1. A
Project report on
Multi-Band Rejection EMI Shield Using
Frequency Selective Surface
Submitted by
Sourav Rakshit [Roll: 25300311094 Reg. No. : 112530110157]
Souradeep Sinha [Roll: 25300311092 Reg. No. : 112530110155]
Souradip Mukherjee [Roll: 25300311093 Reg. No. : 112530110156]
Sumanta Chakraborty [Roll: 25300311107 Reg. No. : 112530110170]
As a partial fulfillment for the award of the degree of Bachelor of
Technology in Electronics and Communication Engineering of
West Bengal University of Technology
Under supervision of
Mr. Syed Majdur Rahim
Department of Electronics and Communication Engineering
Supreme Knowledge Foundation Group of Institutions
Mankundu, Hooghly, W.B.-712139, India
(Affiliated to West Bengal University of Technology)
May, 2015
2. CERTIFICATE OF APPROVAL
This is to certify that the final year project report entitled “Multi-Band Rejection EMI Shield
Using Frequency Selective Surface”, submitted by Souradeep Sinha (25300311092),
Souradip Mukherjee (25300311093), Sourav Rakhshit (25300311094) and Sumanta
Chakraborty (25300311107) to the Department of Electronics and Communication
Engineering, Supreme Knowledge Foundation Group of Institutions, Mankundu,
Hooghly, West Bengal 712139, India, for the partial fulfillment of the B. Tech. course is
bonafide record of work carried out by his/her under my guidance and supervision.
SIGNATURE OF HEAD OF THE DEPARTMENT SIGNATURE OF THE SUPERVISOR
Mr. Soumen Khatua
Head of the Department,
Department of ECE,
Supreme Knowledge Foundation Group of
Institutions, Mankundu, Hooghly,
West Bengal 712139, India.
Mr. Syed Majdur Rahim
Teaching and Technical Assistant,
Department of ECE,
Supreme Knowledge Foundation Group of
Institutions, Mankundu, Hooghly,
West Bengal 712139, India.
3. Declaration
We, Souradeep Sinha, Souradip Mukherjee, Sourav Rakshit and Sumanta Chakraborty of Fourth Year
(8th
Semester), B.Tech in Electronics and Communication Engineering, Supreme Knowledge
Foundation Group of Institutions (SKFGI), hereby declare that the project entitled “Multi-Band
Rejection EMI Shield Using Frequency Selective Surface” has been carried out independently by
us at SKFGI Campus, Mankundu during our fourth year under the valuable guidance of Mr. Syed
Majdur Rahim (Teaching and Technical Assistant, Electronics and Communication Engineering,
SKFGI).
No part of this has been submitted for the award of degree or diploma in any way of the university or
institutions previously.
Date:
Place: Mankundu
__________________ __________________ __________________ __________________
SOURADEEP SINHA SOURADIP MUKHERJEE SOURAV RAKSHIT SUMANTA CHAKRABORTY
Department of Electronics and Communication Engineering
Supreme Knowledge Foundation Group of Institutions (SKFGI)
Mankundu
4. Acknowledgement
First of all, we express our gratitude to Prof. (Dr.) Abhijit Lahiri, Campus Director,
Supreme Knowledge Foundation Group of Institutions (SKFGI), Prof. (Dr.) T. K. Sengupta,
Chief Technical Director, Supreme Knowledge Foundation Group of Institutions (SKFGI) And
Prof. (Dr.) B. N. Biswas, Chairman (Education Division) SKFGI, for giving us the opportunity to
carry out our B.Tech final year project work at SKFGI.
There are number of people we owe deeply for this work Foremost in our mind is our
project advisor Mr. Syed Masdur Rahim (Teaching and Technical Assistant, Electronics and
Communication Engineering, SKFGI) who set an exceptional example of institution, perseverance
and inspiration.
We are personally thankful to Mr. Soumen Khatua (HOD, Electronics and
Communication Engineering, SKFGI), for giving us the opportunity to undergo our B.Tech final
year project.
