electromagnetic earthing and coupling, electromagnetic shielding
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Mühendislik fakültesi
ELEKTRONİK VE HABERLEŞME
MÜHENDİSLİĞİ
Electromagnetic Shielding
A COURSE OFFERED BY
Prof. Dr. Mustafa MERDAN
Electromagnetic Earthing and Coupling
Submitted by
MSc. Student
Mohammed Mahdi AboAjamm
Student No. 1330145006
What is electromagnetic shielding?
EM shielding (electromagnetic shielding) is the practice of
surrounding electronics and cables with conductive or magnetic
materials to guard against incoming or outgoing emissions of
electromagnetic frequencies (EMF).
EM shielding is conducted for several reasons. The most common
purpose is to prevent electromagnetic interference (EMI) from
affecting sensitive electronics. Metallic mesh shields are often used
to protect one component from affecting another inside a particular
device. In a smartphone, for example, a metallic shield protects
electronics from its cellular transmitter/receiver. Radiation shields
in mobile phones also decrease the amount of radio frequency (RF)
energy that might be absorbed by the user.
To increase the security of air gapped systems, EM shielding is
advised. Conventionally, physical isolation and a lack of external
connectivity have been considered adequate to ensure their
security. However, proof-of-concept attacks have demonstrated
that acoustical infection can be enabled by exploiting the
electromagnetic emanations of the system’s sound card.
Air-gapping is used in the military, government and financial
systems like stock exchanges. The measures are also used by
reporters, activists and human rights organizations working with
sensitive information.
A number of different materials and techniques are used for EM
shielding. Wires may be surrounded by a metallic foil or braid
shield to block errant EMI from the cased wires. Audio speakers
often have inner metallic casing to block EMI produced by the
drivers so they don’t affect TVs and other electronics. Complete
continuous enclosure is not necessary so long as any openings are
smaller than the electromagnetic waves that need to be blocked.
Special conductive paints can be used to prevent the EMF from
networks escaping the originating business to prevent
eavesdropping or wireless attacks. These techniques are like a
miniature Faraday cage, which can prevent signal corruption that
would cause electronics to perform unexpectedly.
Electronics may also have connections filtered for EMI by use of
electronic components like capacitors, ferrules and grounded wires
to minimize the effects of EMI noise -- even twisting wires
together with grounds can reduce lower interference.
Magnetic materials must be used for EM shielding in environments
where the magnetic fields are slowly varied below the 100Khz
range as a Faraday cage-type solution is ineffective in that
situation. With magnetic materials, the EMI is drawn into the
magnetic field of the shielding.
Fundamental Concepts
Knowledge of the fundamental concepts of EMI shielding will aid
the designer in selecting the gasket inherently best suited to a
specific design. All electromagnetic waves consist of two essential
components, a magnetic field, and an electric field. These two
fields are perpendicular to each other, and the direction of wave
propagation is at right angles to the plane containing these two
components. The relative magnitude between the magnetic (H)
field and the electric (E) field depends upon how far away the
wave is from its source, and on the nature of the generating source
itself. The ratio of E to H is called the wave impedance, Zw.
If the source contains a large current flow compared to its
potential, such as may be generated by a loop, a transformer, or
power lines, it is called a current, magnetic, or low impedance
source. The latter definition is derived from the fact that the ratio
of E to H has a small value. Conversely, if the source operates at
high voltage, and only a small amount of current flows, the source
impedance is said to be high, and the wave is commonly referred
to as an electric field. At very large distances from the source, the
ratio of E to H is equal for either wave regardless of its origination.
When this occurs, the wave is said to be a plane wave, and the
wave impedance is equal to 377 ohms, which is the intrinsic
impedance of free space. Beyond this point all waves essentially
lose their curvature, and the surface containing the two
components becomes a plane instead of a section of a sphere in the
case of a point source of radiation. The importance of wave
impedance can be illustrated by considering what happens when an
electromagnetic wave encounters a discontinuity. If the magnitude
of the wave impedance is greatly different from the intrinsic
impedance of the discontinuity, most of the energy will be
reflected, and very little will be transmitted across the boundary.
Most metals have an intrinsic impedance of only milliohms. For
low impedance fields (H dominant), less energy is reflected, and
more is absorbed, because the metal is more closely matched to the
impedance of the field. This is why it is so difficult to shield
against magnetic fields. On the other hand, the wave impedance of
electric fields is high, so most of the energy is reflected for this
case. Consider the theoretical case of an incident wave normal to
the surface of a metallic structure as illustrated in Figure below. If
the conductivity of the metal wall is infinite, an electric field equal
and opposite to that of the incident electric field components of the
wave is generated in the shield. This satisfies the boundary
condition that the total tangential electric field must vanish at the
boundary. Under these ideal conditions, shielding should be perfect
because the two fields exactly cancel one another. The fact that the
magnetic fields are in phase means that the current flow in the
shield is doubled.
Shielding effectiveness of metallic enclosures is not infinite,
because the conductivity of all metals is finite. They can, however,
approach very large values. Because metallic shields have less than
infinite conductivity, part of the field is transmitted across the
boundary and supports a current in the metal as illustrated in
Figure below. The amount of current flow at any depth in the
shield, and the rate of decay is governed by the conductivity of the
metal and its permeability. The residual current appearing on the
opposite face is the one responsible for generating the field which
exists on the other side.
RADIO FREQUENCY SHIELDING DEFINITIONS:
In any technical field of knowledge, a certain amount of special
terms unique to that field must be understood in order to
comprehend what is being presented. Therefore, this section is
placed deliberately here in the first chapter so that a working
vocabulary necessary to understanding the material presented can
be easily acquired by those not familiar with shielding prior to the
introduction of the technical concepts. Further definitions and
supporting terminology are given in Appendix A-I.
Absorber a material which absorbs electromagnetic energy by
converting the wave energy into heat.
Absorption Loss The attenuation of an electromagnetic wave as it
passes through a shield. This loss is primarily due to induced
currents and the associated heat loss. Ambient Level. Those levels
of radiated and conducted energy existing at a specified location
and time when a test sample is deenergized. Atmospheric noise
signals, both desired and undesired, from other sources and the
internal noise level of the measuring instruments all contribute to
the "ambient level." Antenna. A device employed as a means for
radiating or receiving electromagnetic energy. Aperture. An
opening in a shield through which electromagnetic energy passes.
Attenuation A general term used to denote a decrease in magnitude
of power or field strength in transmission from one point to
another caused by such factors as absorption, reflection, scattering,
and dispersion. It may be expressed as a power ratio or by decibels.
Bond. The electrical connection between two metallic surfaces
established to provide a low-resistance path between them.
Bonding The process of establishing the required degree of
electrical continuity between the conductive surfaces to be joined.
Conductive Interference. Undesired signals that enter or leave an
equipment along a conductive (wire or metallic) path.
Coupling Energy transfer between circuits, equipments, or
systems. Coupling, Free-Space. Energy transfer via
electromagnetic fields not in a conductor. Cutoff Frequency. The
frequency below which electromagnetic energy will not propagate
readily in a waveguide. dB. Decibel, a unit of voltage or power
ratio. Defined as follows: dB = 10 log P'21P 1 for power or dB =
20 log V2IVI for voltage. HdB" is commonly used to specify
shielding effectiveness since very large differences in the
input/output fields are generally required by the shielding
specification.
This means that if one watt of power impinges on the shield, then
only one millionth to one ten trillionth of a watt exits on the other
side. Degradation. A decrease in the quality of a desired signal
(i.e., decrease in the signalto- noise ratio or an increase in
distortion), or an undesired change in the operational performance
of equipment as the result of interference.
Earth Electrode System. A network of electrically interconnected
rods, plates, mats, or grids installed for the purpose of establishing
a low-resistance contact with earth.
The design objective for resistance to earth of this subsystem
should not exceed 10 O.
Electric Field. A vector field about a charged body. Its strength at
any point is the force which would be exerted on a unit positive
charge at that point.
Electromagnetic Compatibility (EMC). The capability of
equipment or systems to be operated in their intended operational
environment at designed levels of efficiency without causing or
receiving degradation owing to unintentional electromagnetic
interference.
Electromagnetic compatibility is the result of an engineering
planning process applied during the life cycle of the equipment.
The process involves careful considerations of frequency
allocation, design, procurement, production, site selection,
installation, operation, and maintenance.
Electromagnetic Interference (EMI). Any conducted, radiated, or
induced voltage which degrades, obstructs, or repeatedly interrupts
the desired performance of electronic equipment.
