A low-blockage dipole array reflector antenna feed for the lower microwave frequencies

An antenna is described which was devised as a feed for a small axisymmetrical paraboloid reflector at an operational frequency of just over 1 GHz. The antenna consists of a broadside array of two dipoles on a printed circuit board (PCB), joined by a common transmission line. Each dipole is backed by a small strip reflector. With this simple array, which has one central feed point, theH-plane radiation pattern can be varied independently of theE-plane pattern. In addition, the aperture blockage of the feed is small.     

FDTD-derived correlation of maximum temperature increase and peak SAR in child and adult head models due to dipole antenna

This paper investigates the correlation between the peak specific absorption rate (SAR) and the maximum temperature increase in head models of adults and children due to a dipole antenna. Much attention is paid to the effect of variation of electrical and thermal constants on the correlation for the child models, since these constants of child tissues are different from those of adult tissues. For investigating these correlations thoroughly, a total of 1400 situations are considered for the following six models: 3-year-old child, 7-year-old child, and adult models developed at the Nagoya Institute of Technology and the Osaka the University. The numerical results are analyzed on the basis of statistics. We find that the maximum temperature increases in the head can be estimated linearly in terms of peak SAR averaged over 1- or 10-g of tissue. In particular, no clear difference is observed between the adult and child models in terms of the slopes correlating the maximum temperature increase with the peak SAR. Also, the effect of electrical and thermal constants of tissue on these correlation is found to be marginal. Further, we discuss possible maximum temperature increases in the head and brain for SAR limits prescribed in safety guidelines. For the adult model developed at the Osaka Univ., these are found to be 0.26/spl deg/C and 0.10/spl deg/C at the SAR value of 1.6 W/kg for 1-g cubic tissue and 0.59/spl deg/C and 0.21/spl deg/C at the SAR value of 2.0 W/kg for 10-g cubic tissue. Similarly, for the 3-year-old child model at Osaka Univ., these are 0.23/spl deg/C and 0.11/spl deg/C for the value of 1-g SAR and 0.53/spl deg/C and 0.20/spl deg/C for the value of 10-g SAR.     

Dielectric properties of human colostrum at microwave frequencies.

This article communicates the study of both the dielectric properties of human colostrums and breast milk at microwave frequencies. The colostrum samples were taken immediately after child birth and breast milk samples were collected at weekly intervals following the delivery. Rectangular cavity perturbation technique is used for the measurements of dielectric properties at the S-band of microwave frequency. The dielectric constants of the colostrums samples and breast milk samples are found to increase as weeks elapse, which is attributed to the reduced fat content and increased lactose concentration. The conductivity of these samples is similarly found to increase due to the increased dilution.     

SAR distributions in interstitial microwave antenna arrays with asingle dipole displacement

The use of interstitial microwave antenna array hyperthermia (IMAAH) as a treatment for cancer, in conjunction with radiation therapy and chemotherapy, has been investigated widely. The heating pattern produced by a coherently phased 915 MHz asymmetric antenna array displays the maximum power deposition in the array center. The authors report on the effect of variable insertion depth between antennas of an array on the heating patterns produced. The study of this heating behavior demonstrates a similar effect to that of the variably phased arrays, showing a shift of the heating peak towards the periphery of the tumor, offering a more simple method for the clinical treatment of such tumors     

Electromagnetic fields due to a horizontal electric dipole antenna laid on the surface of a two-layer medium

With applications to geophysical subsurface probings, electromagnetic fields due to a horizontal electric dipole laid on the surface of a two-layer medium are solved by a combination of analytical and numerical methods. Interference patterns are calculated for various layer thickness. The results are interpreted in terms of normal modes and the accuracies of the methods are discussed.     

