4. • Shielding is produced by putting a metallic
barrier in the path of em waves between the
culprit emitter and receptor.
• The em waves while penetrating through the
metallic barrier experiences an intrinsic
impedance of metal given by
Zm = (0/2)1/2 (1-j)
• The value of this impedance is extremely low for
good conductors at frequency below optical
region.
5. • Two basic mechanisms are responsible for
major part of shielding.
1.Reflection loss,
2.Absorption loss.
• Therefore shielding theory is mainly based
on transmission behaviour through metals
and reflections from the surface of metals.
6.
7. • Total shielding effectiveness SE(dB) of a
solid conducting barrier can be expressed
as the sum of reflection loss α R(dB),
absorption loss αA(dB) and internal
reflection losses αIR(dB).
8. Shielding theory & Shielding
effectiveness:
Practical shielding performance depends on
number of parameters such as frequency,
distance of interference from the shielding
walls, polarization of the fields, discontinuties
in shield.
• The regions located close to the radiating
source have high intensity fields and fields can
have both longitudinal & transverse
components.
9. • The two fields are related by the wave
impedance which is defined as the ratio of
tangential component of E field and H field.
• Predominantly wave impedance for E field is
very large and H field is very small.
• At sufficiently large distance from
source the em waves become plane waves
with wave impedance
10. Impedances of E,H& plane wave fields:
For Plane Wave (free space) Field :
Zw = 0= 120 ohm
For E-field ZE>> 0
For H-field ZH<< 0
11. Shielding effectiveness measurements:
• Plane Wave SE (dB) = 10 log 10 (P1/P2)
Reflection Loss >> Absorption loss
• E - Field SE (dB) = 20 log 10 (E1/E2)
Reflection Loss >>> Absorption loss
• H - Field SE (dB)= 20 log 10 (H1/H2)
Absorption Loss >> Reflection loss
12. Single shield:
For a conductor used below optical frequencies,
the conduction current is normally much greater
than displacement current.
The electrical parameters of a metal for an em
wave incident at any angle as follows:
• The propagation constant inside a metal along
normal direction is
13. • The attenuation constant inside a metal
along normal direction is
• The phase velocity and wavelength inside
the shield are
14. • By definition, the reflection loss is
expressed as
where T – Net transmission coefficient
V = 0 /zm
• The absorption loss of wave passing
through shield of thickness t is given by
15. • The internal reflection loss term is
expressed by
Internal reflection losses can be neglected
for cases when absorption loss αA>15dB.
22. • When thick copper shields are used and
frequency range is such that
then
Absorption loss is high enough(>15dB) to
avoid internal reflections.
23. E-Field & H-Field Shielding effectiveness:
Using wave impedances & intrinsic impedance of a
metal equations the reflection losses for E & H fields can
be expressed as
24. The reflection losses for plane wave fields is
where r is relative permeability w.r.t air,
δr is relative conductivity w.r,t copper,
f is frequency in hertz,
r is distance from source to the
shielding barrier in meters.
26. For given values of r and t, the E field and plane wave field reflection
losses for both copper & iron decrease with an increase of
frequency whereas magnetic field
27.
28. SHIELDING PROPERTIES
• ABSORPTION :
• REFLECTION :
• REFLECTION OF :
E -FIELD
• REFLECTION OF :
H-FIELD
• REFLECTION OF PLANE
Increases with increase
in frequency ,barrier
thickness, barrier
permeability and
conductivity
In general: Increases
with increase in
conductivity and
decrease in
permeability.
Increases with a
decrease in frequency
and distance.
Increases with an
increase in frequency
and distance
Increases with a
decrease in frequency.
31. Apertures in shielding wall:
SE reduced considerably when the size of
discontinuities become resonant size is /2 for
plane wave field.
32.
33. HONEY COMB AIR VENTS:
Metallic hexagonal honey comb openings used for air
ventilation in shielded enclosure- each honeycomb
acts as waveguide below cut-off
For 100 dB attenuation
d /3.4, t 3d; where
d = mean dia of a cell.
SE (dB) = 20log10 (fc /f)
-10log 10 n+27.3 t/d
fc = cut off frequency
34. SEAMS
The total SE of a shielded compartment is
limited by the failure of seams to make current
flow in the shield.
• The shielding performance of seams depends
on ability to create a low contact resistance
across the joint.
• contact resistance is a function of materials , the
conductivity of their contaminants and the
contact pressure.
• Considerations that increase the SE:
1.conductive contact
2.seams overlap
3.gasket/seam contact points
35. EMI Gasket materials:
1. Knitted wire mesh: used for providing shielding
for electronic enclosure joints , door contacts ,
and cables.