Electromagnetic Pulse (EMP). A large impulsive-type
electromagnetic wave generated by nuclear or chemical explosions
Facility A building or other structure, either fixed or transportable
in nature, with its utilities, ground networks, and electrical
supporting structures
Far Field The region of the field of an antenna where the radiation
field predominates, and where the angular field distribution is
essentially independent of the distance from the antenna. A variety
of guidelines is used; for some shielding calculations, 1/6th of a
wavelength has been found useful.
Fault An unintentional short circuit or partial short circuit (usually
of a power circuit) between energized conductors or between an
energized conductor and ground.
Field Strength A general term that means the magnitude of the
electric field vector (in volts per meter) or the magnitude of the
magnetic field vector (in ampere-turns per meter). As used in the
field of EMC/EMI, the term "field strength" shall be applied only
to measurements made in the far field and shall be abbreviated as
FS.
For measurements made in the near field, the term 44electric field
strength" (EFS) or "magnetic field strength" (MFS) shall be used,
according to whether the resultant electric or magnetic field,
respectively, is measured.
Filter. A device for use on power or signal lines, specifically
designed to pass only selected frequencies and to attenuate
substantially all other frequencies.
Ground the electrical connection to earth through an earth
electrode subsystem. This connection is extended throughout the
facility via the facility ground system, consisting of the signal
reference subsystem, the fault protection subsystem, and the
lightning protection subsystem.
Noise
In signal processing, noise is a general term for unwanted (and, in
general, unknown) modifications that a signal may suffer during
capture, storage, transmission, processing, or conversion.
Sometimes the word is also used to mean signals that are random
(unpredictable) and carry no useful information; even if they are
not interfering with other signals or may have been introduced
intentionally, as in comfort noise.
The IEEE Standard Dictionary defines noise as unwanted
disturbances superposed upon a useful signal that tend to obscure
its information content. This definition is versatile, since it applies
both to intrinsic and extrinsic noise (intrinsic noise is the noise
generated inside a system, while the term extrinsic noise designates
noise originating elsewhere).
Intrinsic Noise
This term refers to the noise generated inside an investigated
device or circuit. In linear systems the physical origin of noise is
the discrete nature of charge carriers. Consequently, the number of
carriers in some specific plane fluctuates in time. These
fluctuations are both universal and unavoidable.
A typical example is thermal noise, originating from the random
motion of free electrons inside a piece of conductive material.
Extrinsic Noise
The sources of extrinsic noise are situated outside the investigated
circuit, which merely acts as a receiving antenna; for this reason
this kind of noise is also called extraneous signals, or spurious
signals, or perturbations. According to its possible origins, two
main categories exist:
Environmental perturbations
such as sky noise (which includes strong broadband noise sources
like the Sun and the Milky Way), atmospheric noise (caused
mainly by lightning discharges in thunderstorms), manmade noise
(due to electric motors, arc welding, power lines, neon signs,
electrostatic discharge, electrical power equipment, radio and TV
broadcast services, motors, switches, spark plugs in ignition
systems, household appliances, cellular telephones, mobile radios,
etc.). All industrial perturbations are characterized by relatively
high amplitudes, and a spectrum that cuts off before reaching
visible wavelengths. Many are regular in form and periodic.
Crosstalk noise
Signals which are useful in one circuit, but unfortunately pass via
parasitic coupling into nearby circuits, where they are undesired
and therefore act as perturbations. As a general rule, the user
discovers the interference (i.e., the undesirable effect of spurious
signals) during operation and not before! Sometimes, such
coupling can be reduced by modifying the relative position of
various cables or equipment in a rack.
Thermal Noise
The physical origin of this noise is the thermal motion of free
electrons inside a piece of conductive material, which is totally
random.
Thermal noise is a consequence of the discrete nature of charge
and matter. At a microscopic level, thermal motion is a general
property of matter, regardless of temperature. Physical systems
containing a huge number of identical particles have a large
number of degrees of freedom, corresponding to the number of
ways in which energy can be stored in the system.
Diffusion Noise
The physical origin of diffusion noise is carrier velocity
fluctuations caused by collisions.
This noise is related to the diffusion process that results from
nonuniform carrier distribution. If carrier density increases at one
end of a semiconductor (for instance by illumination), it is a
natural tendency for the carriers to move towards the opposite end,
where the population is reduced. This is a diffusion process. Note
that no applied voltage is necessary to sustain it.
However, during their trip the electrons are scattered via collisions
with the lattice or with ionized impurity atoms. As a result, carrier
velocities are modified, and this represents a perturbation of the
regular diffusion tendency. As scattering occurs randomly, the
instantaneous value of the diffusion current is also random. This is
the mechanism of diffusion noise.
Quantum Noise
The quantified (discrete) nature of electromagnetic radiation
represents the fundamental origin of quantum noise. According to
quantum mechanics, electromagnetic energy is radiated or
absorbed in small discrete quantities, called photons. The energy of
a photon is related to the frequency of its associated radiation by
E = hf
Where h is Planck’s constant (h = 6.63 • 10−34 Js).
Consider a system exchanging thermal or optical energy with its
environment The bilateral flow of energy between the detector and
its environment has a fluctuation, according to the random number
of photons released or absorbed per second. This is called quantum
noise.
External Noise Sources
External noise sources can be grouped into two categories:
Natural noise sources,
Man-made noise sources.
Natural Noise Sources
For convenience, atmospheric noise, precipitation static, solar
noise, Galactic noise and hot-Earth noise are all grouped together
under ―Sky noise.‖ In practice, all quoted noise sources combine
with the celestial background in the radiation pattern of the
receiving antenna. Magnetic storms induced by solar flares have
the ability to induce damaging surges in power-line voltage,
destroying electrical equipment over a huge area. They also
dramatically affect signal propagation and sky noise.
Atmospheric Noise
This is defined as noise having its source in natural atmospheric
phenomena, mainly lightning discharges in thunderstorms. Their
location is time-variable, depending on time of the day, season of
the year, weather, altitude, and geographical latitude. As a general
rule, they are more frequently encountered in the equatorial region
than at temperate latitudes and above. However, the
electromagnetic waves produced by thunderstorms propagate at
thousands of kilometers via ionospheric sky wave. In the time
domain, this noise is characterized by large spikes against a
background of short random pulses. Its frequency spectrum
extends up to 20MHz and the spectral density is proportional to
1/f. Consequently, it mainly affects long-range navigation systems
(maritime radio), terrestrial radio broadcasting stations (LW, MW,
and SW) and to a considerably lesser extent, FM and TV reception.
Precipitation Static
This kind of noise is encountered in rain, snow, hail, and dust
storms in the vicinity of the receiving antenna. Its frequency
spectrum peaks below 10MHz. It can be substantially reduced by
eliminating sharp metallic points from the antenna and its
surroundings, and by providing paths to drain static charges that
build up on an antenna and in its vicinity during storms.
Galactic Noise
This is defined as noise at radio frequencies caused by disturbances
that originate outside the Earth or its atmosphere. Galactic noise
sources can be grouped into two classes: discrete sources and
distributed sources.
In the former category the chief source is the Sun, together with
thousands of known discrete sources, such as supernova remnant
Cassiopeia A, one of the most intense sources of cosmic radio
emission as viewed from Earth. The Sun is the most powerful
noise source, with its temperature of about 6000◦C and its
proximity to Earth. Its energy is radiated in a continuous mode,
and the frequency spectrum mainly covers the range from several
MHz up to several GHz. During quiescent periods, the Sun’s noise
temperature is about 700,000K at 200 MHz, and about 6000K at
30GHz. However, during sunspot and solar-flare activity these
values are considerably higher.
The distributed noise sources are the ionized interstellar gas clouds
in our Galaxy and a considerable number of extragalactic sources
known as radio galaxies. Depending on emission mechanisms, the
distributed noise sources are thermal or nonthermal. So-called
thermal noise sources are associated with random encounters of
electrons and ions in gas clouds, mostly ionized hydrogen.
Nonthermal noise sources (also called synchrotron radiation)
involve electrons moving in magnetic fields. This is a general
galactic phenomenon, encountered even in interstellar space.
Automotive Ignition Systems
There are two major sources: spark plugs and the current flowing
through the ignition system. Both are responsible for radiated
electromagnetic energy, which comes in bursts of short duration
pulses (nanoseconds), the burst width ranging from microseconds
to milliseconds. The frequency of the bursts depends on the
number of cylinders in the motor and the angular motor speed
(RPM). It can be reduced by using spark plugs with a built-in
interference suppressor and shielding the entire ignition system
(when possible).