6. Abstract
For almost a century, the subject of electromagnetic shielding was limited to extremely low
frequency and very low frequency. The interest originated from the necessity to protect circuits
and radio-receiving apparatus from disturbing effects of radiated fields. Electromagnetic
interference (EMI) shielding research is highly consolidated nowadays due to the emerging
mobile phone and satellite technologies. The radio devices directly or indirectly impose
potential hazards to human health and there exists a growing concern in malfunctioning of
high-speed communication systems due to radiated interference.
This has led to find out the solutions for effective isolation from the interference signals.
Although the main objective is to obtain all stop filter characteristics, the ventilation
requirements force the enclosure to allow certain band of EMI to pass.
This paper presents the design and fabrication of an ultra-thin and flexible electromagnetic
interference (EMI) shield that is capable of rejecting multiple unwanted frequencies. The
design starts with the idea of unit cell model that leaded us to determine the initial geometrical
dimensions of the rings efficiently. Then it followed by full-wave electromagnetic simulation
to fine-tune the final dimensions for the desired frequency response. Impacts of various
geometrical designs on the EMI shielding performance of the concentric ring design are
analyzed and discussed. With these results, an ultra-thin and flexible EMI shield is fabricated
using the screen printing technique. Good correlation between measurement and simulation is
demonstrated in this paper.
9. i
List of Figures
Figure No. Title Page No.
1.1 Cross-section through a coaxial cable showing shielding . . . . . . . . . . . . . . . . .2
And other layers
1.2 Military electronic equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Medical electronic device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Cables with the electromagnetic shielding to Separate . . . . . . . . . . . . . . . . . . . 6
the wires from outside environments
2.2 Sheet metal used to make electromagnetic shielding . . . . . . . . . . . . . . . . . . . . 7
2.3 Microwave oven having electromagnetic shielding . . . . . . . . . . . . . . . . . . . . . 7
2.4 Current through a receiver circuit on a PC card . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 Field penetrating metallic shield barrier causing current . . . . . . . . . . . . . . . . . 9
to get attenuated
2.6 Applying AC voltage source into a pair of parallel plates . . . . . . . . . . . . . . . . 9
2.7 Lines of flux with respect to E and H field . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
2.8 Placing a shielding barrier in the path of the EM field . . . . . . . . . . . . . . . . . . 11
2.9 Field penetrating barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.10 The flow of current through a shield including a gasket interface . . . . . . . . . .12
2.11 The current flowing across a gasketed maintenance cover . . . . . . . . . . . . . . . .14
3.1 FSS types and response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
3.2 Radomes at the Cryptologic Operations Center, Misawa, Japan . . . . . . . . . . . 16
3.3 Two-dimensional periodic array of patch elements . . . . . . . . . . . . . . . . . . . . . 17
3.4 Periodic structures comprising of complimentary elements, . . . . . . . . . . . . . 20
patches and slots (wire-grid), and their surface impedance
3.5 A variety of FSS elements over past decades . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
3.6 A periodic array of infinitely long metallic strips (top) . . . . . . . . . . . . . . . . . . 24
And an infinite array of closely spaced dipoles (bottom)
3.7 The first and the second resonant modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.8 Typical FSS Elements classified in four major groups. . . . . . . . . . . . . . . . 26
based on their shapes
10. ii
3.9 Reflection coefficient curve for closely packed FSS of. . . . . . . . . . . . . . . . . .27
three-legged loaded elements
3.10 Same as in Fig. 3.9 but frequency range 0 to 40 GHz. . . . . . . . . . . . . . . . . . . . 28
3.11 Same FSS as in Fig. 3.9 but normal angle of incidence and. . . . . . . . . . . .29
frequency range 0 to 40 GHz
3.12 Current distribution on a three-legged loaded element. . . . . . . . . . . . . . . . . . 30
at the (a) first even resonance at about 11 GHz; (b) second
even resonance from about 30 to 40 GHz; (c) first odd
resonance at about 25 GHz
3.13 By reducing the transmission line spacing we obtain a. . . . . . . . . . . . . . . . .31
significant reduction in bandwidth compared to Fig. 3.8
3.14 Reflection coefficient curves for an FSS of closely packed. . . . . . . . . . . .32
hexagon elements
3.15 Application of FSS as radome covers in the aircraft technology. . . . . . . . . . . 33
for reducing the antenna RCS
3.16 Dual-frequency reflector antenna using an FSS as the sub reflector. . . . . . . .34
3.17 An active L-C array comprising metallic strips interrupted. . . . . . . . . . . . 35
by gaps in a periodic fashion.