2. Oriented wire mesh: silicon rubber gasket used
in military, industrial and commercial.
3. Conductive elastomer: This is silver-aluminum
filled silicon elastomer that provides high
shielding effectiveness & improved corrosion
resistance.
4. SPIRAL METAL STRIP: Designed to place
between two flat surfaces which is highly
conductive, corrosion resistant spring material.
39. 1.MIL-STD-285
Principle: It is a substitution method using two antennas,
one transmitting and one receiving.
• Frequency range: 100kHz to 10 GHz.
40. Measurements recommended:
1. Low impedance magnetic field shielding :150- 200kHz.
The Transmitting and receiving antennas are 12-in
diameter loop, placed perpendicular to wall at a distance
12-in from the walls.
2. High impedance shielding:200kHz,1MHz,18MHz.the
Transmitting and receiving antennas are 41-in rod
antennas, placed parallel to shielding wall at a distance
12-in from the walls.
3. Plane-wave field shielding:400MHz.Transmitting and
receiving antennas are electric dipole, and are placed
parallel to the shielding walls. Transmitting antenna is
kept greater than 2λ distance. Receiving antenna is
placed 2-in capacitive coupling.
41. 2.The coaxial Holder Method
It is recommended by the ASTM, for SE measurement of
samples of conducting coating of composite materials
and conductive loaded plastics.
42. • The advantage of this method is that far
field testing of sample is possibly by taking
the measurement in a TEM mode field
inside a coaxial line.
• The SE of the material is obtained from
readings, one with and one without test
sample but with reference in place.
• The coaxial holders are limited in
frequency.
43. 3.Dual TEM Cell Method
It is important to test for near field shielding
performance, especially for intra system EMI and
compatibility.
such measurements can be performed for both electric
field and magnetic field shielding using dual TEM cell.
44. • The SE of material for electric and magnetic
fields is obtained from the insertion loss
expressions for sum and difference signals.
Where αe is the electric polarizability in the
direction normal to the aperture and
αm is the magnetic polarizability tangential
to the aperture and normal to the direction of
propagation in the TEM cell.
45. 4.Time Domain Method
• To get plane wave
shielding effectiveness
data at higher
frequencies.
• A short pulse is
transmitted through the
transmitting antenna,
and direct path signals
through the aperture are
without and with MUT
are measured to find ‘T’.
SE (db)=20log|1/T|.
• Antennas are placed at a
distance of λ/2П from
MUT to achieve far-field
condition for
measurement.
46. ELECTRICAL BONDING
• Electrical bonding is a process in which components
or modules of an assembly, equipment of
subsystems are electrically connected by means of a
low impedance conductor.
• Ideally, the interconnections should be made so that
the mechanical and electrical properties of current
path determined by the connected members and not
by the joints.
• The joints must maintain its mechanical and electrical
properties over an extended period of time.
• The purpose is to make the structures homogeneous
with respect to the flow of RF currents.
47. Factors influence the EMI performance of
the bonding :
1. Generation of intermodulation products because of
nonlinear effects at contacts between similar metals.
2. Development of potential difference caused by DC and
AC resistances and inductance of a given length of the
bond strap.
3. Adverse impedance response because of inductance
and the residual capacitance of the bond strap.
Methods for bonding:
1. A bond is achieved by joining two metallic items or
surfaces through the process of welding or brazing,
2. A bond is obtained by metallic interfaces through
fasteners or by direct metal to metal contact.
3. A bond is achieved by bridging two metallic surfaces with
a metallic bond strap.
48. SHAPE AND MATERIAL FOR BOND STRAP
DC and AC resistance of a bond conductor are
inversely proportional to the cross-sectional area
and the perimeter of the conductor.
• The total impedance of bond conductor is,
CIRCULAR CONDUCTOR: AC and DC resistance of
circular conductor are given by
• For the conductor above height ‘h’
51. GUIDELINES FOR GOOD BONDS:
• All bond surfaces should be smooth and clean,
and no non conductive finishes should be given
at the contact.
• To avoid corrosion and intermodulation
generation, bonding should be made with similar
metals.
• Replaceable washers should be used when
joining with nuts and bolts.
• Solder joints should be avoided.
• Protective finishes should be given to protect
the bond from moisture and other corrosion
causes.
• Self tapping screws are avoided
52. CONCLUSION
shielding and bonding techniques are
useful to protect device from EMI.
• Several practical aspects of shielding
were described using simple physical and
mathematical models.
• These will help in reducing the
engineering time and cost required to
achieve system EMC