Arc Welders
Typically, arc welders use an RF arc whose fundamental frequency
is around 2.8MHz . Their spectrum covers the 3 kHz to 250MHz
frequency range, but the fundamental remains at a significant level
even at a distance of several hundred meters. In radio receivers,
this noise is perceived as a ―frying‖ noise. The emission is
considerably reduced by improving welder grounding, using short
welding leads, shielding wiring, and avoiding proximity to power
lines.
Electric Motors
All high-power motors involved in electric transport systems
(underground, trains, conveyor belts, elevators, etc.) generate noise
when switched, but also do so in steady-state operation. Switching
produces transients which can reach several hundred volts as a
result of current interruption in an inductive load. In the steady
state, motor brushes are responsible for arc production, which
increases with aging. Besides radiation, these sparks generate
spurious signals that are conducted and distributed to nearby
systems by the power supply lines. The same problem (although
less aggressive) appears in all household appliances that use
electric motors (washing machines, vacuum cleaners, ventilators,
etc.). This noise can be reduced by inspecting the motor brushes
and changing them when necessary, as well as by adding a
capacitor of about 1 μF in parallel, to suppress sparks.
High-Voltage Transmission Lines
Noise from transmission lines peaks at 50 (60) Hz, and it can cause
interference at distances of several hundred meters. It is especially
perceptible in AM receivers. Transients associated with switching
of loads occur in bursts, and have rise times of a few nanoseconds;
amplitude spikes larger than 2KV are seldom observed. Another
major source of noise is the Corona effect, which consists in a
large number of discharges around the conductors of a power line.
This occurs when the electric field around the conductor exceeds
the value required to ionize the ambient gas (air), but is insufficient
to cause a spark. Discharges are initiated by the presence of small
irregularities in the conductor surface (like dust, pollen, snow, ice
crystals, etc.) and the resulting noise mainly affects AM
communication systems.
AC Supply Lines
The 220 (or 110)V supply lines connect all the rooms of a building
in a power distribution network, as well as all nearby buildings.
Besides its proper 50 (or 60) Hz fields and the transients caused by
switching various loads, the mains wiring constitutes an excellent
antenna, which picks up noise radiated in one room from
perturbing equipment and delivers it to all other rooms sharing the
same line. Hence, it propagates perturbations from one site to
another. In order to protect sensitive equipment, filters and surge
suppressors must be provided so that the bulk of energy is
absorbed before it claims victims. Various types of surge
suppressors exist, such as gas-discharge devices (which can handle
high power, but are slow) and semiconductor devices (using Zener
diodes).
Noise is generated in two distinct areas:
1) In the ionized gas column, which presents a small but
fluctuating resistance when the light is on?
2) In the associated circuitry, which includes a starter. Usually, the
starter is made of a bimetallic strip, which bends when the
temperature changes and abruptly breaks the current flowing
through an inductor. A voltage spike occurs, which is used to
trigger the discharge; however, this spike is also a source of
interference for nearby systems.
ISM Equipment
This category includes industrial equipment (such as relay-
controlled devices, electrical switching gear, laser cutters,
microwave ovens, etc.), scientific equipment (for instance, all sorts
of computer facilities), and medical equipment for intensive care
units, physical therapy facilities, electrosurgical units, diathermy,
CAT scanners, etc. The frequency spectrum of these noise sources
can extend up to several megahertz or even gigahertz.
Radio, Television, and Radar Transmitters
These are intentional emitters of electromagnetic waves that can
interfere with systems not intended for any form of reception. All
such transmitters have considerable power, since they must cover a
large area. Less powerful (but no less harmful) are electromagnetic
waves emitted by CB transmitters, cellular phones, mobile radios,
portable computers, and so forth.
Power Supplies
The major noise sources belonging to this category are DC/DC
converters and switching-mode power supplies. Both employ
switching transistors operating at frequencies up to 100 kHz. The
frequency spectrum is dominated by the fundamental and its
harmonics, but it extends well above the switching frequency
fundamental.
Triboelectric effect
This entails generating electrostatic charges of opposite sign when
two materials are rubbed together and separated, leaving one
positively charged and the other negatively charged. The term
triboelectricity refers to electricity produced by friction of two
dissimilar solids (as by sliding). In practice, this phenomenon
affects mainly the dielectric material within a coaxial cable. When
the cable bends, the metallic conductors slide along the dielectric
used to separate them (if the dielectric does not maintain
permanent contact with the metallic parts). A charge accumulation
appears in the equivalent capacitor formed between the metallic
shield and the inner conductor, separated by the insulator. This
charge fluctuates according to the rhythm of mechanical flexing of
the cable and hence acts as a noise source. This phenomenon is
especially pertinent when the coaxial cable is used to connect a
generator with high internal impedance to a high-value load (like
an electrometer). In this situation, the discharge of the equivalent
capacitor through the terminal impedances is slow, and additional
flexing of the cable causes additional charge to accumulate. For
instance, a coaxial cable terminated by 10-MΩ resistances
generates noise voltages by intermittent flexing which fluctuate
during 50% of the monitoring time around a few millivolts;
however, if the terminal resistances are lowered to 1MΩ, the noise
voltage level decreases to several hundred microvolts. If the cable
is terminated by low impedances, triboelectricity is no longer a
factor.
This kind of noise is critical in cables employed in vehicles,
satellite or airborne instruments, rockets, and military applications,
where vibration is unavoidable. The best solution is to reduce
vibration whenever possible; otherwise, special low-noise cable
can be used, where friction is reduced by an additional layer of
graphite.
Piezoelectric Effect
This is defined as the generation of a potential difference in a
crystal when a strain is introduced. In piezoelectric materials the
converse effect is also observed, namely that a strain results from
the application of an electric field. In practice, some circuit board
materials exhibit this effect. Consequently, they are vibration-
sensitive, and noise voltages can appear between conductors
connected to opposite sides, or between tracks situated on opposite
sides. To avoid this kind of noise, the only solution is to carefully
select circuit boards employing insulators that do not exhibit the
piezoelectric effect.
Noise Parameters
Normalized Power
Let us denote by V the effective (rms) value of the signal v(t)
applied across a resistor R. In this case the power dissipated is
P = V2/ R = RI2
Where I is the effective (rms) value of the current through the
resistor. By definition, the normalized power is the power
dissipated by a one-ohm resistance.
Noise Bandwidth
Is the bandwidth ( Δf ) of an ideal circuit (with rectangular power
transfer characteristic) such that the total transmitted noise power
is equal to that transmitted by the actual circuit.
IEEE Definition: the equivalent noise bandwidth is the frequency
interval, determined by the response frequency characteristics of
the system, that defines the noise power transmitted from a noise
source of specified characteristics.
Equivalent noise resistance
The equivalent noise resistance Rn is a quantitative representation
in resistance units of the spectral density Sv of a noise voltage
generator at a specified frequency.
And the relation between the equivalent noise resistance and the
spectral density Sv of the noise generator is
Rn = (πSv) / kTo
With To = 290 K.
Noise Ratio
The noise ratio N of a one-port is the ratio of: (1) the noise spectral
density (or mean square value) generated by the actual one port, to
(2) the same quantity generated by a hypothetical one-port of
identical impedance which produces only thermal noise.
Signal-to-Noise Ratio
The S/N ratio is the ratio of the value of the signal to that of the
noise (the two being expressed in a consistent way, as for example
peak signal to peak noise ratio, rms signal to rms noise ratio, peak-
to-peak signal to peak-to-peak noise ratio, etc.).
An exception occurs in television transmission, where the S/N ratio
is defined as the ratio of: (1) the maximum peak-to- peak voltage
of the video signal, including synchronizing pulse, to (2) the rms
value of the noise (because of the difficulty of defining the rms
value of the video signal or the peak-to-peak value of random
noise).
What is the earhing(grounding) system?
The electric potential of the conductors relative to the Earth's
conductive surface. The choice of earthing system can affect the
safety and electromagnetic compatibility of the power supply. In
particular, it affects the magnitude and distribution of short circuit
currents through the system, and the effects it creates on equipment
and people in the proximity of the circuit. If a fault within an
electrical device connects a live supply conductor to an exposed
conductive surface, anyone touching it while electrically connected
to the earth will complete a circuit back to the earthed supply
conductor and receive an electric shock.
Regulations for earthing system vary considerably among
countries and among different parts of electric systems. Most low
voltage systems connect one supply conductor to the earth
(ground).
