Brian B. Beard

Comparisons of computed mobile phone induced SAR in the SAM phantom to that in anatomically correct models of the human head

The specific absorption rates (SAR) determined computationally in the specific anthropomorphic mannequin (SAM) and anatomically correct models of the human head when exposed to a mobile phone model are compared as part of a study organized by IEEE Standards Coordinating Committee 34, Sub-Committee 2, and Working Group 2, and carried out by an international task force comprising 14 government, academic, and industrial research institutions. The detailed study protocol defined the computational head and mobile phone models. The participants used different finite-difference time-domain software and independently positioned the mobile phone and head models in accordance with the protocol. The results show that when the pinna SAR is calculated separately from the head SAR, SAM produced a higher SAR in the head than the anatomically correct head models. Also the larger (adult) head produced a statistically significant higher peak SAR for both the 1- and 10-g averages than did the smaller (child) head for all conditions of frequency and position.

Dosimetric comparison of the specific anthropomorphic mannequin (SAM) to 14 anatomical head models using a novel definition for the mobile phone positioning

This paper presents new definitions for obtaining reproducible results in numerical phone dosimetry. Numerous numerical dosimetric studies have been published about the exposure of mobile phone users which concluded with conflicting results. However, many of these studies lack reproducibility due to shortcomings in the description of the phone positioning. The new approach was tested by two groups applying two different numerical program packages to compare the specific anthropomorphic mannequin (SAM) to 14 anatomically correct head models. A novel definition for the positioning of mobile phones next to anatomically correct head models is given along with other essential parameters to be reported. The definition is solely based on anatomical characteristics of the head. A simple up-to-date phone model was used to determine the peak spatial specific absorption rate (SAR) of mobile phones in SAM and in the anatomically correct head models. The results were validated by measurements. The study clearly shows that SAM gives a conservative estimate of the exposure in anatomically correct head models for head only tissue. Depending on frequency, phone position and head size the numerically calculated 10 g averaged SAR in the pinna can be up to 2.1 times greater than the peak spatial SAR in SAM. Measurements in small structures, such as the pinna, will significantly increase the uncertainty; therefore SAM was designed for SAR assessment in the head only. Whether SAM will provide a conservative value for the pinna depends on the pinna SAR limit of the safety standard considered.

On the mechanisms of interference between mobile phones and pacemakers: parasitic demodulation of GSM signal by the sensing amplifier

The aim of this study was to investigate the mechanisms by which the radiated radiofrequency (RF) GSM (global system for mobile communication) signal may affect pacemaker (PM) function. We measured the signal at the output of the sensing amplifier of PMs with various configurations of low-pass filters. We used three versions of the same PM model: one with a block capacitor which short circuits high-frequency signals; one with a ceramic feedthrough capacitor, a hermetically sealed mechanism connecting the internal electronics to the external connection block, and one with both. The PMs had been modified to have an electrical shielded connection to the output of the sensing amplifier. For each PM, the output of the sensing amplifier was monitored under exposure to modulated and non-modulated RF signals, and to GSM signals (900 and 1800 MHz). Non-modulated RF signals did not alter the response of the PM sensing amplifier. Modulated RF signals showed that the block capacitor did not succeed in short circuiting the RF signal, which is somehow demodulated by the PM internal non-linear circuit elements. Such a demodulation phenomenon poses a critical problem because digital cellular phones use extremely low-frequency modulation (as low as 2 Hz). which can be mistaken for normal heartbeat.

Review and standardization of cell phone exposure calculations using the SAM phantom and anatomically correct head models.

We reviewed articles using computational RF dosimetry to compare the Specific Anthropomorphic Mannequin (SAM) to anatomically correct models of the human head. Published conclusions based on such comparisons have varied widely. We looked for reasons that might cause apparently similar comparisons to produce dissimilar results. We also looked at the information needed to adequately compare the results of computational RF dosimetry studies. We concluded studies were not comparable because of differences in definitions, models, and methodology. Therefore we propose a protocol, developed by an IEEE standards group, as an initial step in alleviating this problem. The protocol calls for a benchmark validation study comparing the SAM phantom to two anatomically correct models of the human head. It also establishes common definitions and reporting requirements that will increase the comparability of all computational RF dosimetry studies of the human head.

