Low frequency magnetic fields from MTHR exposure systems

Mobile phones produce ELF magnetic fields as the RF circuitry draws current from the battery. The magnetic field from the battery current is pulsed, with a repetition frequency of approximately 217 Hz, matching the pulse-modulation of the RF emissions from the phone. TETRA devices are also expected to produce pulsed magnetic fields, with a pulse repletion frequency or approximately 16 Hz. The MTHR exposure systems were specified to produce low frequency magnetic fields in GSM/TETRA exposure mode, and these have now been characterised by MCL.

The magnetic fields from the devices were measured with a Bartington 03MS100 fluxgate magnetometer with manufacturer’s calibration. The Bartington magnetometer has three orthogonal sensors and provides a DC output for each individual sensor, proportional to the incident magnetic flux density. It has a specified frequency range of 0 Hz to 3 kHz. To enable the measurement of comparatively small alternating magnetic flux densities in the presence of the geomagnetic field, the magnetometer has a switchable high pass filter which attenuates signals below 0.1 Hz. This filter was used for the measurements reported here. The output from the magnetometer was monitored using a Fluke 199 Scopemeter portable digital storage oscilloscope, with manufacturer’s calibration. The magnetometer sensors have dimensions on the order of a centimetre. The measurements of magnetic flux density from the MTHR exposure systems were made with the centre of each magnetometer sensor 2.5 cm above the surface of the device. The three orthogonal spatial components of magnetic flux density were measured and recorded separately. Their magnitudes have been combined on an rss basis to give a single scalar amplitude.

Figure 1 shows the waveform of the magnetic field from the GSM device in full power, GSM mode. Figure 2 shows a single pulse from the device in more detail.

Figure 3 shows the waveform of the magnetic field from the TETRA device in full power, TETRA mode. Figure 4 shows two pulses from the device in more detail. There is a substantial amount of noise in the signal in some of the TETRA pulses; this is digitisation noise from the oscilloscope operating at a low sample rate.

Figure 1 GSM magnetic field waveform

Figure 2 GSM magnetic field waveform: single pulse

Figure 3 TETRA magnetic field waveform

Figure 4 TETRA device magnetic field waveform: 2 pulses

Figure 5 shows the resultant (rss summed) magnetic flux density at a distance of 2.5 cm from the front of an MTHR GSM device on full power in GSM radiating mode. Figure 6 shows the corresponding data for an MTHR TETRA device. These data are maximum magnetic flux density in the pulse, not rms. Each of the spatial components of flux density was determined at each measurement position by using the oscilloscope cursors to determine the amplitude of the pulse. For example, the amplitude of the pulse shown in figure 2 would be recorded as 4.5 mT. The greatest source of uncertainty in these measurements is likely to be the quantisation noise in each pulse.

Figure 5 Peak magnetic flux density at 2.5 cm from MTHR GSM device

Figure 6 Peak magnetic flux density at 2.5 cm from MTHR TETRA device

The major source of magnetic field for both devices appears to be the power supply, which is situated beneath the spatial peak at bottom left of the keyboard.

There was no measurable pulsed magnetic field from either device in full power sham CW mode.

The measured magnetic field waveforms and their amplitudes are consistent with those from real phones and radios (http://tinyurl.com/8bvgr)