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Two major sets of exposure limits for radiofrequency (RF) radiation, those of the International Commission on Nonionizing Radiation Protection (ICNIRP 2020) and the Institute of Electrical and Electronics Engineers (IEEE C95.1–2019), have recently been revised and updated with significant changes in limits above 6 GHz through the millimeter wave (mm-wave) band (30–300 GHz). This review compares available data on thermal damage and pain from exposure to RF energy above 6 GHz with corresponding data from infrared energy and other heat sources and estimates safety factors that are incorporated in the IEEE and ICNIRP RF exposure limits. The benchmarks for damage are the same as used in ICNIRP IR limits: minimal epithelial damage to cornea and first-degree burn (erythema in skin observable within 48 h after exposure). The data suggest that limiting thermal hazard to skin is cutaneous pain for exposure durations less than ≈20 min and thermal damage for longer exposures. Limitations on available data and thermal models are noted. However, data on RF and IR thermal damage and pain thresholds show that exposures far above current ICNIRP and IEEE limits would be required to produce thermally hazardous effects. This review focuses exclusively on thermal hazards from RF exposures above 6 GHz to skin and the cornea, which are the most exposed tissues in the considered frequency range.

Exposures to radiofrequency (RF) energy above 6 GHz are characterized by shallow energy penetration, typically limited to the skin, but the subsequent increase in skin temperature is largely determined by heat transport in subcutaneous layers. A detailed analysis of the energy reflection, absorption, and power density distribution requires a knowledge of the properties of the skin layers and their variations. We consider an anatomically detailed model consisting of 3 or 4 layers (stratum corneum, viable epidermis plus dermis, subcutaneous fat, and muscle). The distribution of absorbed power in the different tissue layers is estimated based on electrical properties of the tissue layers inferred from measurements of reflected millimeter wavelength energy from skin, and literature data for the electrical properties of fat and muscle. In addition, the thermal response of the model is obtained using Pennes bioheat equation as well as a modified version incorporating blood flow rate-dependent thermal conductivity that provides a good fit to experimentally-found temperature elevations. A greatly simplified 3-layer model (Dermis, Fat, and Muscle) that assumes surface heating in only the skin layer clarifies the contribution of different tissue layers to the increase in surface skin temperature. The model shows that the increase in surface temperature is, under many circumstances, determined by the thermal resistance of subcutaneous tissues even though the RF energy may be deposited almost entirely in the skin layer. The limits of validity of the models and their relevance to setting safety standards are briefly discussed. Bioelectromagnetics. 39:173-189, 2018. © 2018 Wiley Periodicals, Inc.

In this paper, mechanisms of RF energy absorption by body tissue in close proximity to wireless equipment, are studied using numerical simulations at frequencies above 24 GHz. It is shown that at millimeter-wave (mmW) frequencies, of relevance for 5G mobile communications, and for realistic source to body separation distances, the contribution from the reactive near-field to the energy deposition in the tissue is small. Furthermore, the interaction between the source and the exposed body is modest. The results suggest that the effects of the near-field body interactions are small when evaluating electromagnetic field compliance at mmW frequencies.

We conducted an electromagnetic-thermal analysis of Petri dishes filled with different medium volumes under different radio frequency exposure conditions with the aim of identifying linear and non-linear parameters that might explain contradictory results of many in vitro bioelectromagnetic experiments. We found that power loss density and temperature depend on shape, size, and orientation of the exposed sample with respect to direction of incident energy, showing that the liquid medium acts as a receiving antenna. In addition, we investigated the possibility of convection from thermodynamic principles within the liquid medium. For a 35 mm diameter Petri dish, a 2 or 4 ml medium volume is too small to support vertical convection. Conversely, horizontal convective motion is possible for H-polarization exposures at 1.8 GHz. Bioelectromagnetics. 36:527–537, 2015. © 2015 Wiley Periodicals, Inc.

Potential demodulating ability in biological tissue exposed to radio-frequency (RF) signals intrinsically requires an unsymmetrical diode-like non-linear response in tissue samples. This may be investigated by observing possible generation of the second harmonic in a cavity resonator designed to have fundamental and second harmonic resonant frequencies with collocated antinodes. Such a response would be of interest as being a mechanism that could enable demodulation of information-carrying waveforms having modulating frequencies in ranges that could interfere with cellular processes. Previous studies have developed an experimental system to test for such responses: the present study reports a circuit model to facilitate calibration of any non-linear RF energy conversion occurring within a test-piece inside the cavity. The method is validated computationally and experimentally using a well-characterised non-linear device. The proposed model demonstrates that the sensitivity of the measurement equipment plays a vital role in deciding the required input power to detect any second harmonic signal, which is expected to be very weak. The model developed here provides a lookup table giving the level of the second harmonic signal in the detector as a function of the input power applied in a measurement. Experimental results are in good agreement with the simulated results.

