Electromagnetic shielding is the practice
of reducing the electromagnetic field in a
space by blocking the field with barriers
made of conductive or magnetic materials.
Shielding is typically applied to enclosures
to isolate electrical devices from the 'outside
world', and to cables to isolate wires from
the environment through which the cable runs.
Electromagnetic shielding that blocks radio
frequency electromagnetic radiation is also
known as RF shielding.
The shielding can reduce the coupling of radio
waves, electromagnetic fields and electrostatic
fields. A conductive enclosure used to block
electrostatic fields is also known as a Faraday
cage. The amount of reduction depends very
much upon the material used, its thickness,
the size of the shielded volume and the frequency
of the fields of interest and the size, shape
and orientation of apertures in a shield to
an incident electromagnetic field.
Materials used
Typical materials used for electromagnetic
shielding include sheet metal, metal screen,
and metal foam. Any holes in the shield or
mesh must be significantly smaller than the
wavelength of the radiation that is being
kept out, or the enclosure will not effectively
approximate an unbroken conducting surface.
Another commonly used shielding method, especially
with electronic goods housed in plastic enclosures,
is to coat the inside of the enclosure with
a metallic ink or similar material. The ink
consists of a carrier material loaded with
a suitable metal, typically copper or nickel,
in the form of very small particulates. It
is sprayed on to the enclosure and, once dry,
produces a continuous conductive layer of
metal, which can be electrically connected
to the chassis ground of the equipment, thus
providing effective shielding.
RF shielding enclosures filter a range of
frequencies for specific conditions. Copper
is used for radio frequency shielding because
it absorbs radio and magnetic waves. Properly
designed and constructed copper RF shielding
enclosures satisfy most RF shielding needs,
from computer and electrical switching rooms
to hospital CAT-scan and MRI facilities.
Example applications
One example is a shielded cable, which has
electromagnetic shielding in the form of a
wire mesh surrounding an inner core conductor.
The shielding impedes the escape of any signal
from the core conductor, and also prevents
signals from being added to the core conductor.
Some cables have two separate coaxial screens,
one connected at both ends, the other at one
end only, to maximize shielding of both electromagnetic
and electrostatic fields.
The door of a microwave oven has a screen
built into the window. From the perspective
of microwaves this screen finishes a Faraday
cage formed by the oven's metal housing. Visible
light, with wavelengths ranging between 400 nm
and 700 nm, passes easily through the screen
holes.
RF shielding is also used to prevent access
to data stored on RFID chips embedded in various
devices, such as biometric passports.
NATO specifies electromagnetic shielding for
computers and keyboards to prevent passive
monitoring of keyboard emissions that would
allow passwords to be captured; consumer keyboards
do not offer this protection primarily because
of the prohibitive cost.
RF shielding is also used to protect medical
and laboratory equipment to provide protection
against interfering signals, including AM,
FM, TV, emergency services, dispatch, pagers,
ESMR, cellular, and PCS. It can also be used
to protect the equipment at the AM, FM or
TV broadcast facilities.
How electromagnetic shielding works
Electromagnetic radiation consists of coupled
electric and magnetic fields. The electric
field produces forces on the charge carriers
within the conductor. As soon as an electric
field is applied to the surface of an ideal
conductor, it induces a current that causes
displacement of charge inside the conductor
that cancels the applied field inside, at
which point the current stops.
Similarly, varying magnetic fields generate
eddy currents that act to cancel the applied
magnetic field. The result is that electromagnetic
radiation is reflected from the surface of
the conductor: internal fields stay inside,
and external fields stay outside.
Several factors serve to limit the shielding
capability of real RF shields. One is that,
due to the electrical resistance of the conductor,
the excited field does not completely cancel
the incident field. Also, most conductors
exhibit a ferromagnetic response to low-frequency
magnetic fields, so that such fields are not
fully attenuated by the conductor. Any holes
in the shield force current to flow around
them, so that fields passing through the holes
do not excite opposing electromagnetic fields.
These effects reduce the field-reflecting
capability of the shield.
In the case of high-frequency electromagnetic
radiation, the above-mentioned adjustments
take a non-negligible amount of time, yet
any such radiation energy, as far as it is
not reflected, is absorbed by the skin, so
in this case there is no electromagnetic field
inside either. This is one aspect of a greater
phenomenon called the skin effect. A measure
of the depth to which radiation can penetrate
the shield is the so-called skin depth.
Magnetic shielding
Equipment sometimes requires isolation from
external magnetic fields. For static or slowly
varying magnetic fields the Faraday shielding
described above is ineffective. In these cases
shields made of high magnetic permeability
metal alloys can be used, such as sheets of
Permalloy and Mu-Metal, or with nanocrystalline
grain structure ferromagnetic metal coatings.
These materials don't block the magnetic field,
as with electric shielding, but rather draw
the field into themselves, providing a path
for the magnetic field lines around the shielded
volume. The best shape for magnetic shields
is thus a closed container surrounding the
shielded volume. The effectiveness of this
type of shielding depends on the material's
permeability, which generally drops off at
both very low magnetic field strengths and
at high field strengths where the material
becomes saturated. So to achieve low residual
fields, magnetic shields often consist of
several enclosures one inside the other, each
of which successively reduces the field inside
it.
Because of the above limitations of passive
shielding, an alternative used with static
or low-frequency fields is active shielding;
using a field created by electromagnets to
cancel the ambient field within a volume.
Solenoids and Helmholtz coils are types of
coils that can be used for this purpose.
Additionally, superconducting materials can
expel magnetic fields via the Meissner effect.
Mathematical model
Suppose that we have a spherical shell of
a diamagnetic material with permeability , with
inner radius and outer radius . We then put
this object in a constant magnetic field:
Since there are no currents in this problem
except for possible bound currents on the
boundaries of the diamagnetic material, then
we can define a magnetic scalar potential
that satisfies Laplace's equation:
where
In this particular problem there is azimuthal
symmetry so we can write down that the solution
to Laplace's equation in spherical coordinates
is:
After matching the boundary conditions
at the boundaries, then we find that the magnetic
field inside the cavity in the spherical shell
is:
where is an attenuation coefficient that depends
on the thickness of the diamagnetic material
and the magnetic permeability of the material:
This coefficient describes the effectiveness
of this material in shielding the external
magnetic field from the cavity that it surrounds.
Notice that this coefficient appropriately
goes to 1 in the limit that . In the limit
that this coefficient goes to 0, then the
attenuation coefficient takes on the simpler
form:
which shows that the magnetic field decreases
like .
See also
Electromagnetic interference
Electromagnetic radiation and health
Radiation
Ionising radiation protection
Mu-metal
MRI RF shielding
Permalloy
Electric field screening
Faraday cage
References
External links
All about Mu Metal Permalloy material
Mu Metal Shieldings Frequently asked questions
magnetic permeability
Clemson Vehicular Electronics Laboratory:
Shielding Effectiveness Calculator
Shielding Issues for Medical Products — ETS-Lindgren
Paper
Guide To Solving AC Power EMF Problems — VitaTech
Engineering Paper
Practical Electromagnetic Shielding Tutorial
Simulation of Electromagnetic Shielding in
the COMSOL Multiphysics Environment