3.18 Modes of operation for an active L-C array (a) Transmission mode. . . . . . 36
for adjusting the wave intensity (b) Reflection mode for changing
the phase
4.1 (a) Top surface: cross dipole element (b) Bottom . . . . . . . . . . . . . . . . . . . . . 37
surface: ring patch Element (c) Perspective view: both
4.2 Measurement setup showing the FSS prototype, two ridge. . . . . . . . . . . . . . . .38
horn antennas and network analyzer connected via 50-Ω SMA
cables
4.3 Comparison of the numerically computed transmission through . . . . . . . . . . .38
the FSS, with only the cross dipoles, with only the rings and with
both cross dipoles and rings
4.4 An element of the FSS coated on a flat glass . . . . . . . . . . . . . . . . . . . . . . . .39
4.5 Measurement data of reflection (S11) for the square flat Glass . . . . . . . . . . . .40
coated with and without the proposed FSS
4.6 Unit cell geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
11. iii
4.7 (A): TE and TM mode characteristics (B): Comparison of the. . . . . . . . . . . . 41
transmission characteristics
4.8 Attenuation peak. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
5.1 Geometry of a unit cell and periodic array of single-ring and multiple. . . . . 43
concentric rings
5.2 Geometry of a unit cell of multiple concentric rings. . . . . . . . . . . . . . . . . . . . .45
5.3 Equivalent circuit and two-port ABCD network representations. . . . . . . . . . . .45
of single ring
5.4 3D Model of a unit cell of multiple concentric rings. . . . . . . . . . . . . . . . . . . . .46
5.5 Parameters and geometrical dimensions concentric ring. . . . . . . . . . . . . . . 47
5.6 Experimental and simulation result of tri-band rejection EMI shield. . . . . . . .48
5.7 First stop band measured at -3 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.8 First stop band measured at -10 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.9 Second stop band measured at -3 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 49
5.10 Second stop band measured at -10 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.11 Third stop band measured at -3 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.12 Third stop band measured at -10 dB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.13 Full-wave simulated and estimated transmission coefficients. . . . . . . . . . . . . .52
for different radii
13. Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
1
Introduction 1__
1.1 Overview
With the exponential growth of wireless communications, the number of base stations is
expected to increase for better service coverage. As a result, there is a strong likelihood of
electromagnetic interference (EMI) from these wireless communications to other sensitive
electronic devices. EMI mitigation, such as shielding, has been gaining attention. To protect
sensitive equipment from potential EMI threat, there is an increasing demand for architectural
shielding. The conventional shielded enclosures are heavy and add structural loadings to
existing buildings hence, light weight EMI shields that are ultra-thin, highly flexible and can
be applied to existing walls of a building will be an attractive solution. Besides the weight
issue, conventional metallic enclosures have no frequency selective shielding feature and
practically block out electromagnetic waves of all frequencies. For EMI shield that only block
out several undesirable frequencies, frequency selective surface (FSS) design maybe applied
to offer such capability. The FSS design also allows void areas to be implemented on the shield,
leading to another desirable property, the optical transparency. It is to be noted that in order to
achieve multi-band frequency selective surfaces, cascaded FSSs are usually used. The cascaded
nature of the design leads to relatively thick shield. In order to implement the design onto a
single layer, loop design is a good choice. By taking advantage of printed periodic elements to
provide the frequency selective feature, the EMI shield can reject specific unwanted
frequencies without affecting other wireless services. Here describes the design procedure of
an ultra-thin and flexible multiple-band rejection EMI shield. Screen-printing technique is
adopted for the fabrication of the EMI shield because of its roll-by-roll mass production
capability. A prototype based on screen-printing is fabricated and its multi-band rejection
capability is demonstrated experimentally.
