Proc.

of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013

Low Frequency Magnetic Fields Encountered

on Road Network

Michael Magued and Patrick Staebler Dominique Lecointe

EMC – RF Department Department of Electromagnetism
Renault S.A.S Supélec
Guyancourt, France Gif-sur-Yvette, France
michael.magued@renault.com

Abstract— While crossing a semi-urban area, a vehicle This work is a part of a PhD thesis whose topic relates to
undergoes a varied electromagnetic environment. This paper the exposure of persons on board of the vehicle. It provides an
focuses particularly on low frequency magnetic fields. These order of magnitude of the fields other than those produced by
fields are measured on board of a vehicle traveling at a variable the vehicle. It will be a useful starting point when the fields
speed. Measured field sources are identified and classified. produced by vehicles will be measured according to their type
of engine and their embedded equipment.
Keywords— magnetic field; road network; ELF; measurements
For convenience, the magnetic induction B could thereafter
I. INTRODUCTION be called “magnetic field”.
Ambient electromagnetic pollution has very diverse origins
and covers a wide frequency range from extremely low II. MATERIALS & METHODS
frequencies (ELF) to tens – or even hundreds – of gigahertz. The measurements described below were performed on a
On the road network, it derives, inter alia, from external vehicle using the same semi-urban path over several days. The
artificial sources as Base Transceiver Stations (BTS) or RMS values of the B-field evaluated over a sliding window of
broadcast antennas (for radio and television) continuously 1s have been recorded using a field meter (Narda’s Exposure
emitting electromagnetic radiations in the radio frequency (RF) Level Tester ELT-400 [4]) which is a hand-held device with a
range, from High Voltage and Extra High Voltage (HV and digital display that measures magnetic induction in workplace
EHV) electric power transmission lines, from transformers or and public spaces (see block diagram in Fig. 1). The instrument
also from overhead lines for rail systems producing mainly low is equipped with an isotropic triaxial magnetic sensor,
frequency fields. operating in the frequency range of 1 Hz to 400 kHz, and is
placed on the floor in the center of the vehicle. Simultaneously,
It derives also from some natural sources as the static the position and the moving speed measures issued from a
Earth's magnetic field and other natural phenomena such as Global Positioning System (GPS) receiver have been stored.
lightning and cloud-to-cloud activity which can produce
discharges spanning a broad spectrum (from a few kilohertz up
to tens of megahertz).
Finally should be mentioned the vehicles themselves which
embark internal sources as engine and radio transmitters
(telephone module, WiFi, etc.).
A French national project CEERF1 conducted in the 2000s
characterizing the electromagnetic environment on the French
road network focused on intentional, fixed or mobile,
transmitters in the RF domain and sources existing on the road
infrastructure from an electromagnetic compatibility (EMC)
point of view [1].
In addition, other studies on magnetic field in low Fig. 1. Simplified block diagram of the B-field RMS value measurement
frequency domain were performed close to habitations mode
(immobile field meter) [2] and on the railway network [3].
In order to compare the measured values with the reference
The investigation is restricted here to the identification of levels stated in the standard, this field meter contains a mode
some sources of magnetic fields present on the road network. called "Exposure STD" which displays directly the exposure
level as a percentage of the reference levels defined in
1
Caractérisation de l’Environnement Electromagnétique Routier en
France

978-1-4673-4980-2/13/$31.00 © 2013 IEEE 84

 Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013

the ICNIRP guidelines (1998) [5]. For that, it uses the 25

alternative option to the spectral method proposed in [6-7]; Record of the rms value over a path
weighting the B-field with a filter function which is related to Selection threshold of significant variations
the reference levels. Thus, frequency measurement is not
See Fig. 5
required as the filter converts the frequency information into 20
the appropriate attenuation which is a particularly useful

Magnetic induction Brms (μT)

method when the signal shape is unknown (e.g. multiple
frequency fields, pulsed fields) as the phase information is not
lost. 15

These records show fields of varying intensity levels and

varying durations according to the nature of the source and the
moving speed of the vehicle. 10

