Printable VersionEdition Wissenschaft
Edition No. 3 January 1996
Experimental Set-up for Studying Electromagnetic Alternating Fields
Professor R. Elsner, Doctor of Engineering,
U. Neibig, Doctor of Engineering
Technical University of Braunschweig
The suspicion raised by many parties that electromagnetic alternating fields from mobile telephones may have harmful effects on living organisms has given rise to several research projects in Germany, among other countries. In principle, the purpose is to study the effects of electromagnetic fields on biological systems, such as cells. Such projects have proven to be interdisciplinary, since in addition to medical researchers and biologists, physicists and electrical engineers are involved. The particular task for electrical engineers is to develop equipment that meets the particular conditions for the planned trials. A research project done at the Communications Engineering Institute at the Technical University of Braunschweig was dedicated to the exposure equipment for such tests. The assumptions to be made regarding this experimental equipment played a particularly significant role in that project. These assumptions are sketched out briefly below.
The frequencies of the electromagnetic wave field that are used in radio networks must be generated and metrologically recorded with these devices. These are frequencies of 450 MHz, 900 MHz, and 1.8 GHz, with the corresponding bandwidths of a few dozen MHz. These frequencies are now used in mobile radio networks of the C, D, and E types.
Since special conditions apply with regard to the structure of the fields and
the amplitudes that characterize them, TEM and GTEM cells (TEM refers
to transversal electromagnetic, and G refers to gigahertz) were used for
these experiments. In keeping with the laws of physics, the electrical
and magnetic field components are perpendicular to each other, and both
run transversally with respect to the direction of propagation of the
waves. To achieve a uniform (homogenous) field, they must run parallel
in all areas of the measurement space. The tests were designed so that
the same field strength values that occur in practice occured during the
experiment, as well. The measurement spaces must be shielded in both directions;
on the one hand, the biological systems must be protected from external
electromagnetic fields, and on the other hand, the high-frequency field
generated in the testing apparatus must not radiate into the environment.
In keeping with the task at hand, objects are located inside the measurement
spacenamely, the material under test and the means necessary for containing
it. Since the structure and amplitude of each field is modified when objects
are present, efforts must be made to make the best with as little sample
material as possible, and the sample holder must also occupy only a small
part of the overall volume. In addition to the conditions imposed for
metrological reasons, there are also conditions related to the biological
nature of the samples. Working with living cells requires that the temperature
be kept constant, at 37°, for example. Finally, the apparatus must allow
the necessary application and measuring instruments to be used. Usually
this takes place from the outside, but the housing of the measurement
space must be designed, through transparent wall sections, for example,
so that, among other things, microscope observations can be carried out.
Apparatus mounted outside the measuring space is available for generating
the high-frequency electromagnetic energy. The measuring spaces are so-called
TEM or GTEM cells. The TEM cells can be thought of as coaxial lines with
a large, rectangular crosssection. In the lower frequency range, the desired
homogenous TEM waves exist in the middle section of the TEM cell. Interference
occurs at higher frequencies because of the waveguide waves, but this
interference can be rendered ineffective if appropriate measures are taken.
GTEM cells are similar in construction; the waveguide is terminated with
respect to the broadband energy by means of resistors and absorbers. Their
purpose is to suppress reflections that could result in heterogeneous
fields. In the course of the development works, four test arrangements
were designed to examine the influence of high-frequency electromagnetic
fields on human cells. In the arrangement for the Institute for Human
Biology at the Braunschweig Technical University, the purpose was to examine
the influence of 450 MHz alternating fields on white blood cells (lymphocytes).
Since undesirable resonance occurred in the TEM cell at the prescribed
450 MHz, the tests were done at 440 MHz, a difference that does not cause
any appreciable difference in the results from a biological perspective.
To maintain the temperature of the nutritive medium, a white oil circuit
was used, whereby the temperature was kept constant by means of a bath
thermostat. In the studies done at the Physiological Institute II of the
University of Bonn, the focus was on the influence of 900 MHz and 1.8
GHz wave fields on cells of the heart muscle. In preliminary trials, it
was determined that a TEM cell could be used at the prescribed frequencies.
One special requirement was that the sample had to be observable by microscope
as the fields were causing their effects. A clear acrylic container was
used as a sample holder; it contained the nutritive medium and the cells,
and was placed directly on the floor of the TEM cell. Observations were
made through a hole one centimeter in diameter that was made in the cell.
The hole was filled with a finemesh conductive gauze, which acted as a
blocking device for the high-frequency fields. At the Institute for Clinical
Chemistry and Clinical Biochemistry at the Free University of Berlin,
the study focused on the influence of 1.8 GHz alternating fields on a
special type of cell (promyelocytes). In this sequence of measurements,
several test tubes filled with the nutritive medium and cells were to
be exposed to the high-frequency field. Since a larger volume is required
for this purpose, a GTEM cell had to be used as a measuring chamber instead
of a TEM cell. The test tubes could be arranged vertically, horizontally,
or at 45° angles in the GTEM cell. The signal generator was completed
with a pulse modulator. This makes it possible to turn the carrier frequency
on and off manually, and allows for pulse modulation with an external
or internal modulation signal. Additional tests applied to the sample
container, which exceeded the recommended dimensions. It turned out that
the reaction on the field was still within tolerable range. Another measurement
arrangement was developed for the Institute for Human Biology at the Braunschweig
Technical University, specifically again for measuring the effects of
900 MHz and 1.8 GHz alternating fields on blood cells—in this instance,
peripheral lymphocytes. The arrangement used in this case was analogous
to the one developed for Berlin. The only difference was the fact that,
this time, the test tubes were placed into the GTEM cell only in the vertical
position. In implementing these trials in general, field strengths that
were lower than the limit values set forth in DIN 0848 were selected.
This made it possible to examine athermal effects.