The Swinburne RF Laboratory
The purpose-built RF Laboratory, comprising an Anechoic Chamber and associated preparation areas, was funded by Telstra Corporation and Swinburne University of Technology. It now houses much of the equipment used by the Telstra EME Safety Group in their former location, including the items described above. As well as providing a service to Telstra in checking compliance of Telstra’s assets, the Laboratory is available for research projects on RF dosimetry and to provide consultancy services for external clients. Aspects of RF dosimetry will also be taught using the laboratory for ACRBR and Swinburne-run courses, meeting a key goal of the ACRBR to develop new researchers in this highly specialised field.
Telecommunications in the modern world are made possible by the transmission and reception of electromagnetic energy, usually in the Radiofrequency (RF) range. RF transmission is nothing new – the first radio broadcasts took place over 100 years ago – but the increasing use of mobile telecommunications systems has enlarged the variety and complexity of our everyday exposure to these transmissions. There have been some concerns that the increased exposure of humans to these forms of energy could underlie increased susceptibility to certain types of disease. International safety standards have been developed to set limits for human exposure to RF energy, taking into consideration huge numbers of scientific studies on biological effects. The demonstration of compliance of particular devices to these standards is not a trivial matter – the RF energy is absorbed by the body in a complex way because of the differing electromagnetic properties of different tissues at different frequencies, and also the way RF energy from the source spreads out or is reflected off nearby objects. RF dosimetry is the science of accurate measurement of RF energy absorbed by specific regions of the body. It requires a number of chief components explained below. These are: an Anechoic Chamber; an Anthropomorphic Phantom; a Source Simulator; and a Specific Absorption Rate (SAR) Robot.
Many devices adjust the amount of RF energy they produce to give optimal service: for example a mobile phone handset will prolong time between battery charges by adaptively controlling RF output. Phone handsets can also be controlled by commercial software and a cable connection to give a constant output. Other sources of RF energy such as phone base-stations, TV and radio transmitting masts, satellite dishes etc. cannot be directly reproduced in an anechoic chamber, so the characteristics of the RF waves are simulated by feeding the output from a RF signal generator to an antenna. Many signals (for example 3G network signals) are very complicated and it requires very expensive equipment to adequately reproduce the base-station signals in the anechoic chamber.
RF energy is absorbed by the body in the form of heat. Some way of assessing the amount of absorbed energy in the body is required to demonstrate compliance with safety standards but the use of thermometers for this purpose is impracticable. Hence, the induced electric field (E-field) in the simulated tissue, which is related to the resultant temperature change from energy absorption, is measured instead. The E-field is measured using a small probe which simultaneously measures RF fields from all directions. Because it is typically necessary to measure these fields at hundreds of points within the phantom, this is done automatically, using a robotic arm controlled by a computer. The software is also able to identify the points in the body where the maximum absorption is obtained and is able to average the results over a standard small volume for comparison with human exposure limits.
Since RF dosimetry is concerned with measuring the effects of RF energy deep within the body, an ethical means of assessing this is needed. Although some measurements are carried out on laboratory animals, these are not directly comparable to humans, because of the differing body size and shape. A good compromise is to construct a realistic life-sized model of the human body (or ‘phantom’) which can be composed of material with similar electrical absorption properties as human tissue. The phantom is often constructed from a fibreglass shell filled with a solution of sugar, salt and water which matches human tissue at the frequencies of interest. Although it is possible to construct a multi-layer phantom with each tissue type (muscle, bone, fat etc.) represented by different liquids, it has been demonstrated that a phantom filled with a single liquid (homogeneous phantom) is more conservative than one with different regions (heterogeneous phantom). The extra uncertainties introduced when manufacturing the latter type of phantom thus do not justify going to a more life-like representation. In fact, because of the very localised absorption of RF energy in many cases, it has been found that it is quite satisfactory to have only one half of the body represented, or just a region, such as the head.
Telstra and RF Dosimetry
As the largest mobile communications provider in Australia, Telstra understands community concern about Electromagnetic Energy (EME). Telstra has been actively involved in EME safety research for over 20 years, and is proud to continue this commitment through their involvement in the ACRBR. The co-operative venture between Telstra and Swinburne University allows leading academic research to be combined with the most significant body of network and telecommunications engineering skills in Australia. The expertise of both Telstra and Swinburne University will lead to rigorous and relevant research to help inform regulators, policy makers and the community in an area which is of significant interest to the general public. The collaboration also forms part of the activities of the Australian Centre for RF Bioeffects research (ACRBR) a multi-institution and multi-disciplinary centre for research excellence, funded since 2004 by the National Health and Medial Research Council.
In testing how RF energy interacts with the human body, the first requirement is to ensure that the measurements are not affected by stray energy from elsewhere, or from reflections or absorption of energy by the environment. This requires measurements to be made in a special room which shields the interior from the external environment, and where all RF energy inside the room is absorbed once it reaches the walls or ceiling. This is achieved with what is effectively a metal box where all the openings, including the doors and connections to external power supplies, are designed to prevent RF energy from getting in or out. The internal walls and ceiling are lined with special pyramid-shaped absorbers (carbon impregnated expanded rubber foam) so that for RF energy, of whatever wavelength, there is no reflected energy (or ‘echoes’) in the room. Hence the name ‘anechoic chamber’. The room has to be big enough to accommodate the testing equipment, and also needs power, lighting and adequate air-conditioning to allow personnel to conduct fairly lengthy and exacting measurements.
RF dosimetry is also involved in determining the levels at which RF energy begins to affect the normal working of electronic and electromechanical equipment. This is very important, for example, in hospitals, where it is necessary to know if life support systems will be affected. Hearing aids and cardiac pacemakers may likewise be affected. It is very important to conduct testing of equipment in an anechoic chamber using calibrated RF sources, so that the level at which malfunction occurs can be accurately measured. This information may then be used to modify equipment designs to make them more immune to the sorts of RF signals they may encounter in their normal working environment, or it may be used to introduce management practices to ensure such signals are not produced in areas where critical equipment may be affected.
Mathematical Models in RF Dosimetry
Another aspect of the work of the Telstra EME Safety Group is that of using complex computer models of the human body to simulate the energy absorption patterns produced from exposure to RF EME. These incorporate anatomical details from clinical MRI and CT scans and data on the electrical properties of specific tissues at different frequencies. These models have detail down to a few millimetres and special computer systems are required to run the simulations. The most sophisticated models also incorporate the effects on absorption from the convection associated with blood flow. These models have been used to predict the effects of metallic implants on the pattern of energy absorption, for example. One aspect of Swinburne’s commitment to the ACRBR program and to the RF Laboratory, is the provision of high-performance hardware and software to assist in the expansion of this work as new research projects, with joint supervision by Telstra and Swinburne personnel. Part of this work will assist other ACRBR projects carried out at other institutions.