MCL MTHR GSM and TETRA handset exposure systems for human volunteer studies. 

 

 

 



MTHR GSM device. SAR: 0 dB = 2.4 W kg-1

 

 

Introduction

In 2001 MCL was asked by the Programme Management Committee of the UK’s Mobile Telecommunications Research programme (MTHR) to provide exposure systems for the programme’s human volunteer studies.   These systems are intended to produce exposures emulating those from GSM phones and TETRA radios.

 

The original design criteria were proposed by MCL and the final specification agreed by the PMC. 

 

The exposure systems were made in two basic variants with either GSM or TETRA modulation.  

 

System design criteria

 

  • The systems should produce human exposures which are representative of exposures of real phone users ie it should have phone-type near-field radiation characteristics.

 

  • The waveform of the radiated fields should contain all the significant characteristics of a real mobile phone signal.

 

  • The systems should be compatible with double-blind-type investigations.  The facility must exist for both real and sham exposures to be achieved without cues to the researcher as to which is which.  

 

  • Exposures to experimental subjects must be within ICNIRP public exposure guidelines. 

 

 

Exposure system

Each exposure system consists of a self-contained, head-band mounted generic handset.  There are no external connections and the devices do not require any RF expertise to operate.  The handsets can radiate power with GSM/TETRA modulation, or without modulation (CW mode). DTX capability has been provided for the GSM handsets, but this was not be used in the MTHR Programme.

 

It is possible to divert power in either mode to an internal load to provide sham RF exposure conditions with heating and low-frequency magnetic field similar to the exposure modes.  The power can also be diverted, via approximately 20 dB of attenuation, to a power-monitoring port (an SMA socket on the top of the handset), where power in either CW or GSM/TETRA mode can be measured with a suitable power meter.  A GSM handset is shown below; the TETRA handset looks identical apart from the antenna, which is longer.

 

 


GSM handset, front

GSM handset, back

On-off and emission mode switches, and “power-on” LED

 

There are 27 possible different emission modes – 100 % power, 50 % power and 25 % power, the three possible modulation regimes (GSM/TETRA, CW and DTX)  and the three routes for the power (antenna, internal load or power monitoring port).  These modes have been randomly and multiply assigned to various hexadecimal values.  There are 10 hex values for each mode, apart from the power-monitoring modes, which each have  a unique hex value.  The hex modes are set using the two rotary switches on the back of each handset, labelled MSB and LSB.   Each hex value has the form nn where n in each case is a number between 0 and 9 or a letter between A and F.  So to set the hex value 1F, for example, the left-hand (MSB) dial is set to 1 and the right-hand (LSB) dial is set to F. 

 

The mean power in the GSM and CW modes is the same: this is to ensure that the ICNIRP guidelines are not exceeded in CW mode.  The consequence is that the radiated power in CW mode will be approximately 1/8 of the peak power in GSM mode (1/4 for TETRA).  The mean radiated power of the handsets is 250 mW; TETRA has 1 W peak radiated power and GSM has 2 W peak radiated power.

 

In GSM/TETRA mode, the signals produced by the exposure system will have the lower frequency modulation components arising from the super- and hyper-frame structure as well as the frame-related burst modulation. 

 

The currents drawn by the handsets in GSM/TETRA and CW radiating modes have been matched so that the heating of the cases is comparable.  Tests by MCL and the University of Sheffield indicate that the case temperatures are similar to better than 1 degree celcius.

 

An audible battery warning sounds when there is a few minutes of useful battery life left.

The output of the handset will be stable to within +/- ˝ dB between 3 minutes from switch-on of a new battery and 3 minutes after the battery warning.  It is recommended that the handset is left switched on to “settle” for around 3 minutes after the fitting of a new battery before exposures take place so that the SAR levels remain stable.

 

The handsets can be mountable on the headset in either right- or left-handed positions. The handset is designed to be mounted against the subject's head in the CENELEC(1) ”cheek"  geometry:  this is one of the two standard assessment geometries for mobile phones.


 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 1:  CENELEC "cheek" measurement position

 

The standard exposure position for the left-hand has the antenna touching or within a few mm of the head, above and slightly behind the ear.

 

To  achieve this positioning with different subjects, it is necessary to use different position settings of the device on the mounting bracket and on the headband.  Instructions on this are given to the researchers when the handsets are delivered.

 

 

 

The right-hand side of the head has a slightly different positioning system:  the antenna is arranged to be in the same place as for the left-hand mounting;  because the handsets are not symmetrical, this is achieved by displacing the whole handset up by approximately 4 cm using a spacing arrangement

 

Dosimetry

Assessment of exposure from each  handset was made using a robot-mounted, computer-controlled miniaturised probe to map out the internal electric field strengths in a headshell phantom filled with tissue-simulating material.  The tissue-simulating materials will be formulated and tested to ensure that they are matched to tissue electrical conductivity and permittivity at the frequency of operation of the handset.

 

The complete dosimetry system and investigation procedure used was consistent with CENELEC EN50360/1 specifications(1,2) for the SAR testing of mobile telephone handsets: the standardised exposure geometry allows the use of a CENELEC standard assessment position.  The test facility used was fully calibrated test facility, with United Kingdom Accreditation Service accreditation.   

 

SAR levels

The maximum SAR (100% power) in the GSM and TETRA handsets occurs close to the antenna.  Its value is 1.3 W kg-1, ± 30%,  averaged over 10 g.  This can be compared with the ICNIRP basic restriction(3) on public exposure of 2 W kg-1 in any 10 g of tissue of the head. 

 

The 10 g-average SAR from the handset body is typically 0.3 kg‑1.  10 g-average SARs in sham mode are approximately 0.002 W kg-1.


SAR peak close to antenna

SAR distribution from handset body

 

Detailed investigations of the distributions of SAR and low-frequency magnetic field from the handsets have been made.

 

Support

MCL provided a "package"  for researchers consisting of the exposure system,  dosimetric assessments in accordance with CENELEC EN50360/1 before, at the end and at approximately 4-monthly intervals during the course of each project as well as on-site commissioning and follow-up support throughout the life of the MTHR Programme.

 

References

 

1           CENELEC EN50361:2001. Basic standard to demonstrate the compliance of handheld mobile phones with the basic restrictions related to human exposure to radio frequency electromagnetic fields.

 

2           CENELEC EN50360:2001. Product standard to demonstrate the compliance of handheld mobile phones with the basic restrictions related to human exposure to radio frequency electromagnetic fields.

3           ICNIRP, 1998.  Guidelines on limiting exposure to time-varying electric, magnetic and electromagnetic fields. Health Physics, 74(4) 494.