Statistics of intensities and durations of the measured See Fig. 6

magnetic fields are extracted from these records.
5
III. RESULTS
A. Studying Different Field Levels Observed along a Given
Path
0
The realized record, projected on a map (see excerpt in 0 2000 4000 6000 8000
Position relative to starting point (m)
Fig. 2), shows the presence of several sources of low frequency
magnetic field on the road network, from magnetic induction Fig. 3. Record of the magnetic induction carried on a moving vehicle over a
loops to power transmission lines. A characterization of some 8-km-long path.
sources is presented. Therefore, as the sensor is placed inside the vehicle, its
emplacement was chosen to be as far as possible of the internal
A record of the measurements performed over an 8-km- sources to minimize their effect.
long path is presented (Fig. 3). There is no magnetic
disturbance (above background noise) between 2 km and 3 km, To be sure that there is no influence on the results by the
whatever the vehicle direction is, due to the lack of road fields produced by the vehicle itself, the same measurements
structures on this route section. are repeated with two different vehicles over the same path at
the same speed as shown in Fig. 4. We note that, the two
B. Minimizing the Influence of Vehicle Internal Sources curves completely coincide regardless of the vehicle, which
Emissions proves that the influence of vehicle internal sources emissions
The study being restricted to the fields generated by is minimized during measurement.
external sources present on the road network, the influence of
20
internal radiating sources must not be taken into account. Record using vehicle A
18 Record using vehicle B
Overhead power
16
Railway
bridge
Magnetic induction Brms (μT)

14
Highway
bridge 12

10

4
Magnetic field (µT)
█ B > 15
2
█ 10.5 < B < 15
█ 5.5 < B < 10.5
█ 1 < B < 5.5 Highway 0
0 2000 4000 6000 8000
█ B<1 bridge Position relative to starting point (m)

Fig. 2. Magnetic fields measured along the path. Fig. 4. Records of the magnetic induction carried on two different moving
vehicles over the same 8-km-long path.

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 Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013

C. Comparison of Different Measurements Carried on the 8

Same Route Record at 70 → 95 km/h
Different measurements were performed with the same Record at 65 → 90 km/h
7
vehicle on the same route at different speeds conditioned by Record at 60 → 85 km/h
traffic. The overlay graphs of some measurements allow
distinguishing two types of magnetic field variations recorded 6

Magnetic induction Brms (μT)

by the probe:
5
• Variations independent of the speed of the vehicle as
illustrated in Fig. 5, where all the curves nearly
coincide showing the same field levels at the same 4
localizations regardless the speed of the vehicle.
These fields originate from radiating sources (e.g. 3
overhead railway lines or electric power distribution
network). An analysis of some of these sources is 2
presented below once their localization has been
known.
1
• Variations dependent of the speed of the vehicle (see
Fig. 6), corresponding to magnetic anomalies resulting
0
from the local deformations of the Earth's magnetic 6800 7000 7200 7400 7600 7800
field caused by different road structures such as Position relative to starting point (m)
bridges, tunnels or underground pipes. Fig. 6. Records of the magnetic induction carried out by a moving vehicle
The probe cannot measure the Earth's magnetic field as over a 1-km-long path containing magnetic anomalies – levels
dependent of vehicle speed.
its lower cutoff frequency (1 Hz) doesn’t allow
perceiving static fields. Nevertheless, it is sensitive to
These two types of variations are mainly distinguished by
Earth's magnetic field local variation, because the
comparing the measured value of the B-field at the same point
vehicle is moving during the measurement. Thus, the
at different speeds, and correspond to visual sources and road
measured variations depend on the vehicle speed and
structures encountered.
are not associated with a radiating source of low
frequency magnetic field. D. Overall Summary of the Results
20 These results were then subjected to statistical analysis to
Record at 110 km/h study the varying intensities and durations of the observed
18 Record at 80 km/h fields depending on the moving speed of the vehicle on the
Record at 95 km/h
nature of the source as explained before. These statistics were
16 calculated from measurements covering a 50-km-long semi-
urban path. Only significant variations exceeding 1 microtesla
Magnetic induction Brms (μT)

14
were considered hereafter (see selection threshold, Fig. 3).
12 1) Varying intensities of encountered fields
In Fig. 7, a histogram represents the distribution of the
10 fields’ intensities detected over the whole path. It shows that
most of sources have negligible magnitudes, and that rare
8
sources emit more than few microteslas. These encountered
6
field intensities will to be compared with those emitted by
sources within the vehicle.
4 2) Varying durations of encountered fields
The histogram shown in Fig. 8 represents the distribution of
2
the fields’ durations observed over the same path. It provides
an order of magnitude of the durations to which the vehicle and
0
200 0
400 600 800 1000 its passengers are exposed. We note that, in 95% of the cases
Position relative to starting point (m) they endure for less than 4 seconds, and that outstanding
Fig. 5. Records of the magnetic induction carried out by a moving vehicle observed sources may be perceived for 8 seconds for normal
over a 1-km-long path containing radiating sources – levels independent traffic conditions over this 50-km path. These durations may
of vehicle speed.
also be compared with the vehicle internal sources emission.

