ELF Magnetic Field Mitigation by Active Shielding
COnCettina BuCCdk&Memher. IEEE, Mauro Feliziani, Member. IEEE. VhCenZO F u k
Dept. of Electrical Eng., Univ. of L'Aquila, Poggio di Roio, 67040 L'Aquila, Italy,
Abstrncr -- The reduction of extremely low frequency (ELF)
magnetic field in an area excited by power frequency currents is
investigated by active shielding techniques. The magnetic field
inside a shielded area is measured in simple test configurations.The
performances of the proposed field-controlled active shield are
An interesting problem from a scientific and technical point
of view is that of reducing the magnetic field produced by a
power frequency source in a delimited region of space.
Often, especially in the immediate proximity of an
electromagnetic source, the magnetic field can be large
enough to violate some electromagnetic field exposure
standards or to degrade the functionality of electrical and
electronic systems. In this case it is necessary to reduce the
electromagnetic field strength to an acceptable level. In
general, two different techniques exist to attenuate the
surrounding magnetic fields: passive shielding and active
The passive shielding is extensively used for protection
against high frequency fields. To shield static and extremely
low frequency (ELF) magnetic fields, materials such as
ferromagnetic shields and thick eddy-current shields can be
used [I]-[Ill. However, in the design of large volume
shielded enclosures, the ferromagnetic shield becomes too
heavy and too expensive for the amount of material needed,
while the eddy-current shield provides a very poor shielding
at low frequency.
Active shielding is an alternative attenuation method n
which the disturbing magnetic field can be reduced by
superposing other magnetic fields having about the same
magnitude, but in the opposite direction .The opposite
fields are generally produced by currents injected into
adequately designed active coils. The principal problem of
active shielding can be given by the high currents requested
in order to obtain a significant value of the shielding
effectiveness. Furthermore, active shielding works only at a
given frequency, normally at the power frequency, and there
is not any magnetic field attenuation at other frequencies:
higher order harmonic or radio frequency fields are not
The active shielding technique is here investigated.
Experimental results in test configurations are showed.
11. MATHEMATICAL MODEL
where Ar = r-Y'is the difference between the position vector
r o f the point of observation P and the position vector r'of
the element d / ' . From (2) it is possible to derive the
magnetic field produced by a square loop current. Assuming
the square loop of dimensions 2 a X 2b and parallel to the+
plane at a distance d, the coordinates of the loop four
corners are: C,(d, -b. -U), C2(d4,U ) , C,(d, h, a ) and C,(d, b.
-U) as shown in Fig. 2. The components ofthe magnetic flux
density produced by the active shielding, B,,,, B,,., and B,,,,
are given at generic point P(YJ,z) by :
x - d
B(,j.=-Z(-l)'+'-P O I
B,, =-Z(-l);+'P o l -
rj(1.j +z;)4n ;=I
(1; +y; )
Fig. 2 - Square loop on plane y-z.
where y i ,zi are respectively the projections along the y-
axis and zaxis of the distance between the generic point
P(s.yz) and the i-tli corner, and rj = -,,/.I;! +z? + ( 5 -d)’ is
the distance between the i-th corner Ciand the point P. For
several active coils the components of the magnetic field can
be obtained applying the superposition principle.
If B,(B,,,,B,,?.,BJ is the magnetic flux density produced by
active shielding and B,(B,,.B,),BJ is the incident magnetic
flux density. the total magnetic flux density B, can be given
B, = B,, +B, (3)
If B,,is opposed to B,,a reduction of the total magnetic field
B, occurs. Obviously, the shielding effectiveness depends
on the characteristic of the incident field and on the
capability to generate a reaction field in opposition to that
incident. Previous equations, derived exactly for static fields,
can be successfully utilized for ELF time-hamionic fields,
when the geometrical dimensions are negligible with respect
to the wavelength and when the magnetic field produced by
the eddy currents is neglected.
