Cynthia Furse

Electromagnetic absorption in the human head and neck for mobile telephones at 835 and 1900 MHz

The authors have used the finite-difference time-domain method and a new millimeter-resolution anatomically based model of the human to study electromagnetic energy coupled to the head due to mobile telephones at 835 and 1900 MHz. Assuming reduced dimensions characteristic of todays mobile telephones, the authors have obtained SAR distributions for two different lengths of monopole antennas of lengths /spl lambda//4 and 3/spl lambda//8 for a model of the adult male and reduced-scale models of 10- and 5-year-old children and find that peak one-voxel and 1-g SARs are larger for the smaller models of children, particularly at 835 MHz. Also, a larger in-depth penetration of absorbed energy for these smaller models is obtained. The authors have also studied the effect of using the widely disparate tissue properties reported in the literature and of using homogeneous instead of the anatomically realistic heterogeneous models on the SAR distributions. Homogeneous models are shown to grossly overestimate both the peak 1-voxel and 1-g SARs. Last, the authors show that it is possible to use truncated one-half or one-third models of the human head with negligible errors in the calculated SAR distributions. This simplification will allow considerable savings in computer memory and computation times.

Design of implantable microstrip antenna for communication with medical implants

The objective of this paper is to design a microstrip patch antenna for communication with medical implants in the 402-405-MHz Medical Implant Communications Services band. Microstrip antenna design parameters are evaluated using the finite-difference time-domain method, and are compared to measured results. The effects of shape, length, size, location of feed point and ground point, substrate and superstrate materials, and their thicknesses are evaluated. An extensive study of the performance of the antennas to changes in these parameters was undertaken. The results of this paper provide guidance in the design of implantable microstrip antennas.

Analysis of spread spectrum time domain reflectometry for wire fault location

Spread spectrum time domain reflectometry (SSTDR) and sequence time domain reflectometry have been demonstrated to be effective technologies for locating intermittent faults on aircraft wires carrying typical signals in flight. This paper examines the parameters that control the accuracy, latency, and signal to noise ratio for these methods. Both test methods are shown to be effective for wires carrying ACpower signals, and SSTDR is shown to be particularly effective at testing wires carrying digital signals such as Mil-Std 1553 data. Results are demonstrated for both controlled and uncontrolled impedance cables. The low test signal levels and high noise immunity of these test methods make them well suited to test for intermittent wiring failures such as open circuits, short circuits, and arcs on cables in aircraft in flight.

Computations of SAR distributions for two anatomically based models of the human head using CAD files of commercial telephones and the parallelized FDTD code

The finite difference time domain (FDTD) method is well suited for the computation of bio-electromagnetic effects and has become the method of choice for most researchers in this area. There does however remain some limitations on its use. Firstly the FDTD method requires large amounts of memory and computational power. The size of the model is dependent upon both the physical size of the model and its resolution. Higher frequencies of operation require higher resolutions. This can place the solution of some problems outside the capabilities of the technique. Secondly the representation of the problem (i.e. the head and the telephone) can cause some difficulties. Often the telephone has to be represented by a series of boxes which approximate the shape of the actual device. The paper addresses these two problems. The problem size is accommodated by the use of a parallelized version of the FDTD method, which is run on large parallel processing machines such as the IBM SP-2. Additionally a method of inputting data from the computer aided design (CAD) files of the telephone has been developed. These two techniques are used in combination with two head models which have been developed from MRI images of two human subjects. The usefulness of the techniques developed and comparisons of the specific absorption rates (SARs) in the two models is discussed.

Down to the wire [aircraft wiring]

Aging, brittle wiring within aircraft poses a hidden hazard that emerging technologies aim to address. Among the most promising technologies are advanced reflectometry methods, for routine maintenance; so-called smart wire systems, for continual, on-the-spot wire testing; and arc-fault circuit breakers and advanced fire suppression techniques, for minimizing damage and injury should a fault occur. Remaining challenges include detecting the minuscule insulation breaks that encourage arcing; optimizing the benefits and mitigating the risks of the various wire testing techniques; and getting a better handle on the labyrinthine complexity of aircraft wiring systems.