Correlation of maximum temperature increase and peak SAR in the human head due to handset antennas

This paper attempts to correlate the maximum temperature increase in the head and brain with the peak specific absorption rate (SAR) value due to handset antennas. The rationale for this study is that physiological effects and damage to humans through electromagnetic-wave exposure are induced by temperature increases, while the safety standards are regulated in terms of the local peak SAR. For investigating these correlations thoroughly, the total of 660 situations is considered. The numerical results are analyzed on the basis of statistics. We find that the maximum temperature increases in the head and brain can be estimated in terms of peak SARs averaged over 1 and 10 g of tissue in these regions. These correlations are less affected by the positions, polarizations, and frequencies of a dipole antenna. Also, they are reasonably valid for different antennas and head models. Further, we discuss possible maximum temperature increases in the head and brain for the SAR values prescribed in the safety standards. They are found to be 0.31/spl deg/C and 0.13/spl deg/C for the Federal Communications Commission Standard (1.6 W/kg for 1 g of tissue), while 0.60/spl deg/C and 0.25/spl deg/C for the International Commission on Non-Ionizing Radiation Protection Standard (2.0 W/kg for 10 g of tissue).     

一种UHF频段弯折偶极子RFID天线的设计

针对目前射频识别(RFID)天线小型化要求,设计了一种超高频(UHF)弯折偶极子RFID标签天线。天线的构型采用了经典的电感耦合环匹配模型,结构简单且易实现阻抗匹配;在天线宽度上进行二次弯折降低长宽比例,具有小型化优势。利用HFSS仿真软件对该天线进行仿真分析,结果表明该天线具有较宽的阻抗带宽,能够覆盖920~925 MHz超高频频段,并且其回波损耗值及方向性均良好,能够满足RFID的实际生产和应用的要求。     

Measurements of crosspolarisation and gain reduction due to dust on a microwave reflector antenna

9 GHz measurements of effects of dust accretion on the surface of a reflector antenna confirm an earlier prediction of a larger gain reduction when the dust layer covers half the area. Crosspolarisation levels were higher than expected theoretically. Deposits on the primary feed also cause high crosspolarisation.     

Electronic tuning of a dielectric resonator at microwave frequencies

This aper deals with electronic tuning of a 950MHz shielded dielectric resonator by using a varactor-loop circuit. A theoretical investigation based on an accurate model circuit is resented. henomena such as jumps in frequency and amplitude, and degradation of the DR qualities are described     

Temperature dependence of permittivity and loss tangent of lithium tantalate at microwave frequencies

Lithium tantalate (LiTaO/sub 3/) exhibits excellent electro-optical, piezoelectric, and pyroelectric properties and a very low thermal expansion. In this paper, we report measurements of loss tangent and the real part of the relative permittivity /spl epsiv//sub r/spl perp// measured in c-axis LiTaO/sub 3/ crystals in the temperature range from 14 K to 295 K at a frequency of 11.4 and 10 GHz. Microwave properties of LiTaO/sub 3/ were determined by measurements of the resonance frequency and the unloaded Q/sub o/ factor of a TE/sub 011/ mode cylindrical cavity containing the sample under test and accounting for uncalibrated cables and adaptors inside the cryocooler. The permittivity of LiTaO/sub 3/ was found to increase from 38.9 to 41.1 and the loss tangent to change from 1.1/spl times/10/sup -4/ to 6.5/spl times/10/sup -4/ over the full temperature range. Due to its low loss and relatively high permittivity, LiTaO/sub 3/ is suitable for microwave applications.     

Magnetoresistance of the Corbino disk at microwave frequencies

The behavior of a Corbino disk of InSb in a static, transverse magnetic field and a high frequency electric field is investigated. Measurements and analysis show that, owing to an internal magnetic field effect, the impedance of the Corbino disk shows a frequency dependence even at microwave frequencies. Measurements performed on rectangular samples show the same effect even more pronounced. It is concluded that, in the case of medium to high Hall angles, this effect may determine an upper frequency limit of Hall-effect devices which is considerably lower than the dielectric relaxation frequency.     

Nonlinear effects in coaxial cables at microwave frequencies

A study of the generation of intermodulation products due to nonlinearities in braided coaxial cables at L-, S-and C-band frequencies is in progress and some results are presented. The different cable parameters responsible for intermodulation products at microwave frequencies have been examined and their relative contributions towards the total nonlinearities assessed.     

