterns from this antenna are both wider and deeper than with the other microwave antenna systems and radio-frequency electrodes.     

Effective permittivity at the interface of dispersive dielectrics in FDTD

A simple treatment of E-field component tangential to dispersive media interfaces in FDTD is introduced. The method uses concepts from the auxiliary differential equations method to average the constitutive parameters. The cases of a wave propagating in a coaxial line and of an open-ended coaxial line radiating into the dispersive media are investigated. Results show that the simulations could be significantly erroneous if the interface is not handled properly.     

Absorption efficiency of receiving antennas

A receiving antenna with a matched load will always scatter some power. This paper sets an upper and a lower bound on the absorption efficiency (absorbed power over sum of absorbed and scattered powers), which lies between 0 and 100% depending on the directivities of the antenna and scatter patterns. It can approach 100% as closely as desired, although in practice this may not be an attractive solution. An example with a small endfire array of dipoles shows an efficiency of 93%. Several examples of small conical horn antennas are also given, and they all have absorption efficiencies less than 50%.     

A planar diversity antenna for handheld PCS devices

A polarization diversity antenna (PDA) is described, and its performance is compared to that of a monopole antenna at frequencies near 900 MHz. Numerical modeling of each antenna, using the finite-difference time-domain (FDTD) technique, incorporates a cellular telephone handset in the vertical orientation, the user's head and hand, and the mobile communications environment. Results indicate that the two modes of the PDA are sufficiently uncorrelated for diversity operation and that, overall, the values of the mean effective gain (MEG), efficiency, and averaged specific absorption rate (SAR) in the head are better for the PDA than for the monopole antenna. However, in terms of the MEG, the PDA is more sensitive than the monopole antenna to the presence of the user's body. For the PDA, most of the power absorbed in the user's body is deposited in the hand, whereas for the monopole antenna, most of the absorbed power is deposited in the head. For both antennas, the MEG depends on the environment (urban or suburban)     

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     

A numerical solution for the near and far fields of an annular ring of magnetic current

A simple method for calculating the near and far zone fields from an annular ring of circumferentially directed magnetic current which may be used to represent coaxial apertures is presented. Near-field contours are given for two ring sizes. The utility of the method has been illustrated by its application in several practical antenna problems where the magnetic ring current serves as the primary source. Among these are the analysis of dipole antennas mounted on a conducting sphere or cylinder, the impedance of a coaxially fed Yagi-Uda antenna, a coaxially driven wire loop, and the radiation from a coaxial aperture at the base of a cone.     

Theoretical and experimental SAR distributions for interstitialdipole antenna arrays used in hyperthermia

Theoretical predictions and experimental measurements of power deposition in muscle tissue phantoms are compared for various arrays of microwave dipole antennas used for hyperthermia cancer therapy. The antennas are linear coaxial dipoles which are inserted into small nylon catheters implanted in the tumor volume. The specific absorption rate (SAR) patterns for a 2-cm square array of four 915-MHz antennas are presented for both resonant and nonresonant dipoles. Arrays of dipoles with lengths much shorter than the resonant half-wavelength have a far more reactive input impedance and a much smaller absolute SAR magnitude in the array center than is seen for arrays of resonant dipoles, and the maximum SAR shifts from the array center to the antenna surfaces. The absolute length of the volume heated by the small-diameter antennas with the longer half-wavelength was longer than that of the larger-diameter antennas. SAR distributions for 4-cm square arrays of eight and nine antennas fed with equal amplitude and phase are also compared. It is shown that much of the array volume has a power deposition less than 25% of the maximum SAR and that the distribution is nonuniform for both the eight- and nine-antenna configurations     

Input impedance of a probe-fed circular microstrip antenna with thick substrate

A method of computing the input impedance for the probe fed circular microstrip antenna with thick dielectric substrate is presented. Utilizing the framework of the cavity model, the fields under the microstrip patch are expanded in a set of modes satisfying the boundary conditions on the eccentrically located probe, as well as on the cavity magnetic wall. A mode-matching technique is used to solve for the electric field at the junction between the cavity and the coaxial feed cable. The reflection coefficient of the transverse electromagnetic (TEM) mode incident in the coaxial cable is determined, from which the input impedance of the antenna is computed. Measured data are presented to verify the theoretical calculations. Results of the computation of various losses for the circular printed antenna as a function of substrate thickness are also included.