Oscar M. Garay

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. >

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.

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.

An antenna for a portable data terminal

A Portable Data Terminal has radiation requirements somewhat different from those of a traditional portable radio. A portable transmitter is normally required to radiate in the hands or on the belt of the user when the operator depresses the Push-To-Talk (PTT) switch. The radiation environment, the objects in the immediate vicinity of the radiator (say less than six inches distant) and the position of the radio with respect to such objects, is predictable. For example, the designer can expect that the radio case will be in the hand of the user, the antenna in front of the face of such person, and that there are a few feet of free space around the head of the user. The user of the radio can be expected to operate the transmitter in such conditions. This is not the case for a portable data terminal whose RF transmissions are controlled not by an operator but by interrogation by a base station. In these conditions, the terminal is required to radiate from inside a briefcase, on a table, on a metal slab, etc. In general, one can see that the terminal is expected to radiate from any position in the close vicinity of conducting or non-conducting surfaces. Traditional portable antennas, e.g., whips and helices, cannot satisfy the condition of radiating efficiently when placed close to and parallel to conducting surfaces.

RF energy in cars from window-mounted antennas

The measurements of the E-field energy density inside and around a car equipped with window-mounted antennas are reported. The 800 MHz window-mounted antennas produce inside a metal car energy densities comparable to those due to an antenna mounted in the center of the roof. In the 450 MHz band, window-mounted antennas support inside the vehicle energy densities about 2-4 times higher than those from center of the roof mounted antennas. At VHF, with the exception of few localized spots, the E field intensity in the car is less than 20 V/m (∼100 µW/cm2). These field levels are very similar to those found in previous tests by Motorola and the NBS (1) using center of the roof mounted antennas.

The near field of dipole and helical antennas

This paper describes some recent E-field measurements performed in the close vicinity of antennas commonly used in conjunction with portable communication equipment. The study was motivated by the criteria of the proposed ANSI Radio Frequency Protection Guides, which are based on the measurement of mean squared electric (|E|2) field strength. In the far field of radiators the criteria are sound because the value of |E|2is directly related to power flow and so to a potential hazard. In the close proximity of sources the safety criteria are no longer valid because the value of |E|2is not directly related to power flow, but, rather, to the level of electric energy stored around the radiator. This reactive energy, because of its high impedance, is not all available for penetration and deposition in human tissue. So, in the near field of radiators a Safety Standard based only on the value of |E|2can be exceedingly restrictive, as shown by the measurements presented in the paper.