A protective earth (PE), known as an equipment grounding
conductor in the US National Electrical Code, avoids this hazard
by keeping the exposed conductive surfaces of a device at earth
potential. To avoid possible voltage drop no current is allowed to
flow in this conductor under normal circumstances. In the event of
a fault, currents will flow that should trip or blow the fuse or
circuit breaker protecting the circuit. A high impedance line-to-
ground fault insufficient to trip the overcurrent protection may still
trip a residual-current device (ground fault circuit interrupter or
GFCI in North America) if one is present. This disconnection in
the event of a dangerous condition before someone receives a
shock, is a fundamental tenet of modern wiring practice and in
many documents is referred to as automatic disconnection of
supply (ADS). The alternative is defence in depth, where multiple
independent failures must occur to expose a dangerous condition -
reinforced or double insulation come into this latter category.
In contrast, a functional earth connection serves a purpose other
than shock protection, and may normally carry current. The most
important example of a functional earth is the neutral in an
electrical supply system. It is a current-carrying conductor
connected to earth, often, but not always, at only one point to avoid
flow of currents through the earth. The NEC calls it a groundED
supply conductor to distinguish it from the equipment groundING
conductor. Other examples of devices that use functional earth
connections include surge suppressors and electromagnetic
interference filters, certain antennas and measurement instruments.
The main reason for doing earthing in electrical network is for the
safety. When all metallic parts in electrical equipments are
grounded then if the insulation inside the equipments fails there are
no dangerous voltages present in the equipment case. If the live
wire touches the grounded case then the circuit is effectively
shorted and fuse will immediately blow. When the fuse is blown
then the dangerous voltages are away.
Purpose of Earthing:
1. Safety for Human life/ Building/Equipment:
a. To save human life from danger of electrical shock or
death by blowing a fuse i.e. To provide an alternative path
for the fault current to flow so that it will not endanger the
user
b. To protect buildings, machinery & appliances under fault
conditions.
c. To ensure that all exposed conductive parts do not reach a
dangerous potential.
d. To provide safe path to dissipate lightning and short
circuit currents.
e. To provide stable platform for operation of sensitive
electronic equipments i.e. To maintain the voltage at any
part of an electrical system at a known value so as to
prevent over current or excessive voltage on the
appliances or equipment .
2. Over voltage protection:
Lightning, line surges or unintentional contact with
higher voltage lines can cause dangerously high
voltages to the electrical distribution system. Earthing
provides an alternative path around the electrical
system to minimize damages in the System.
3. Voltage stabilization:
There are many sources of electricity. Every
transformer can be considered a separate source. If
there were not a common reference point for all these
voltage sources it would be extremely difficult to
calculate their relationships to each other. The earth is
the most omnipresent conductive surface, and so it was
adopted in the very beginnings of electrical distribution
systems as a nearly universal standard for all electric
systems.
In a high-tech age, why is the concept of Earthing so
important?
What is most profound about Earthing is that it is so natural and
simple, and that it affects every aspect of human physiology. When
you ground yourself, the entire body readjusts to a new level of
functioning, the level, in fact, it seems to have been designed for
throughout evolution. Many people who have lived Earthed for
some years say that they do not want to go back to living
ungrounded. They feel the difference. Living Earthed broadly
elevates your quality of life to a level that seems not otherwise
possible.
Why does the Earth’s electric field transfer so easily to the
body?
The body is mostly water and minerals. It is a good conductor of
electricity (electrons). The free electrons on the surface of the
Earth are easily transferred to the human body as long as there is
direct contact. Unfortunately, synthetically-soled shoes act as
insulators so that even when we are outside we do not connect with
the Earth’s electric field. When we are in homes and office
buildings, we are also insulated and unable to receive the Earth’s
balancing energies.
What is the difference between the Earth’s electric field and
the electric field used to conduct electricity in my home?
The Earth’s electric field is mainly a continuous direct current
(DC) producing field. Throughout history, life on the planet has
attuned our biology to this subtle field. By comparison, home
wiring systems in the U.S. use 60-cycle per second alternating
current (AC). Unless at very low frequency (less that 10 cycles per
second) and/or low power, alternating current is foreign to our
biology. AC, and other forms of man-made environmental
electromagnetic fields (EMFs) are being researched as possible
factors in a variety of stress-related responses. Many people are
sensitive to EMFs. Studies show an ―association,‖ but not cause
and effect, between livings near power lines (or exposure to EMFs
on the job) and higher rates of health problems.
How much “current” is actually being transferred from the
Earth's surface via the wire to a grounding product?
There is no constant measurable current flow beyond the
equalization charge that is instantly transferred to the body when a
person lies on a conductive sheet or makes contact with another
type of Earthing product. We are talking about numbers of
electrons in the trillions and quadrillions. Once the body is
grounded, the rate of influx changes, and the body will only absorb
that amount of electrons needed to maintain the same electrical
potential as the Earth and to restore what is lost in the body’s
metabolic processes. It may take many quadrillions to get the body
stable. As long as a person continues to be grounded, the body can
use the Earth as a natural reservoir, or ―power source,‖ of electrons
to maintain a ―topped up‖ homeostatic level that compensates for
any attrition of internal electrons. With connection to the Earth, it
would thus seem hard for the body to develop any electron
deficiency, and, theoretically, any chronic inflammation. The
actual amount of charge (electrons transferred) would vary
significantly based upon location of the body above the Earth
(voltage) and any potential electrostatic charge that has built up on
the body. The continuing amount of electrons absorbed by the
body to reduce metabolic and immune response free radicals
would also vary significantly between people depending upon their
life style and activity. This is all extremely difficult, if not
impossible, to measure.
What is the difference between the Earthing technology and
the use of magnets?
Although the use of magnets produces some therapeutic effects
when properly applied, magnets cannot provide free electrons, nor
can they connect the body with the naturally balancing electric
frequencies of the Earth. Earthing technology used inside your
home or office connects you with the Earth’s electrons in the same
way as if you were standing barefoot on ground outside.
Can I wear any type of footwear and still be earthed?
No. Standard plastic/rubber or composite soles do not conduct the
Earth’s electric energy. Most shoes today are made from those
materials. You need leather or hide soles, which used to be the
primary footwear materials in the past. Leather itself isn’t
conductive, but the foot perspires and the moisture permits
conduction of the energy from the Earth through the leather and up
into the body. In addition, moisture from walking on damp ground
or sidewalks could permeate up into the leather soled shoe.
Thickness of the sole can also be a factor, and specifically that a
very thick leather sole may not allow the moisture through.
Moccasins are the best type of natural conductive footwear.
Leather isn’t quite as good as bare feet on the ground but certainly
much, much better than standard soles that are insulating.
Hopefully soon shoe companies will begin making grounded
shoes.
Can I ground myself outside by wearing electrostatic discharge
(ESD) footwear?
ESD shoes are primarily designed for discharging static electricity
but to a degree they ground the body beneficially. They are better
than regular shoes but not as good as going barefoot. The
difference between grounding and static discharge is that
grounding instantly equalizes your body at Earth’s potential. Static
discharge, generally called a soft ground or a dissipative ground,
has an inline 1 meg ohm resistor in the ground cord which is
design to slowly bleed off static electrical charges (contact and
separation charges). These charges are created on the body by
clothing and shoes whenever you move your clothing with arm
movement or walk or sit on any synthetic material. The ESD
industry uses dissipative grounding to prevent a rapid discharge of
static electricity that might otherwise blow an electronic circuit or
sensitive chip.
Does Earthing occur if I work, stand, or walk barefooted on a
ceramic tile floor?
It depends on whether the tile floor sits on a concrete slab or on the
ground. If directly on a slab or ground, the energy could come
through. If the tile sits on plywood or some other kind of wood,
plastic, or vinyl understructure, you are not likely to get any
conductivity. It also depends on what kind of tile. Ceramic tile
with a glazed finish on the surface will, like glass, probably
prevent conductivity.
Can Earthing protect me from cell phone frequencies?
The protective potential of Earthing has not been tested yet on cell
phone exposure. There is no research indicating that Earthing will
or will not protect a person from exposure to cell phones signals,
microwave radiation, or radio frequencies. What we know is that
Earthing reduces significantly the induced body voltages generated
by simple exposure to common household 60 Hz EMFs
continuously emitted by all plugged-in electrical cords (even if the
appliance is off), internal wiring, and all ungrounded electrical
devices in the home or office. Based in the cases we have seen of
people extremely sensitive to such EMFs it is prudent to be
grounded as much as possible in the home or office.
If I sleep on the ground in a sleeping bag am I grounded?
The first thing you need to know is that Earthing products do not
operate on electricity, so it does not matter what the electrical
current is in your country (whether 110 volts or 240 volts, etc).