Personal wireless telecommunication devices, such as radiofrequency (RF) electromagnetic field (EMF) sources operated in vicinity of human body, have possible adverse health effects. Therefore, the correct EMF assessment is necessary in their near field. According to international near-field measurement criteria, the specific absorption rate (SAR) is used for absorbed energy distribution assessment in tissue simulating liquid phantoms. The aim of this investigation is to validate the relationship between the H-field of incident EMF and absorbed energy in phantoms. Three typical wireless telecommunication system frequencies are considered (900, 1800 and 2450 MHz). The EMF source at each frequency is an appropriate half-wave dipole antenna and the absorbing medium is a flat phantom filled with the suitable tissue simulating liquid. Two methods for SAR estimation have been used: standard procedure based on E-field measured in tissue simulating medium and a proposed evaluation by measuring the incident H-field. Compared SAR estimations were performed for various distances between sources and phantom. Also, these research data were compared with simulation results, obtained by using finite-difference time-domain method. The acquired data help to determine the source near-field space characterized by the smallest deviation between SAR estimation methods. So, this region near the RF source is suitable for correct RF energy absorption assessment using the magnetic component of the RF fields.

Guidelines for exposure to radiofrequency fields are based largely on thermal effects of these fields. However, controversy remains over the possibility of adverse health effects from exposure to low level fields such as those associated with the use of mobile phones. A plausible mechanism considers the demodulation of amplitude modulated RF fields by biological systems: if this were to operate, they would be exposed to low frequency fields similar to those of biological processes, and therefore with the potential for biological activity. This study performed the experiment originally proposed by Balzano [2002] to test whether biological cells exhibit the nonlinear responses necessary for demodulation. This would be indicated by the generation of a second-harmonic signal upon exposure of the sample to a pure fundamental signal at around 883 MHz. The experiment used a highly sensitive system employing a doubly-resonant cavity. Storage of the high-frequency energy in the highly-resonant cavity with very low noise amplification and the avoidance of any electrode contact with the samples allowed the experiments to approach the ultimate limits of physical sensitivity. Using a wide range of biological materials, the study was not able to demonstrate non-linear conversion of RF energy by biological materials. The results do not support the hypothesis that living cells possess the non-linear properties necessary to rectify or demodulate RF energy. Therefore biological systems do not appear to act as effective radio receivers, at frequencies above a few hundred MHz.

We describe the design, construction, and operating characteristics of a doubly resonant cylindrical microwave cavity. This cavity has been developed to allow a search for nonlinear RF energy conversion in biological cells. Cells with a diode-like nonlinearity could demodulate a modulated RF carrier wave and generate low frequency signals in an exposed biological preparation. The cavity is designed to be resonant on the TE111 mode at about 890 MHz and on the TE113 mode at about 1780 MHz. The cavity performs exactly as designed and has proved capable of detecting the nonlinearity in a microscopic Schottky diode test structure. The sensitivity is sufficient to detect any nonlinearity in a collection of biological cells that could have any potential biological significance. Bioelectromagnetics 29:81–91, 2008. © 2007 Wiley-Liss, Inc.

Some biological experiments report effects that depend on low frequency modulation of a radiofrequency (RF) carrier. Such effects require nonlinear responses in biological preparations, which we show could be observed with great generality by the unique frequency signatures that would appear in the scattered RF energy. Following Illinger [Illinger (1982): Bioelectromagnetics 3:9–16], we considered a two part physical system. The greater part, dominated by the properties of water, interacts linearly with the RF field and is described by equilibrium thermodynamics. However, another, much smaller part, e.g., certain biological molecules and molecular subgroups, supports nonlinear interactions and is described by nonequilibrium thermodynamics. For example, a nonlinear interaction might result from scattering of RF photons from oscillators located in a region of strong field gradients, such as at membrane surfaces. A second nonlinear mechanism could appear if stress (elastic) waves were launched within the confines of the exposure vessel by RF heating. Amplitude modulation at angular frequency Ω of a carrier wave with angular frequency ω (Ω ≪ ω) produces two peaks in elastic stress in the cell structure during each period; that is, there is “full-wave demodulation.” As a result of coherent nonlinear charge motion, modulation products could appear at frequencies ω ± 2Ω and, in general, at ω ± n 2Ω (n = 1, 2,…) if vibrational harmonics at 2 nΩ also are excited. Although in principle microwaves can alter the stability of a thermodynamic system by pumping a chemical transition, the degree of nonlinear coupling required for an observable instability is so great that its probability is effectively zero, unless field intensity is extremely high. A companion paper suggests an extremely sensitive method and the related instrumentation for detection of the spectrum scattered by living cells during exposure to amplitude modulated RF energy. Bioelectromagnetics 24:473–482, 2003. © 2003 Wiley-Liss, Inc.