The shielding effectiveness at a given point P(s,,v,z) inside
the shielded region is given by:
being B,, Bi!.,Biz the components of the incident
magnetic flux density, B,,, Bf!,, B, the components of the
total magnetic flux density, and the symbol * denotes
complex conjugate. The average value of SE,,Bin the volume
/of the shielded enclosure is given by:
where N,. N, and N, are the number of points in thex. y and z
directions inside the volume V of the shielded enclosure,
where calculations or measurements are carried out.
111. EXPERIRIENT.11, PROTOTYPEOF RELD-CONTROLLED
A field-controlled configuration has been realized as shown
in Fig. 3. An active circuit composed by a coil with 20 turns
has been located on the four sides of a 0.50 ni x 0.50 ni
square surface. A controller feeds the active coil in order to
obtain the desired magnetic shielding effectiveness inside
The main problem of the field-controlled active shields is to
construct the waveform of the coil currents as function of
the incident magnetic field.
The idea is to obtain instant by instant a current
proportional to the magnetic field B,(t) measured by the field
The proposed field-controlled supply is realized by the
functional chain shown in Fig. 4. where:
F,(s) is the compensating network
G is an analytical function obtained by (2),which links
the active magnetic field B,, with the current
F2(s)represents the sensor
V,.,,.isthe reference signal, which must be equal to zero.
The one-axis linear magnetic field sensor, used in this
application, is the Honeywell HMC1021Z resistive bridge
device, which works in the field range - 600 ,UT-600 ,UT.
The compensating network (sketched in Fig. 4) has been
realized as shown in Fig. 5, by using bw cost electronic
Fig. 4 -Control system representation.
Fig. 5 - Functional chain realization.
The magnetic field source has been realized by using a
square coil with 30 turns having each side of 1.50 m
crossed by a current of 7 A and frequency of 50 Hz. In the
central region of the coil plane the magnetic flux density
can be considered constant as show in Fig.6. In this zone
is placed the field sensor.
The ELF magnetic field inside the planar region has been
measured by the Wandel & Goltemian EFA-3 field sensor.
The measurements of magnetic field have been carried out
placing the field sensor in the points of a N,xN, point
grid, as shown in Fig. 7, with N , =N, =5, located in the
center of the square region. Five different measurements
of the mis magnetic field have been
acquired for each point of the grid in order to minimize the
influence of possible noise. The measured value has been
assumed to be the average one. The cross section of the
experimental configuration has been also analyzed by
two-dimensional computer codes. The performance of the
proposed field-controlled active coils is shown in Fig. 8,
where the incident and total magnetic flux density maps
are shown. The total flux density is obtained powering
the active coil by a current of amplitude 3 A and required to compensate the incident field at 50 Hz.
frequency 50 Hz. Fig. 9 shows the maximum field
dynamic behavior of the system, that is the active current
-1 -0 8 5
Fig. 6 - Magnetic flux density produced by a square coil.
II 1 l 1 , I :.:: (cm)
-20 -10 0 10 20
Fig. 7 - Point grid measurement.
Fig. 8 - Incident magnetic field (a) and
total magnetic field after shielding (b).
. __..... ._-.. .,-. ...
Fig. 9 - Attenuation of the magnetic field
Fig. IO - Dynamic behavior of system.
The use of active shielding, based on a set of coils where
adequate currents are injected, is efficient in terms of
magnetic field attenuation and it can be preferred in some
The active shielding requires a driving system for the active
coils. It can be technologically complex and needs good
The active shielding is not efficient against high frequency
High currents inside the coils can be necessary to obtain a
good shielding efficiency, but in the proximity of the
conductor coils the level of the magnetic field can be
An experimental set up has been built to measure the
magnetic field inside a planar region excited by an incident
It is possible to obtain a maximum attenuation of about 25
dB for the incident field frequency of 50 Hz.
The attenuation decreases until about 15 dB for the
frequency of incident field of 1 kHz.
The future developments of the work foresee to consider an
arbitrary waveform of incident field, the hybrid shields
(passive/active shields), the optimization and digital control
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