Earthing products simply allow the natural, gentle energy from the
Earth outside to be carried inside. When you make physical —
bare skin — contact with the Earthing product it is the same as if
you were standing or walking barefoot outside. This is what
creates the benefits of Earthing.
Our preference is that the products throughout the world be
connected to Earthing ground rods, however many people like the
idea of simply plugging them into an electrical outlet ground port
in their home or office. For people who live in tall apartment
buildings (high rises), a ground rod may not be feasible, and for
people who do not have a grounded electrical system in their home
or office, the only option is the ground rod.
The following explanation covers two options for connecting
Earthing products. The electrical terms ―ground‖ and ―Earth‖ have
the same meaning.
Earthing examples:
Factors affecting on Earth resistivity:
1. Soil Resistivity:
a. It is the resistance of soil to the passage of electric current.
The earth resistance value (ohmic value) of an earth pit
depends on soil resistivity. It is the resistance of the soil to
the passage of electric current.
b. It varies from soil to soil. It depends on the physical
composition of the soil, moisture, dissolved salts, grain
size and distribution, seasonal variation, current magnitude
etc.
c. In depends on the composition of soil, Moisture content,
Dissolved salts, grain size and its distribution, seasonal
variation, current magnitude.
2. Soil Condition:
a. Different soil conditions give different soil resistivity.
Most of the soils are very poor conductors of electricity
when they are completely dry. Soil resistivity is measured
in ohm-meters or ohm-cm.
b. Soil plays a significant role in determining the
performance of Electrode.
c. Soil with low resistivity is highly corrosive. If soil is dry
then soil resistivity value will be very high.
d. If soil resistivity is high, earth resistance of electrode will
also be high.
3. Moisture:
a. Moisture has a great influence on resistivity value of soil.
The resistivity of a soil can be determined by the quantity
of water held by the soil and resistivity of the water itself.
Conduction of electricity in soil is through water.
b. The resistance drops quickly to a more or less steady
minimum value of about 15% moisture. And further
increase of moisture level in soil will have little effect on
soil resistivity. In many locations water table goes down in
dry weather conditions. Therefore, it is essential to pour
water in and around the earth pit to maintain moisture in
dry weather conditions. Moisture significantly influences
soil resistivity
4. Dissolved salts:
a. Pure water is poor conductor of electricity.
b. Resistivity of soil depends on resistivity of water which in
turn depends on the amount and nature of salts dissolved
in it.
c. Small quantity of salts in water reduces soil resistivity by
80%. Common salt is most effective in improving
conductivity of soil. But it corrodes metal and hence
discouraged.
5. Climate Condition:
a. Increase or decrease of moisture content determines the
increase or decrease of soil resistivity.
b. Thus in dry whether resistivity will be very high and in
monsoon months the resistivity will be low.
6. Physical Composition:
Different soil composition gives different average
resistivity. Based on the type of soil, the resistivity of
clay soil may be in the range of 4 – 150 ohm-meter,
whereas for rocky or gravel soils, the same may be well
above 1000 ohm-meter.
7. Location of Earth Pit :
a. The location also contributes to resistivity to a great
extent. In a sloping landscape, or in a land with made up
of soil, or areas which are hilly, rocky or sandy, water runs
off and in dry weather conditions water table goes down
very fast. In such situation Back fill Compound will not be
able to attract moisture, as the soil around the pit would be
dry. The earth pits located in such areas must be watered
at frequent intervals, particularly during dry weather
conditions.
b. Though back fill compound retains moisture under normal
conditions, it gives off moisture during dry weather to the
dry soil around the electrode, and in the process loses
moisture over a period of time. Therefore, choose a site
that is naturally not well drained.
8. Effect of grain size and its distribution:
a. Grain size, its distribution and closeness of packing are
also contributory factors, since they control the manner in
which the moisture is held in the soil.
b. Effect of seasonal variation on soil resistivity: Increase or
decrease of moisture content in soil determines decrease or
increase of soil resistivity. Thus in dry weather resistivity
will be very high and during rainy season the resistivity
will be low.
9. Effect of current magnitude:
a. Soil resistivity in the vicinity of ground electrode may be
affected by current flowing from the electrode into the
surrounding soil.
b. The thermal characteristics and the moisture content of the
soil will determine if a current of a given magnitude and
duration will cause significant drying and thus increase the
effect of soil resistivity.
10. Area Available:
a. Single electrode rod or strip or plate will not achieve the
desired resistance alone.
b. If a number of electrodes could be installed and
interconnected the desired resistance could be achieved.
The distance between the electrodes must be equal to the
driven depth to avoid overlapping of area of influence.
Each electrode, therefore, must be outside the resistance
area of the other.
11. Obstructions:
The soil may look good on the surface but there may be
obstructions below a few feet like virgin rock. In that
event resistivity will be affected. Obstructions like
concrete structure near about the pits will affect
resistivity. If the earth pits are close by, the resistance
value will be high.
12. Current Magnitude:
A current of significant magnitude and duration will
cause significant drying condition in soil and thus
increase the soil resistivity.
Measurement of Earth Resistance by use of Earth Tester:
1. For measuring soil resistivity Earth Tester is used.
2. It has a voltage source, a meter to measure Resistance in
ohms, switches to change instrument range, Wires to connect
terminal to Earth Electrode and Spikes.
3. It is measured by using Four Terminal Earth Tester
Instrument. The terminals are connected by wires as in
illustration.
4. P=Potential Spike and C=Current Spike. The distance
between the spikes may be 1M, 2M, 5M, 10M, 35M, and
50M.
5. All spikes are equidistant and in straight line to maintain
electrical continuity. Take measurement in different
directions.
6. Soil resistivity =2πLR.
7. R= Value of Earth resistance in ohm.
8. Distance between the spikes in cm.
9. π = 3.14
10. P = Earth resistivity ohm-cm.
11. Earth resistance value is directly proportional to Soil
resistivity value.
Measurement of Earth Resistance (Three point method)
Measure Correct Grounding System Impedance of
Electromagnetic Field:
Measurement of the grounding system impedance of a substation
or power plant is often required immediately after construction, in
order to verify that the design calculations correctly predict the
performance of the system. Years later, new measurements are
sometimes required to check that the performance of the grounding
system has not deteriorated. This type of measurement, however,
which is typically carried out using the fall-of-potential method, is
plagued by a number of potential problems: conductive coupling
between the grid under test and the remote current return electrode,
especially for soil structures with low resistivity over high;
inductive coupling between current and voltage test leads;
inductive coupling between test leads and grounding grid
conductors; inductive coupling between test leads and power line
static or neutral wires; additional grounding provided by power
line static and neutral wires, which lowers the apparent impedance
of the grounding grid.
Plant grounding system, its ground impedance is usually measured,
in order to validate the design calculations. The fall-of-potential
method, also known as the ―3-pin‖ method, is typically used. The
test method consists in essence of causing a test current to flow
through the soil, from the grounding system under test to a remote
current return electrode, while the resulting potential rise of the
grounding system is measured with respect to a sufficiently distant
potential reference electrode. Typically, several potential rise
values are measured for increasingly distant reference electrode
positions, in order to confirm that the full potential rise has been
measured. The potential rise divided by the test current yields the
ground impedance. For small grounding systems that are isolated
from any other ground electrodes and have sufficient clearance
from nearby grounded structures, this test is usually as simple as it
sounds.
On the other hand, for larger grounding systems or those that are
not electrically isolated from other ground electrodes, a plethora of
problems can occur, which is why a distinct IEEE standard was
written to address the testing of such large systems. The following
points must therefore be considered:
1. The ground impedance of a large grounding system tends to be
low. As a result, the measured grid potential rise is small and
undesirable effects, such as induced voltages, that would otherwise
go unnoticed, can become large enough to alter the measured
signal considerably. Stray noise also becomes a greater issue, but
can usually be handled by frequency-selective test gear.
2. Since the potential rise of the grounding system is small, it is
more susceptible to earth potentials transferred from the remote
current return electrode, so this latter must be placed further away
than for a small grounding system. Indeed, IEEE Standard 81.2
indicates that a separation distance of 6.5 times the maximum
diagonal of a rectangular grounding system is required to achieve
95% accuracy. As this paper will show, the actual required
separation distance is a function of soil structure.
3. As a result of this large separation distance, long test leads are
required.
4. A long lead carrying an ac test current is apt to induce
significant voltages in the test lead used to measure the potential
rise of the grounding system, if the two leads are run parallel to
one another or at an acute angle to one another. Induced voltages
do not decrease rapidly as a function of separation distance
between leads, so increasing the spacing between them is of
limited effectiveness.