Biological experiments that expose living cells or tissues to RF energy must have an aqueous medium to provide essential water, ions, nutrients, and growth factors. However, as we show here, the medium inherently functions as a receiving antenna that conveys RF energy to the biological entity in a manner entirely determined by exposure vessel geometry, orientation to the incident RF flux, frequency, and dielectric properties of the medium. We show for two common experimental arrangements that basic antenna theory can predict electromagnetic energy patterns that agree well with those otherwise obtained by computationally intensive methods that require specialized resources.

Demodulation of amplitude modulated radio frequency (RF) energy has been proposed as a mechanism for the biological responses to these fields. The experiment proposed here tests whether the electric and magnetic structures of biological cells exhibit the nonlinear responses necessary for demodulation. A high Q cavity and very low noise amplification can be used to detect ultraweak nonlinear responses that appear as a second harmonic of a RF field incident on the sample. Nonlinear fields scattered from metabolically active biological cells grown in monolayer or suspended in medium can be distinguished from nonlinearities of the apparatus. Estimates for the theoretical signal sensitivity and analysis of system noise indicate the possibility of detecting a microwave signal at 1.8 GHz (2nd harmonic of 900 MHz) as weak as one microwave photon per cell per second. The practical limit, set by degradation of the cavity Q, is extremely low compared to the much brighter thermal background, which has its peak in the infrared at a wavelength of about 17 microm and radiates 10(10) infrared photons per second per cell in the narrow frequency band within 0.5% of the peak. The system can be calibrated by introduction of known quantities of nonlinear material, e.g., a Schottky diode. For an input power of 160 microW at 900 MHz incident on such biological material, the apparatus is estimated to produce a robust output signal of 0.10 mV at 1.8 GHz if detected with a spectrum analyzer and a 30-dB gain low noise amplifier. The experimental threshold for detection of nonlinear interaction phenomena is 10(10) below the signal produced by a Schottky diode, giving an unprecedented sensitivity to the measurement of nonlinear energy conversion processes in living tissue.

This paper investigates the character of the average power density in the close proximity of base-station antennas, where human exposure to electromagnetic fields radiated from such radiofrequency (RF) sources is the highest. The concept of average power density is used since, in proximity of a large array antenna, the direction of the power flow at a given point is not readily predictable because of the substantially diverging path lengths and direction of propagation of the energy arriving from different array elements. This quantity Is shown to have a marked cylindrical decay near the antennas, which converts to spherical in the far field. On this basis, a set of simple prediction formulas is derived to allow the estimation of the average power density with good precision. The latest IEEE C95.1-1999 Standard for RF safety calls for spatially averaged measurements of incident power density to verify compliance to maximum permissible exposure limits. The advantage of using the concept of average power density and the resulting prediction formulas is that the evaluation of the exposure of humans near cellular base-station antennas becomes extremely simple during surveys when large computerized scanning equipment may not be available.

Epidemiologists are now embarking on the evaluation of the hypothesis that exposure to radio frequency energy from low-power wireless communication devices, such as portable cellular telephones, causes brain cancer and other adverse health outcomes. Even in the laboratory, exposures from radio frequency sources are difficult to quantify; their measurement in large populations for epidemiologic study is challenging. In this paper, we outline the nature and magnitude of these exposures and discuss the prospects for obtaining useful measures of exposure for epidemiologic research.

Radio frequency (RF) dosimetry is the quantification of the magnitude and distribution of absorbed electromagnetic energy within biological objects that are exposed to RF fields. At RF, the dosimetric quantity, which is called the specific absorption rate (SAR), is defined as the rate at which energy is absorbed per unit mass. The SAR is determined not only by the incident electromagnetic waves but also by the electrical and geometric characteristics of the irradiated subject and nearby objects. It is related to the internal electric field strength (E) as well as to the electric conductivity and the density of tissues; therefore, it is a suitable dosimetric parameter, even when a mechanism is determined to be "athermal." SAR distributions are usually determined from measurements in human models, in animal tissues, or from calculations. This tutorial describes experimental techniques that are used commonly to determine SAR distributions along with the SAR limitations and unresolved problems. The methods discussed to obtain point, planar, or whole-body averaged SARs include the use of small E-field probes or measurement of initial rate of temperature rise in an irradiated object.