International intercomparison of specific absorption rates in a flat absorbing phantom in the near-field of dipole antennas

This paper reports the results of an international intercomparison of the specific absorption rates (SARs) measured in a flat-bottomed container (flat phantom), filled with human head tissue simulant fluid, placed in the near-field of custom-built dipole antennas operating at 900 and 1800 MHz, respectively. These tests of the reliability of experimental SAR measurements have been conducted as part of a verification of the ways in which wireless phones are tested and certified for compliance with safety standards. The measurements are made using small electric-field probes scanned in the simulant fluid in the phantom to record the spatial SAR distribution. The intercomparison involved a standard flat phantom, antennas, power meters, and RF components being circulated among 15 different governmental and industrial laboratories. At the conclusion of each laboratorys measurements, the following results were communicated to the coordinators: Spatial SAR scans at 900 and 1800 MHz and 1 and 10 g maximum spatial SAR averages for cubic volumes at 900 and 1800 MHz. The overall results, given as mean standard deviation, are the following: at 900 MHz, 1 g average 7.850.76; 10 g average 5.160.45; at 1800 MHz, 1 g average 18.44plusmn1.65; 10 g average 10.14plusmn0.85, all measured in units of watt per kilogram, per watt of radiated power

Precise dielectric property measurements and E-field probe calibration for specific absorption rate measurements using a rectangular waveguide

Specific absorption rate (SAR) measurements require accurate calculations of the dielectric properties of tissue-equivalent liquids and associated calibration of E-field probes. We developed a precise tissue-equivalent dielectric measurement and E-field probe calibration system. The system consists of a rectangular waveguide, electric field probe, and data control and acquisition system. Dielectric properties are calculated using the field attenuation factor inside the tissue-equivalent liquid and power reflectance inside the waveguide at the air/dielectric-slab interface. Calibration factors were calculated using isotropicity measurements of the E-field probe. The frequencies used are 900 MHz and 1800 MHz. The uncertainties of the measured values are within ±3%, at the 95% confidence level. Using the same waveguide for dielectric measurements as well as calibrating E-field probes used in SAR assessments eliminates a source of uncertainty. Moreover, we clearly identified the system parameters that affect the overall uncertainty of the measurement system.

Radio frequency immunity testing of hearing aids

For many Equipment Under Test (EUT), such as the hearing aids examined in this study, the desired RF immunity measurement result is that which would be measured in the most sensitive EUT orientation relative to an applied RF field. This is generally approximated from measurements at a number of predetermined orientations within a GTEM cell. This paper presents new 6 and 12-orientation “maximal sum” methods of small EUT immunity measurement, which may be considered extensions to present sorted three-input vector summation techniques. Experimental results for the new methods approached the established reference goal more consistently than did other approaches examined employing a comparable number of contributing measurements.

RF interference in hearing aids from cellphones part 1: Near-field cellphone emissions measurements and the effects of hands

Cellular telephones (cellphones) are currently categorized for hearing aid compatibility based on a calculated value (metric) obtained from the measurement of near-field, radio-frequency emissions according to a procedure described in ANSI Standard C63.19 “Measurement of Compatibility between Wireless Communications Devices and Hearing Aids”. There has been a lack of documentation, however, that relates this metric to a cellphones potential for interference in actual use, that is, when it is held at the ear in a normal-use position by a hearing aid wearer. In Part 1 of this two-part series, we compare the ANSI C63.19 metric to simpler metrics, still based on the near-field test procedure of the standard, and to near-field measurements made when the cellphones are hand-held. The results justify employing a simpler no-hand metric than the exclusion area procedure presently specified by the standard, but not the addition of a test hand to the procedure. The further effect of the head and interaction with the hearing aid is examined in Part 2 of the series.

An alternative method for determination of low-frequency specific absorption rate patterns in homogeneous phantoms

We propose a new application of voltage gradient measurements to determine specific absorption rate (SAR) at low frequencies where quasi-static electromagnetic conditions apply. This method, which we call the voltage gradient method, relies on direct measurement of the voltage field rather than measurement of the electric field or thermal transients. The voltage gradient method is fast and can be implemented with voltmeters of moderate cost. We tested the voltage gradient method using normal saline, in a phantom with simple geometry, and a sine wave voltage source at 5, 10, 20 and 50 kHz. Compared to the SAR measured thermally in the same phantom, the voltage gradient method produced almost identical curves when normalized. When the results of the voltage gradient method were scaled to the same power level used for the thermal SAR, the agreement was compatible with typical thermal SAR accuracy.

RF interference in hearing aids from cellphones Part 2: Comparing in-use RF coupling to predictions

Cellphones and hearing aids are presently tested for their near-field RF emissions and RF immunity, respectively, to predict their mutual compatibility when used together. In the concluding part of this two-part series, we examine the relationship between these independent device measurements and the resultant in-use coupled RF interference, which may be heard as audio frequency noises by the hearing aid wearer. The established standards are seen to be generally reasonable in meeting the compatibility goals (i.e., ensuring a low level of perceived audio interference), but the combined effects of the relative device positioning, the hand, and especially the head add a high degree of uncertainty to the relationship between the actual in-use RF interference coupling and predictions based on individual emissions and immunity measurements.