Quirino Balzano

Electromagnetic energy exposure of simulated users of portable cellular telephones

Describes a method to quantify the RF exposure of the users of portable cellular phones in terms of specific absorption rate (SAR). The method involves a robotic system to accurately position an isotropic E-field probe within equivalent biological tissue. The user of cellular phones is simulated by a simple human model (a phantom) consisting of a thin shell of fibreglass filled with a liquid having the complex dielectric constant of human brain tissue. The authors present the results of the dosimetric assays conducted using current and previous models of cellular telephones. The peak SAR values detected using the measurement method described are below the limits recommended by the National Council for Radiation Protection and Measurements (NCRP) Report 86 for the protection of humans exposed to RF electromagnetic energy. >

An efficient RF exposure system with precise whole-body average SAR determination for in vivo animal studies at 900 MHz

A radial electromagnetic cavity has been designed and optimized for the in vivo whole-body exposure of mice to 900-MHz RF fields. Parallel circular plates shorted around the perimeter form the cavity, which is fed at the center in order to excite a cylindrical TEM wave. Plastic housings allow the insertion and equidistant positioning from the exciter of 40 mice, with the electric field parallel to the body axis. The resulting exposure system is highly efficient, featuring more than 80% of the incident power dissipated in the mice. The whole-body average SAR can be determined with remarkable precision by means of straightforward power balance since the RF power leakage from the cavity is extremely low. Fairly uniform exposure of the mice, individually and collectively, has been achieved by means of the symmetric arrangement. This exposure system has been adopted in a replication study on transgenic mice currently being carried out in South Australia, and is being considered for upcoming animal studies in Europe.

Dosimetry in mice exposed to 1.6 GHz microwaves in a carrousel irradiator.

We have developed a carrousel irradiator for mice which delivers a head-first and near-field radiofrequency exposure that more closely simulates cellular telephone and radio use than conventional whole body exposure systems. Mouse cadavers were placed on the carrousel irradiator and exposed with their noses 5 mm from the feedpoint of a 1.6 GHz antenna. Local measured specific absorption rates (SAR) in brain regions corresponding to the frontal cortex, medial caudate putamen, and midhippocampal areas were 2.9, 2.4, and 2.2 W/kg per watt of irradiated power, respectively. In addition, average SAR was estimated to be 3.4 W/kg per watt along the sagittal plane of the brain, 2.0 W/kg per watt along the sagittal plane of the body, and between 6.8 and 8.1 W/kg per watt at peak locations along the sagittal plane at the body surface. This detailed SAR information in mice is critical to the interpretation of biological studies of IRIDIUM exposure, and similar analysis should be included for all studies of in vivo exposure of small animals to microwaves.

Human exposure to cellular base station antennas

The human exposure to RF power radiated by cellular base station antennas can be assessed by means of the incident power density averaged over the body. The convenience of adopting this quantity lies in its well-behaved decay away from the antenna. As a consequence, the average power density decay can be predicted using simple formulas, which remain valid even in the near-field of the antenna, where the spatial distribution of the power density is highly non-uniform. The average power density is shown to have a marked cylindrical character in the vicinity of the antenna, which gradually converts into the spherical far-field behavior.

The near field of dipole antennas, part I: Theory

The theoretical and experimental evaluation of the electromagnetic fields in the immediate vicinity of resonant dipole antennas is presented. This type of antenna is widely used with portable and mobile radio transmitters. The work presented herein has been motivated by the concern that future Radio Frequency Protection Guides with respect to human exposure to nonionizing electromagnetic radiation might be expressed strictly in terms of the intensity squared of the electric or magnetic fields. It is shown in the results that it is possible to detect relatively high intensity electromagnetic (EM) fields in close proximity to resonant dipoles even for very low levels of radiated power (1 mW and less). The paper is divided into a theoretical section and an experimental section because its goals are twofold. First, the formulas for the correct evaluation of the EM fields in the close proximity to dipole antennas are established. Second, it is shown that such EM fields, which can be theoretically predicted and experimentally verified with satisfactory accuracy, are indeed strong enough to violate proposed Radio Frequency Protection Guides even for very low levels of radiated power. Thus portable radios are rendered virtually useless, although the same guides permit exposures to much higher levels of power in the far field. Part I of the paper is essentially theoretical and expresses the fields near dipole antennas in terms of cylindrical waves, which lend themselves to closed form integration. The asymptotic expressions of some components of the field are particularly simple for close distances (in terms of wavelength) from the antenna. The correctness of the solution is checked by evaluating how closely boundary conditions are satisfied. Results have shown that previously used formulas for evaluating field intensity very near dipole antennas can give incorrect values.

The near field of dipole antennas, part II: Experimental results

The results of an experimental program to evaluate the electric field near dipole antennas are presented. The measured field intensities are compared with the numerical values computed using the theory developed in Part I of this paper. The theoretical and measured field intensities are in excellent correlation even for observation points spaced from the axis of the dipole less than one hundredth of a wavelength. For thin dipoles (radius ≃ .002λ) the experimental measure of the E-field at the antenna surface or at one antenna radius distance has not been possible because of the practical limitation of available instruments. The experimental and theoretical results show that the field intensities near some parts of a dipole antenna are higher than predicted by commonly used formulas.

Characterization of miniaturized E-field probes for SAR measurements

SAR compliance tests of wireless products are performed measuring the electromagnetic energy deposition pattern in human-like shaped fiberglass shells filled with liquid tissue simulants. SAR measurements are performed by means of miniaturized electric field probes calibrated in the liquid. The overall uncertainty of SAR tests is strongly dependent on the isotropic response and the spatial resolution of dosimetric probes. We devised experimental procedures to determine these important quality parameters for miniaturized E-field probes immersed inside human tissue simulants. These procedures have been incorporated by the IEEE SCC-34/SC-2 in the upcoming IEEE recommended practice for assessing SAR compliance of hand-held wireless communication devices.

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.

Peak and average RF safety compliance levels near radio base station antennas-prediction formulas and numerical validation

A numerical analysis of different classes of radio base station antennas has been carried out to validate power density prediction formulas for the assessment of the RF safety compliance at basestation sites. Measuring or estimating the equivalent incident power density can assess the compliance distance near radio basestation antennas. Formulas for the prediction of the average or peak equivalent power density may lead to reliable and cost-effective ways to perform simple radio basestation site compliance assessments according to both IEEE and ICNIRP guidelines criteria.

Measurement of the magnetic field very close to an RF source

Described here is an automated system for measuring the distribution of the magnetic near-field close to an RF source. It utilizes a new miniature (10mm-diameter) broadband magnetic-field probe which is transparent to electromagnetic energy. The system provides accurate repeatability of probe placement, fast data acquisition, and several types of data presentation.