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. >

A number of possible physical mechanisms for radiofrequency (RF) interactions with biological systems have been proposed over the past 50 years. Evaluation of proposed mechanisms of interaction suggests there is no physical reason to expect a low-level non-thermal effect between 100 MHz and 150 GHz, possibly excepting those conditions suitable for radical pair effects on biochemical rates. Experimental and theoretical work suggest that demodulation of RF below a few MHz is possible but practical only at much lower frequencies. In all cases, externally applied fields must be on the order of or exceed endogenous fields to affect biological processes. Most likely, low-level non-thermal RF bioeffects do not exist between 10 MHz and several hundred gigahertz.

Directional, high capacity tactical networks with reconfigurable, steerable, narrow-beam agile point-to-point RF links can be deployed on families of distributed fixed and/or mobile C2 platforms, ranging from small UAVs to aerostats to HUMVees to ships at sea. In this paper, we describe the design, implementation and evaluation of: 1) lightweight (less than 1 lb) and low-cost directional RF flat panel antennas; 2) agile gimbals with integrated stabilization, pointing, acquisition and tracking (SPAT). We have designed directional point to point communication (Ethernet) links that provide sustained data rates of up to 18.5 Mb/s and use a high gain, light weight antenna mounted on a gimbals system that can steer the antenna beam in a short time interval comparable with that of electronic switching. With a light weight antenna, the beam steering is much more cost effective, lighter and simpler than a system of RF switches and/or phase shifters. To provide node stabilization and pointing on mobile or unstable platforms, we describe our designs for new advanced gimbals with active loop angular position control based on local GPS and inertial sensors. We will present the results of experiments using the directional RF antennas as well as the pointing, acquisition and tracking in the final version of this paper.

The presence of harmonic products due to possible nonlinear interaction of amplitude modulated RF signals in living cells is best detected by using a cavity with high quality factor. Harmonic products generated by elementary oscillators can be trapped and accumulated in a cavity, permitting detection sensitivity much greater than in an open environment, where they would be radiated in all directions. The experimental method described herein is a systematic approach to detection of the non-Planck RF energy (if any) emitted by an exposed sample of living cells. Balzano and Sheppard [Balzano and Sheppard (2003): Bioelectromagnetics 24:473–482] classified the non-Planck RF emissions from living cells as coming from (1) nonlinear interactions and (2) inelastic interactions. Nonlinear harmonic products would appear in the band at twice the frequency of an amplitude modulated RF carrier. Inelastic interaction products resulting from the interaction between the incident RF energy and normally occurring mechanical vibrations are found in the band immediately adjacent to the carrier. Detection of the latter signals is difficult because of this close spectral proximity, for example, 1 part in 107 for 100 Hz modulation of a GHz carrier. Modern audio spectrum analyzers have excellent selectivity, providing 60 dB rejections only 2 kHz away from the carrier. By judicious selection of the amplitude modulation (AM) frequency, frequency of the RF carrier, and size of the biological sample, it is possible to achieve very high sensitivity (about −90 dBm) with commercially available instrumentation. The presence (or absence) of harmonics in the band adjacent to the amplitude modulated RF carrier would establish (or negate) the existence of coherent interactions between mechanical vibrations in the cell ensemble and the incident RF signal. Bioelectromagnetics 24:483–488, 2003. © 2003 Wiley-Liss, Inc.

Nine small radiation shields made to adhere to the case of mobile phones were tested at 914 and 1880 MHz. Five popular products were tested because advertisements typically claim they are up to 99% effective in blocking radio frequency (RF) radiation emitted from mobile phones. Also, four other conceptually unusual products were tested because advertisements typically claim they emit oscillations that counteract the RF radiation from mobile phones. Each shield was tested on the same mobile phone, and measurements were made to compare the absorption of RF energy in the head with and without each shield attached to the phone. The phone was positioned against a head model, and an automated measurement process was used to determine specific absorption rate (SAR) in the same way it is used at Motorola to test the compliance of mobile phones with respect to human exposure limits. The location of the peak SAR was not observed to change with any of the shields attached to the phone, and the 1 g, peak spatial average SAR did not change by any statistically significant amount. These results indicate the small shields are ineffective in reducing the exposure of the head to RF energy emitted by a mobile phone.

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.

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.

The digital signalling of certain communication products (e.g., trunked radios) requires that high speed logic and RF circuits be associated. Very often the close proximity of logic devices to RF boards can cause undesired desense phenomena, which, if not identified early in the design phase, add costs and delays to the delivery of a new product. This paper describes a simple device that can help the designer identify potential EM interference sources and characterize their strength and nature. Some EM signatures, over a broad band and at a single frequency, of a high speed logic board are shown.

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.

Exposure standards currently recommended by international health organizations and under consideration by U.S. Governmental Agencies restrict the occupational exposure to 500 µW/cm2and the environmental (general public) limit to 200 µW/cm2in the band of land mobile frequencies. This paper discusses the options open to the manufacturers and the designers of mobile and personal transmitters in this rapidly changing regulatory scenario.