MTHR - Mobile Telecommunications and Health Research

Final Report

Project Title:
Assessment of Specific Energy Absorption Rate (SAR) in the Head from TETRA Handsets

Project Start Date:
1 March 2002

Project End Date:
28 February 2003

Final Report Date:

May 2003

Cost:
£75,000

Principal Investigator:
Dr Peter Dimbylow

Contact Details:
Radiation Protection Division

HPA Centre for Radiation, Chemical, and Environmental Hazards

Health Protection Agency
Chilton
Didcot
OX11 0RQ

Executive Summary:

The purpose of this project was to calculate the distribution of power absorbed in an anatomically realistic model of the head from a representative TETRA handset.

TETRA (Terrestrial Trunked Radio) is a modern digital private mobile radio system designed to meet the requirements of professional users, such as the police and fire brigade. The current frequency allocations in the UK are 380-385 MHz, 390-395 MHz for the public sector network (BT Airwave) and 410-415 MHz and 420-425 MHz for a commercial network (Dolphin). There are 4 time slots and in normal two-way voice transmission, only one of the slots will be used and this will result in a duty factor of ¼. Handsets are available with peak powers of 1 W and 3 W, giving average powers of ¼ W and ¾ W after consideration of the duty factor. The radios can be held to the ear like a mobile phone or dispatcher-style in front of the face, or mounted remotely on the body but this case will give much lower SAR values to the head than the former positions near to the head.

The TETRA radios can use either a monopole or a helical antenna. The latter is more difficult to model computationally and so it was important to verify the numerical description of the antenna and handset by measurement. A combination of experimental work and computer modelling was therefore used to gain an in-depth understanding of the electrical performance of the helical antennas used with TETRA handsets. A real handset was then set up to radiate continuously under isolated conditions in the absence of the body. Measurements of the near total electric fields in planes at 5-, 10- and 15 cm in front of the handset were made. The comparisons between the calculations and measurements at the three separation distances for both monopole and helical antennas show very good agreement in shape and magnitude.

The power absorbed in the body is quantified in terms of the specific energy absorption rate (SAR) in the unit of watt per kilogram (W/kg). A comprehensive set of calculations of SAR in the head were performed for an ensemble of positions for the handset in front of the face and for both sides of the head, with the handset along the cheek or tilted away from the cheek at an angle of 15° as specified by the European Standard for assessing SAR in physical phantoms.

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) in its guidance on limiting human exposure to radio frequency electromagnetic fields advises restrictions based upon avoidance of adverse effects resulting from whole-body or partial body heating. It recommends that for occupational exposure the SAR should not exceed 10 W/kg in any 10 g of the head. The restriction for public exposure is 2 W/kg. The SAR values are to be averaged over any 6 minute period.

The work described in this report shows that the representative TETRA handset considered, operating at 1 W in normal use with either the helical or monopole antenna will comply with both the ICNIRP occupational and public exposure restrictions.

The handset with a monopole antenna operating at 3 W in normal use will show compliance with both the ICNIRP occupational and public exposure restrictions. The handset with a helical antenna operating at 3 W in normal use will show compliance with the ICNIRP occupational exposure restriction but will be over the public exposure restriction by up to ~ 50 % if kept in the position of maximum SAR for 6 minutes continuously.

Full Final Report:

A pdf file of the full Final Report on this project is available to download (293 kb, 13 pages).