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 Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013

1200 50 -1.5

← Beginning of the bridge

1 920 (~11%) measurements over 40 -2

1000 18 000 performed every 250 ms 50 Hz

exceed 1 μT 30 -2.5
9.5 9.55 9.6 9.65 9.7

Magnetic induction Bz (μT)

Number of occurrence

800 20

10
600
0

End of the bridge →

-10
400
-20

200 -30 1.5

≥ 30 1
↓ -40
0
0 5 10 15 20 25 30 0.5
0.1 0.15 0.2 0.25 0.3

Field magnitude (μT) -50

0 2 4 6 8 10 12
Fig. 7. Histogram of the field intensities exceeding 1 microtesla over a 50- Time (s)
km path.
Fig. 9. Magnetic field variations perceived while crossing an overhead lines’
bridge.
70 Over a total recording time of 75 minutes, As well, variations caused by the overhead lines radiating at
the total duration of fields exceeding 1 μT 50 Hz can be identified by zooming the recorded signal (see
60 is 8 minutes the red part of the curve in Fig. 9). These fields at 50 Hz are not
present before or after crossing the bridge (see the blue part of
the curve).
Number of occurrence

50
We can, thus, recognize both types of magnetic fields on
40 this record.
IV. CONCLUSION
30 This study shows that a vehicle, and its passengers,
traveling on the road network are exposed to magnetic fields
20 generated either by radiating sources as the electric power
distribution network (power lines and transformers) and other
devices present along the roads (inductive-loop traffic
10 detectors, public lighting, etc.), or by magnetic anomalies
generated mainly by the road network structures.
0
0 2 4 6 8 10 12 These anomalies caused by the local variations in static
Field duration (s) field are perceived by the moving field meter as a time-variable
Fig. 8. Histogram of the durations of fields exceeding 1 microtesla over a 50- field measured by an immobile instrument.
km path.
Statistical analyses show that most of encountered fields
E. Characterization of some sources endure for few seconds for normal traffic conditions and have
negligible magnitudes on average.
In order to characterize the different variation sources, the
analog output of the field meter (see Scope (output) in Fig. 1) Characterizing the different sources provides an order of
was displayed and saved using a digital oscilloscope. The magnitude of the fields present in road network to be compared
observed magnetic field while crossing a bridge over overhead with the fields produced by the vehicle itself at the passengers’
lines of the French rail system is shown in Fig. 9. The three of level. This comparison will allow to us to do some assumptions
the magnetic field components being similar, only one is when applied to similar fields produced by vehicles.
illustrated for readability reasons. We note that these works are quite sufficient to be
In this signal, frequencies in the ELF range (~ 1 Hz) can be applicable when establishing a vehicle measurement procedure.
easily identified. These are due to Earth’s static magnetic field It is also important to note that the exposure values are low
deformation caused by the bridge itself and seen through the and comply with the European Council Recommendation
field meter filter, and are detected by the moving field meter. 1999/519/EC [8] on the limitation of exposure of the general

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 Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013

public to electromagnetic fields ; the basic document which Trans. On Electromagnetic Compatibility, vol. 46, no. 1, pp. 12-23,
offers the European Union members a framework for exposure February 2004.
assessment. [4] Narda Safety Test Solutions, “ELT-400 Exposure Level Tester
Operating Manual”, Pfullingen, Germany. [Online]. Available:
http://www.narda-sts.us/
ACKNOWLEDGMENT
[5] International Commission on Non-Ionizing Radiation Protection
This work has been financially supported by the “Association (ICNIRP), “Guidelines for limiting exposure to time-varying electric,
Nationale de la Recherche et de la Technologie” (ANRT) for magnetic, and electromagnetic fields (up to 300 GHz),” Health Phys.,
vol. 74, no. 4, pp. 494-522, 1998. [Online]. Available:
“CIFRE” research agreement. http://www.icnirp.de/
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