5. The current-carrying test lead can also induce voltages in long
grounding grid conductors, if there is any parallelism or an acute
angle between them, thus altering the distribution of test current in
the grounding system and its measured impedance.
6. Test current injected into a long grounding grid can induce
significant voltages in the test lead that measures the potential rise,
if this test lead is run parallel or at an acute angle to the grounding
grid.
7. The above concerns would be eliminated by the use of a very
low frequency test signal. However, for large grounding grids or
even smaller ones in low resistivity soils (the size of concern is
strongly related to soil resistivity), the 50 or 60 Hz reactance of the
grounding grid conductors make a major contribution to the total
grid impedance. A very low frequency test signal would not
reproduce the inductive choke effect that these conductors exhibit
at power frequency and therefore provide false results,
underestimating the grounding system impedance. It is important
that the test signal be carefully chosen to avoid this pitfall.
Guidance in the selection of the appropriate test frequency is one
important part of this paper’s contribution.
8. When a grounding system is connected to other ground
electrodes, as is the case for an operating station, whose grounding
grid is typically connected to lightning shield wires (or other types
of ground return conductors), the effective size of the grounding
system is increased and all of the above problems are compounded,
with the lightning shield wires introducing additional inductive
coupling problems. Furthermore, since the objective of the test is
to measure the ground impedance of the station, it is desirable to
somehow exclude the supplemental grounding provided by the
exterior electrodes.
Ground impedance measurements can be influenced by the
following factors:
1. Size of grounding system.
2. Distance from the grounding system of the remote test
current return electrode and the potential reference
electrode.
3. Soil resistivity and layering.
4. Test signal frequency.
5. Angle and separation distance between test leads.
6. Type of lightning shield wire.
7. Separation between test leads and power lines.
8. Separation between test current injection point and potential
rise measurement point on grounding system.
Coupling
In electronics and telecommunication, coupling is the desirable or
undesirable transfer of energy from one medium, such as a metallic
wire or an optical fiber, to another medium, including fortuitous
transfer.
Coupling is also the transfer of electrical energy from one circuit
segment to another. For example, energy is transferred from a
power source to an electrical load by means of conductive
coupling, which may be either resistive or hard-wire. An AC
potential may be transferred from one circuit segment to another
having a DC potential by use of a capacitor. Electrical energy may
be transferred from one circuit segment to another segment with
different impedance by use of a transformer. This is known as
impedance matching. These are examples of electrostatic and
electrodynamics inductive coupling.
Types of coupling:
1. Electrical conduction:
a. Hard-wire
b. Resistive
c. Natural conductor
2. Electromagnetic induction:
a. Electrodynamic -- commonly called inductive coupling,
also magnetic coupling
b. Electrostatic -- commonly called capacitive coupling
c. Evanescent wave coupling
3. Electromagnetic radiation:
a. Radio -- wireless telecommunications
b. Electromagnetic interference (EMI) -- Sometimes called
radio frequency interference (RFI), is unwanted coupling.
Electromagnetic compatibility (EMC) requires techniques
to avoid such unwanted coupling, such as electromagnetic
shielding.
c. Microwave power transmission
4. Other kinds of energy coupling:
Acoustic.
What is electromagnetic coupling?
Electromagnetic coupling is a phenomenon common to electrical
wiring and circuits where an electromagnetic field in one results in
an electrical charge in another. It is often referred to as inductive
coupling because the process occurs due to electrical inductance,
where a transferring of electromagnetic properties from one
location to another occurs without physical contact taking place. In
order for electromagnetic coupling to take place, there must be a
change in the electromagnetic field that is generating it. For this
reason, direct current (DC) devices do not produce the effect, but it
is common in alternating current (AC) circuits. The principle of
electromagnetic coupling was discovered by Michael Faraday and
Joseph Henry in 1831, and is known as Faraday's Law.
When an AC current in a circuit or wire induces a voltage in
another wire, it is usually due to the fact that they are both in close
proximity to each other, such as in the electrical windings that
transformers have. This is not always true, however, and coupling
at a distance that is unintentional, called cross talk, can occur with
radio and telephone transmissions as well. Intentional
electromagnetic coupling is the principle that transformers are
based upon, where current can be stepped up or stepped down in
voltage in a secondary wire winding based on the current level in a
primary winding on the device.
Since electromagnetic radiation is a dual condition in nature where
electromagnetic waves are composed of both electrical and
magnetic properties, couplings are also of two types. An electrical
coupling results when a positive or negative charge density in a
wire or circuit changes, and this repels like charges in another
circuit wire. The process of repelling like charges in nearby wire
causes them to move within the wire, and this is the definition of
what electrical current is. This form of current flow is often
referred to as charge coupling or capacitance coupling.
Magnetic coupling is the flip side of this effect. As a current flows
in a wire, it generates a magnetic field. With AC current, this
magnetic field will fluctuate and cause a changing magnetic field
in coupled circuits or wires. Magnetic fields are directly
perpendicular to electric fields in electromagnetic coupling, so
altering a magnetic field in one circuit can alter the current flow in
another.
The principle of electromagnetic coupling is what all modern
electric motors, relays, and transformers are built upon. Electrical
generators also utilize it, as do a wide variety of communications-
related devices; from citizen's band (CB) radios to televisions and
wireless door locks for buildings and automobiles. It can also be
detrimental to how a circuit functions and cause interference in
telecommunications. In this case, it's often referred to as
electromagnetic interference (EMI). Not all EMI is unintentional,
however, as it can be used as a form of carrier wave to enhance
signal strength as well.
Electromagnetic coupling is used in communications-related
devices like CB radios.
Coupled Structures:
As frequency of an electrical signal becomes greater, the
corresponding wavelength be-comes commensurable with the
dimensions of the signal-carrying lines. As this happens,
geometrical parameters of the signal-carrying lines may no longer
be neglected when de-signing electronic circuits, for potentials and
currents are no longer constant along the length of the lines and
per-unit-length line parameters, viz. resistance, inductance,
conductance, and capacitance significantly affect propagation of
the signal along the line. Thus, the problem of designing the lines
acquires another domain, spatial, in addition to the time domain.
The signal-carrying lines used in microwave circuits are usually
referred to as transmission lines, as any other line employed to
convey energy, e.g. transmission lines in electrical power
engineering, where the problem of the line design also has to
account for variation of potentials and currents along the length of
the line and per-unit-length parameters, but due to immense
lengths of the lines rather than high operating frequency.
Transmission lines employed in microwave circuits typically have
two main missions: (1) transfer of electromagnetic energy between
circuit elements, and (2) serving as wave guiding media for
creation of separate circuit elements such as couplers, filters,
baluns (balanced-to-unbalanced transformers), etc.
When two or more unshielded transmission lines supporting
propagation of time-varying electromagnetic fields are placed in
close proximity, see figure below, they demonstrate
electromagnetic coupling between each other due to excitation of
the electromotive forces in the line affected by the time-varying
fields of the other line, which gives rise to additional per-unit-
length parameters, viz. mutual capacitance, which describes
interaction between the lines in terms of voltages, and mutual
inductance, which describes interaction between the lines in terms
of currents. Such structures are called parallel edge-coupled
structures, if the lines lie in a common plane, or broadside-coupled
structures, if the lines are placed one above the other.
A general representation of coupled lines
A graphical representation of these in a strip line wave guiding
medium is given in figure below (a strip line waveguide consists of
a signal conductor placed between two ground planes as shown in
the figure). The distance to which the lines should be brought
together in order for appreciable electromagnetic coupling to
appear is defined relative to the distance between the signal lines
and the lines, serving as electric potential reference, referred to as
grounds. Generally, the distance between the signal lines should
not be much greater than the distance between the lines and the
ground in order not to confine electro-magnetic fields only around
the signal lines.
Electromagnetic coupling between transmission lines may be
classified as either desirable or parasitic. Desirable coupling is
created intentionally in the structures that operationally depend on
interaction of the electromagnetic fields supported by the lines.
Such structures include couplers, filters, and baluns. Parasitic
coupling, also referred to as crosstalk, is an unwanted effect
observed in closely spaced interconnecting traces or lumped
elements that operate on either high frequencies or pulses with
sharp edges. High levels of parasitic coupling may significantly
deteriorate electrical performance of the circuits; therefore
parasitic coupling should be accounted for when designing circuits
with high density of arrangement of elements and additional
measures should be taken to decrease the influence of crosstalk.