Temperature increase in human eyes due to near-field and far-field exposures at 900 MHz, 1.5 GHz, and 1.9 GHz

This work investigates the effect of frequency, polarization, and angle of incidence of an electromagnetic (EM) wave on the specific absorption rate (SAR) and maximum temperature increase in the human eye at 900 MHz, 1.5 GHz, and 1.9 GHz. In particular, the temperature increase in the eye is compared for near-field and far-field exposures. The difference of a maximum temperature increase in the lens is also discussed between the head models of an adult and children. Throughout the investigations, our attention is paid to a maximum temperature increase in the lens for SAR values prescribed in safety standards. For the results of our investigation, the SAR and temperature increase in the eye are found to be largely dependent on the separation between the eye and a source, and the frequency, polarization, and angle of incidence of the EM wave. The maximum temperature increase (0.303/spl deg/C-0.349/spl deg/C) in the lens of the adult for the SAR value of 2.0 W/kg for the eye tissue (about 10 g) is marginally affected by the above-mentioned factors. No clear difference of a maximum temperature increase in the lens at the SAR limit is observed between the adult and children models.     

Acoustic-wave amplification in high-mobility semiconductors at microwave frequencies

The use of high-mobility piezoelectric semiconductors for acoustic-wave amplification at microwave frequencies is considered.     

MOSFET degradation due to hot-carrier effect at high frequencies

Hot-carrier-induced degradation due to AC stress on short-channel MOSFETs is discussed. It is observed that pulsed gate voltage stressing with a short falltime (0.7 ns) can cause additional lifetime degradation in the form of drain-current reduction. The AC-enhanced degradation is more pronounced at higher frequencies     

Effects of ear-connection modeling on the electromagnetic-energyabsorption in a human-head phantom exposed to a dipole antenna field at835 MHz

Specific absorption rate (SAR) compliance measurements for wireless personal devices are usually performed in anatomically correct phantoms. The phantoms have a lossless spacer to model the external ear (pinna). The use of a lossless spacer has been questioned. The purpose of this paper was to study the effects of the lossy pinna by E-field and numerical assessments validated with thermal measurements. The measurements were performed in a box with a rectangular well simulating a pinna compressed during phone usage. Various openings were created in the septum separating the box and the well to simulate the connection between the head and pinna. A balanced half-wave dipole was used as the RF source. The results of this study lead to the conclusion that for SAR values averaged over 1 gram, within our current probe resolution, complicated lossy pinna structures are not necessary     

Resistive parametric mixing in GaAs oscillators at microwave frequencies

Resistive mixing in GaAs between a suitably biased microwave pump signal and a small signal at the first subharmonic of the pump is shown to result in an effective negative mobility at the signal frequency. The implications are discussed.     

The impedance of a short dipole antenna in a magnetoplasma

A formula for the impedance of a short cylindrical dipole in a magnetoplasma is derived using quasi-static electromagnetic theory. The formula is valid in a lossy plasma and for any dipole orientation with respect to the magnetic field. The dipole impedance is found to have a positive real part under lossless conditions when the quasi-static differential equation is hyperbolic; this indicates that the quasi-static theory predicts a form of radiation. It is shown that the quasi-static theory can be interpreted in terms of scaled coordinates and that a cylindrical dipole in a magnetoplasma has a free space equivalent with a distorted shape. A conductance correction term obtained from Langmuir probe theory is shown to be significant. Laboratory measurements of monopole impedance are compared with the theoretical calculations.     

Radiation resistance of a center fed dipole antenna with feed points displaced transverse to dipole axis

Theoretical studies are made to investigate the effect of feed-point displacement (transverse to dipole axis) on the radiation resistance of a center fed half-wave dipole antenna. A mathematical expression is developed and the radiation resistance is calculated for various feed-point displacements using the numerical integration method.     

Current distribution on a dipole antenna in a warm plasma

A cylindrical dipole antenna having finite dimensions, and surrounded by a homogeneous warm plasma, has been treated as a boundary-value problem. The current on the antenna has been determined and has been found to possess an oscillatory component, the fluctuations of which have approximately the wavelength of the surface wave on a corresponding infinite cylinder immersed in a warm plasma.