Calculation of electric fields and currents induced in a millimeter-resolution human model at 60 Hz using the FDTD method

The finite-difference time-domain (FDTD) method has previously been used to calculate induced currents in anatomically based models of the human body at frequencies ranging from 20 to 915 MHz and resolutions down to 1.31 cm. Calculations at lower frequencies and higher resolutions have been precluded by the huge number of time steps which would be needed to run these simulations in the traditional way. This paper describes a new method used to overcome this problem and calculate the induced currents in a MRI-based 6-mm-resolution human model at 60 Hz. A new algorithm based on solving two equations with two unknowns is used for calculating magnitude and phase from the CW FDTD simulation. This allows magnitude and phase calculations to be made as soon as steady-state is reached, which is within a fraction of a cycle. For incident electric fields of 10 kV/m, local induced current densities above 16 mA/m/sup 2/ have been calculated in the torso, with even higher values up to 45 mA/m/sup 2/ for the legs. These are considerably higher than the 4 or even 10 mA/m/sup 2/ that have been suggested in the safety guidelines.

Feasibility of spread spectrum sensors for location of arcs on live wires

Spread spectrum methods are an important emerging class of sensors that have the potential to locate small, intermittent faults on energized aircraft power circuit wires. Previous work has demonstrated the use of these methods for hard faults (open and short circuits). This paper extends that work to the location of typical intermittent faults that plague aircraft maintainers. Test results on 200-ft-long realistic aircraft wires demonstrate the feasibility of these techniques to locate both wet and dry arcs while the system is powered with 400-Hz 115-V ac power running a variety of aircraft lighting loads. The capability of the system to function with either the aircraft structure or a paired wire as the return path to ground is demonstrated. These results indicate that spread spectrum methods have significant promise for locating intermittent faults on wires as they occur in flight or other modes of operation, such as landing and takeoff, taxiing, and other critical times when possible vibration, etc., may cause intermittent faults.

Improvements to the finite-difference time-domain method for calculating the radar cross section of a perfectly conducting target

Several improvements to the finite-difference time-domain (FDTD) method for calculating the radar cross section (RCS) of a perfectly conducting target are presented. Sinusoidal and pulsed FDTD excitations are compared to determine an efficient method of finding the frequency response of targets. The maximum cell size, the minimum number of external cells, and a method to eliminate field storage in the shielded internal volume of perfect conductors to reduce the computer storage requirements of FDTD are discussed. The magnetic-field DC offset induced by surface currents on perfectly conducting objects is observed, and its effects are removed by postprocessing to achieve convergence of the RCS calculations. RCS calculations using the FDTD method in two dimensions are presented for both square and circular infinite cylinders illuminated by both transverse electric and transverse magnetic polarized plane waves. The RCS of a metal cube in three dimensions is also presented. Good agreement between FDTD calculations and theoretical values was achieved for all cases, and parameters necessary to achieve this agreement are examined. >

Low-Power STDR CMOS Sensor for Locating Faults in Aging Aircraft Wiring

A CMOS sensor used to locate intermittent faults on live aircraft wires is presented. A novel architecture was developed to implement the Sequence Time Domain Reflectometry method on a 0.5-mum integrated circuit. The sensor locates short or open circuits on active wires with an accuracy of +/-1 ft when running at a clock speed of 100 MHz. A novel algorithm is proposed that utilizes the shape of the correlation peak to account for sub-bit delay, thus increasing the accuracy of fault location. The power consumed by the microchip is 39.9 mW

Power deposition in the head and neck of an anatomically based human body model for plane wave exposures

At certain frequencies, when the human head becomes a resonant structure, the power absorbed by the head and neck, when the body is exposed to a vertically polarized plane wave propagating from front to back, becomes significantly larger than would ordinarily be expected from its shadow cross section. This has possible implications in the study of the biological effects of electromagnetic fields. Additionally the frequencies at which these resonances occur are not readily predicted by simple approximations of the head in isolation. In order to determine these resonant conditions an anatomically based model of the whole human body has been used, with the finite-difference time-domain (FDTD) algorithm to accurately determine field propagation, specific absorption rate (SAR) distributions and power absorption in both the whole body and the head region (head and neck). This paper shows that resonant frequencies can be determined using two methods. The first is by use of the accurate anatomically based model (with heterogeneous tissue properties) and secondly using a model built from parallelepiped sections (for the torso and legs), an ellipsoid for the head and a cylinder for the neck. This approximation to the human body is built from homogeneous tissue the equivalent of two-thirds the conductivity and dielectric constant of that of muscle. An IBM SP-2 supercomputer together with a parallel FDTD code has been used to accommodate the large problem size. We find resonant frequencies for the head and neck at 207 MHz and 193 MHz for the isolated and grounded conditions, with absorption cross sections that are respectively 3.27 and 2.62 times the shadow cross section.