Representation of edge-coupled (a) and broadside-coupled (b)
structures in strip line wave guiding medium
The Need for Compact Tight Couplers
Couplers have been used extensively in a wide variety of
microwave applications, including power division/combining and
signal sampling. The majority of couplers provide coupling levels
within the 8 – 40-dB range. However, several applications require
greater levels of coupling, up to 3-dB, which corresponds to
division of incoming power into two equal halves. One such
application is the active antenna array, where usage of compact
components is critical.
An active antenna array is a combination of transmit/receive
modules (TRM), each per-forming the functions of final
amplification for transmitted signals, preliminary amplification for
received signals, and control of the phase and amplitude of these
signals to electronically steer the antenna beam. In order for an
active antenna array to operate properly within the whole
frequency and spatial range, the spacing between the TRMs should
be no greater than half-wavelength at the highest operating
frequency. For instance, an active antenna array operating in free
space at 30 GHz must have TRMs spaced no further than 5 mm
from each other. This gives appreciation of how compact, yet
complex enough each TRM should be to meet the aforementioned
functional requirements. Moreover, TRMs must be of low cost in
order for active antenna arrays to be economically feasible since
virtually thousands of TRMs are required for high gain active
antenna arrays. This requires increased levels of circuit integration
to make fabrication less expensive.
Notwithstanding being faced with this multitude of engineering
and economic problems, each TRM’s power amplifier is required
to output high power, which is a contradictory requirement for
solid-state active devices, being the backbone of monolithic
microwave integrated circuits (MMIC), to be of small size and
feature a high output power at high operating frequency. Hence,
summation of output power from a number of power amplifiers is
a must for high overall transmit output power of each TRM.
Power combining networks consist of a number of branches with
power amplifiers, each fed from a common signal source through a
power divider that divides the input signal be-tween the branches.
Amplified signals from the output of each branch power amplifier
are combined into a common output signal with the help of a
power combiner. Power-combining networks of this type are called
corporate networks. An example of such a network is given in
below figure. Here, the output power of the network is twice the
output power of each branch. In addition to the advantage of power
combining, this network pro-vides redundancy: if one of the
branch amplifiers fails, the whole network remains operational,
though with half the normal output power.
Both power dividers and power combiners in the corporate power-
combining networks are the same microwave devices, viz. 3-dB
directional couplers, and are four-port reciprocal devices, which
means that any port can serve as the input port and the remaining
ports will serve as relevant output ports.
Power-combining corporate network employing couplers
The most popular couplers are the SML edge-coupled couplers
owing to their simplicity and optimum area utilization. They are
often implemented in the planar wave guiding media, which are
the waveguides of choice in the prevailing majority of small-scale
micro-wave circuits. However, the SML edge-coupled couplers are
unable to attain the 3-dB level of coupling due to prohibitively
narrow gaps between the signal traces required for as tight a level
of coupling as 3 dB, which compromises the very possibility of
fabrication of such couplers. Therefore, microwave designers
having conventional circuit fabrication technologies at their
disposal have to resort to other, more complex coupler
configurations such as multi-dielectric layer broadside-coupled
strip lines (see Fig. a), which greatly increases production
complexity, especially in MMIC’s; multi-line couplers such as the
Lange coupler (see Fig. b), which are not optimum from the area
utilization point of view and require wire jumpers, which add
parasitic inductances; or branch-line couplers (see Fig. c), which
are neither compact in area, nor wide in frequency bandwidth.
increasing the level of coupling with retaining feasibility of the
coupled-line structures may be attained by following one or both of
the following methods: (1) enlarging the area of metal interface in
the field interaction volume of the coupled structure, or (2)
applying dielectric materials with high relative permittivity.
a) Broadside strip line coupler (cross-sectional view)
b) Lange coupler (top view)
Branch-line coupler (top view)
Following the latter approach might significantly deteriorate high-
frequency performance of microwave devices fabricated using
dielectric materials with high relative permittivity.
Therefore, following the first approach could be considered more
beneficial to obtain simple 3-dB couplers.
In MMICs, or small-scale circuit environment, optimum
enlargement of metal interface area with respect to chip area
utilization may be achieved by exploring the third, tradition-ally
unused, dimension, viz. the height of metal structures serving as
signal lines. The fabrication technology that allows production of
small-scale tall metallic structures combined with the precision and
aptitude for large-scale fabrication of integrated circuits is the deep
X-ray lithography (DXRL) and its spin-off techniques such as
LIGA (DXRL with replication).
Principle of Electromagnetic Coupling:
A transmission line representing a system of N+1 non-touching
conductors, where one of the conductors acts as the reference of
zero potential (in other words, N transmission lines with a common
reference placed in proximity to each other) can generally support
N independent, or normal, propagation modes, each one described
with a unique propagation constant and nonzero voltages and
currents [12]. When the media, where the system is located is
homogeneous, i.e. electromagnetic waves propagating along the
conductors do not experience refraction, the modes will be
transverse electromagnetic (TEM) and they, still being
independent, will have identical propagation constants. When one
of the N transmission lines is excited with a time-varying signal, N
sets of voltages and currents will be excited in the system due to
the electromagnetic coupling. The essence of the electromagnetic
coupling may be readily seen from Faraday’s Law (2.1) stating that
any change in the magnetic environment produces opposing
electromotive force:
where electric field intensity E is tangential to contour C and
normal to magnetic flux density B passing through an arbitrary
surface S bounded by C. For a stationary system, counteracting
electromotive force will be induced by any temporal change of
perpendicular magnetic field. Written in terms of field components
after performing vector operations, Faraday’s Law may be
presented as follows:
Where Et and Bn are mutually orthogonal components of electric
and magnetic fields respectively. One of the implications of (2.2) is
that if there are two closely spaced conductors and the first one is
acted upon by magnetic field produced by current in the second
conductor, there will be induced electromotive force emf causing
flow of current in the first conductor in the direction opposing the
change of magnetic field. Thus, electromagnetic coupling is
established between the two conductors via magnetic-flux linkage
as shown in Figure:
Magnetic-flux linkage between two conductors
In case if time-varying magnetic flux is caused by harmonic
current, (2.2) may be written in the phasor form as:
Where ω is the operating angular frequency. It is clear from (2.3)
that in order for electro-magnetic coupling to exist in a stationary
system of conductors, one of them must be driven from an AC
source.
Directional Couplers
A four-port microwave device demonstrating electromagnetic
coupling is called a ―directional coupler‖. The simplest example of
the directional coupler is a pair of unshielded transmission lines
placed closely to each other and having a common reference of
zero potential. Assuming that the lines are identical and the cross
section of the coupled part does not change along its entire length,
i.e. the coupled part is uniform and symmetrical, as shown in Fig.
2.2, then the scattering matrix, [S], of such a network will be as
follows, provided the network is ideally matched to the
characteristic impedance of the network at all four ports:
a) A two-line four-port uniformly coupled symmetrical section
If the network is not built of any nonreciprocal media, it shows
reciprocal properties, which means that any port can serve as input
and remaining ports will serve as appropriate output ports.
Depending on the direction that the coupled wave travels, which in
its turn depends on the geometry of the coupler, coupled power
will appear either at Port 3 or at Port 4 as shown in Fig. a) In the
former case, the coupler will be backward-wave, and in the latter –
forward-wave. Assuming a backward-wave directional coupler, i.e.
the coupler where the coupled wave on the secondary line
propagates in the direction opposite to the direction of the wave on
the primary line, the elements of matrix (2.4) will have the
following meaning when Port 1 acts as the input: S12 –
throughput, S13 – coupling, S14 – isolation. By the virtue of
reciprocity, the remaining elements of matrix (2.4) correspond to
the above as follows: S23 = S14, S24 = S13, S34 = S12. So when
the input is assigned to Port 1, Port 2 becomes direct (through)
port, Port 3 – coupled port, and Port 4 – isolated. Ideally, no power
should appear at the isolated port or S14 should be zero, i.e. all
power should be divided between the direct and coupled ports with
the following relation being true:
In this case, the [S] matrix of the coupler looks like:
Performance of couplers is described with the help of the following
four main parameters:
Where P1, P2, P3, and P4 are levels of power at the input, direct,
coupled, and isolated ports respectively.
The coupling factor K shows the fraction of input power that is
electromagnetically coupled to the output port; the directivity
factor D indicates the ability of the coupler to isolate between the
forward and backward waves, where one of them is the coupled
wave de-pending on the coupler type; and isolation factor I is the
measure of the coupler’s ability to deter the useful power from
traveling to the isolated port. The isolation factor is linearly
connected to the above quantities as follows:
I = K + D, dB
The ideal coupler, as described by [S]-matrix (2.6), would have
infinite isolation and directivity.
From the energy conservation law, in order for a network to be
lossless its [S] matrix should be unitary, which means that each
matrix column should be orthogonal to the conjugates of other
columns and parallel to its conjugate. It can be easily seen from
(2.5) and (2.6) that a directional coupler matched at all four ports is
ideally lossless. Complimentarily, it may be inferred from the
energy conservation condition that any lossless, reciprocal four-
port network matched at all ports is a directional coupler.
Therefore, directional couplers may be designed for the condition
of being simultaneously lossless, reciprocal, and matched at all
ports. Unlike directional couplers, three-port couplers, such as
Wilkinson coupler, have to be lossy in order to be reciprocal and
matched at all ports [13].
A special case of the directional coupler in which the signals at the
two output ports are equal (which corresponds to the 3-dB level of
coupling) and differ in phase by 90 degrees is called ―hybrid‖.
Magnetic field coupling(Inductive coupling):
Magnetic field coupling (also called inductive coupling) occurs
when energy is coupled from one circuit to another through a
magnetic field. Since currents are the sources of magnetic fields,
this is most likely to happen when the impedance of the source
circuit is low.
Consider the two circuits sharing a common return plane shown in
Fig. 1. Coupling between the circuits can occur when the magnetic
field lines from one of the circuits pass through the loop formed by
the other circuit. Schematically, this can be represented by a
mutual inductance between the two signal wires as shown
below:
Two circuits above a signal return plane.
Schematic representation of the circuits in the above fig
including inductive coupling.
In most cases, a convenient closed-form equation for calculating
the mutual inductance will not be available. However, we can often
estimate the mutual inductance by estimating the percentage of the
total magnetic flux generated by the first loop that couples the
second loop. For example, suppose the two wires in the example
above are 20 mm above the plane and separated by 5 mm. We
could visualize the magnetic flux lines that wrap the current in line
1 as shown in figure:
More intuitive schematic representation of the circuits
If the wire radius in the example above is 0.6 mm, we could
calculate the self-inductance of the source circuit using the
equation for the inductance per unit length of a wire over a
conducting plane,
The self-inductance is the total flux divided by the current while
the mutual inductance is the flux that couples both loops divided
by the current. Therefore the mutual inductance can be expressed
as a fraction of the self-inductance,
We might estimate that somewhere between 50% and 80% of the flux
couples both circuits. If we were to assume 60%, then our estimate of
the mutual inductance would be,
Of course, there are more accurate ways of determining the mutual
inductance between two circuits. Electromagnetic modeling
software is often used for this purpose when it is necessary to
determine crosstalk levels more precisely. There are also a number
of closed-form equations that can be applied to specific
(1)
(2)
(3)
geometries. In fact, for the case of two thin wires above an infinite
ground plane, there is a relatively simple closed form expression
[1],
Where h1 and h2 are the heights of the two wires above the plane,
s is the distance between the two wires and the wire radius is small
relative to the height and separation. Applying this equation to the
example above,
The difference between the estimate (3) and the calculation in (5)
is less than 2 dB. Estimates within a few dB are usually accurate
enough to indicate whether a potential crosstalk problem exists.
To calculate the crosstalk due to magnetic field coupling, we start
with the current in the source circuit, since the current is the source
of the magnetic field. The voltage induced in the second circuit can
be expressed as,
(4)
(5)
(6)
VLOOP2 is the voltage induced in the entire loop of the circuit.
The fraction of this voltage that will appear across the load can be
expressed as,
Since, I1 = VRL1/RL1, the crosstalk due to magnetic field
coupling can be expressed as,
(7)
(8)
Magnetic Resonance and Magnetic Induction -
What is the best choice for my application?
Loose coupling or tight coupling between the Tx and Rx coils?
Inductive power transfer works by creating an alternating magnetic
field (flux) in a transmitter coil and converting that flux into an
electrical current in the receiver coil. Depending on the distance
between the transmit and receive coils, only a fraction of the
magnetic flux generated by the transmitter coil penetrates the
receiver coil and contributes to the power transmission. The more
flux reaches the receiver, the better the coils are coupled.
A higher coupling factor improves the transfer efficiency, and
reduces losses and heating. Applications with a larger distance
between the transmit and receive coils operate, by definition, as a
loosely coupled system. In loosely coupled systems, only a fraction
of the transmitted flux is captured in the receiver. That means that
loosely coupled systems have higher electromagnetic emissions,
making them less suitable for applications with tight EMI or EMF
requirements.
Loosely coupled systems trade-off larger distance at the cost of
lower power transfer efficiency and higher electromagnetic
emissions. This may be suitable choice in applications where
tightly aligned coils is impractical, but less suitable for applications
with tight EMI or EMF of efficiency requirements.
Tightly coupled systems, because of their higher efficiency, tend to
produce less heat which is an advantage is products with tight
thermal budgets such as modern smartphones.
The transmit and receive coils are tightly coupled when (a) the
coils have the same size, and (b) the distance between the coils is
much less than the diameter of the coils.
Operate the coils at resonance or off-resonance?
From the beginning of inductive power transmission, resonant
circuits have been used to enhance the efficiency of power
transmission. As early as 1891, Nicola Tesla used resonance
techniques in his first experiments with inductive power
transmission. Systems with a low coupling factor generally use a
resonant receiver and resonant transmitter to improve power
transfer efficiency.
You might expect that operating tightly coupled coils at resonance
offers the best performance. That combination, however, is not
used in practice because two tightly coupled coils cannot be both
in resonance at the same time. This is one of the counter-intuitive
effects that make power electronics such an interesting subject.
Most Qi transmitters use tight coupling between coils. In that
configuration, the best results are achieved by operating the
transmitter at a frequency that is slightly different from the
resonance frequency of the Qi receiver. Off-resonance operation
gets you the highest amount of power at the best efficiency.
Single coil or multi-coil?
Tightly coupled coils are sensitive to misalignment. That’s why
most Qi transmitters use multiple coils. This increases the
complexity of the transmitter design, but improves the horizontal
(X, Y) freedom of positioning. Coil arrays can cover large areas.
for example, ConvenientPower’s WoW5 transmitter.
Another advantage of multi-coil systems is that they help localize
the magnetic flux, reducing EM emissions, and make it possible to
charge multiple receivers concurrently.
Here are some examples of transmitters that use overlapping coils.
Coils don't need to overlap either. Solutions with non-overlapping
coils can be easier easy to assemble.
Multi-coil transmitters can charge several receivers at the same
time, simply by powering the coils underneath the receiver.
Multi-coil transmitters also allow the wireless power ecosystem to
scale with increasing power levels that devices demand, by
powering multiple coils underneath the receiver. The first smart
phones needed 3W, todays require over 7.5W and growing
A loosely coupled system can achieve multi-device charging with
a single transmitter coil; provided it is much larger than the
receiver coils and the provided the receivers can tune themselves
independently to the frequency of the single transmitter coil.
References
1. E. C. Niehenke, R. A. Pucel, and I. J. Bahl, ―Microwave and Millimeter-Wave
Integrated Circuits,‖ IEEE Transactions on Microwave Theory and Techniques,
vol. 50, No. 3, pp. 846–857, March 2002.
2. http://en.wikipedia.org/wiki/Electromagnetism#cite_note-1
3. Anatoly Tsaliovich, Electromagnetic Shielding Handbook For Wired And
Wireless Emc Applications, Kluwer Academic
4. http://en.wikipedia.org/wiki/Noise_(signal_processing)
5. B. A. Kopp, M. Borkowski, and G. Jerinic, ―Transmit/Receive Modules,‖ IEEE
Transactions on Microwave Theory and Techniques, vol. 50, No. 3, pp. 827–834,
March 2002.
6. G. I. Haddad and R. J. Trew, ―Microwave Solid-State Active Devices,‖ IEEE
Transactions on Microwave Theory and Techniques, vol. 50, No. 3, pp. 760–779,
March 2002.
7. F. H. Raab et al, ―Power Amplifiers and Transmitters for RF and Microwave,‖
IEEE Transactions on Microwave Theory and Techniques, vol. 50, No. 3, pp.
814–826, March 2002.
8. Practical Guide to Electrical Grounding, Library Of Congress Catalog Card
Number: 99-72910, 1999 ERICO, Inc.
9. http://en.wikipedia.org/wiki/Inductive_coupling
10. C. R. Paul, Introduction to Electromagnetic Compatibility, 2nd Ed., Wiley Series
in Microwave and Optical Engineering, 2006.
11. http://en.wikipedia.org/wiki/Coupling_%28electronics%29
12. D. M. Pozar, Microwave Engineering, Addison-Wesley Publishing Company,
1990.
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