1.2 Brief History of EMI Regulation
Since the advent of radio communications, EMI’s negative effects have been observed from
both intentional and unintentional sources. The International Electrotechnical Commission
(IEC) met in 1933 in order to recommend that the International Special Committee on Radio
Interference (CISPR) be created in order to help deal with the growing EMI problem. The
committee then created technical documentation that produced the first measurement and
testing techniques to be used in industry along with emission limitations. These regulations
have since evolved into the basic electromagnetic transmission regulations that are in place
14. Introduction [2]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
today. In the United States, the FCC put legal limitations on electromagnetic emissions
throughout the country. Today, most developed countries have some level of EMI regulation
in place to help ensure a higher level of performance across all industries.
1.3 Standards
The International Special Committee for Radio Interference or CISPR (French acronym for
"Comité International Spécial des Perturbations Radioélectriques"), which is a committee of
the International Electrotechnical Commission (IEC) sets international standards for radiated
and conducted electromagnetic interference. These are civilian standards for domestic,
commercial, Industrial and Automotive sectors. These standards form the basis of other
regional and national standards most notably the European Norms (EN) written by CENELEC
(European committee for electrotechnical standardization).
1.4 Example Applications of EMI Shielding
One example is a shielded cable, which has
electromagnetic shielding in the form of a wire mesh
surrounding an inner core conductor. The shielding
impedes the escape of any signal from the core
conductor, and also prevents signals from being added
to the core conductor. Some cables have two
separate coaxial screens, one connected at both ends,
the other at one end only, to maximize shielding of both
electromagnetic and electrostatic fields.
The door of a microwave oven has a screen built into the window. From the perspective
of microwaves (with wavelengths of 12 cm) this screen finishes a Faraday cage formed
by the oven's metal housing. Visible light, with wavelengths ranging between 400 nm
and 700 nm, passes easily through the screen holes.
RF shielding is also used to prevent access to data stored on RFID chips embedded in
various devices, such as biometric passports.
NATO specifies electromagnetic shielding for computers and keyboards to prevent
passive monitoring of keyboard emissions that would allow passwords to be captured;
consumer keyboards do not offer this protection primarily because of the prohibitive
cost.
Figure 1.1: Cross-section through a
coaxial cable showing shielding
and other layers
15. Introduction [3]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
RF shielding is also used to protect medical and
laboratory equipment to provide protection against
interfering signals, including AM, FM, TV, emergency
services, dispatch, pagers, ESMR, cellular, and PCS. It
can also be used to protect the equipment at the AM,
FM or TV broadcast facilities.
Shielding medical electronic devices is significant design
issue due to the pervasiveness of electronic equipment in
the hospital. Autocatalytic selective plating and
conductive paint are applied onto many medical
electronic device enclosures to provide EMI shielding.
1.5 Materials used for EMI Shielding
Typical materials used for electromagnetic shielding include sheet metal, metal screen,
and metal foam. Any holes in the shield or mesh must be significantly smaller than
the wavelength of the radiation that is being kept out, or the enclosure will not effectively
approximate an unbroken conducting surface.
Another commonly used shielding method, especially with electronic goods housed in plastic
enclosures, is to coat the inside of the enclosure with a metallic ink or similar material. The ink
consists of a carrier material loaded with a suitable metal, typically copper or nickel, in the
form of very small particulates. It is sprayed on to the enclosure and, once dry, produces a
continuous conductive layer of metal, which can be electrically connected to the chassis ground
of the equipment, thus providing effective shielding.
RF shielding enclosures filter a range of frequencies for specific conditions. Copper is used for
radio frequency (RF) shielding because it absorbs radio and magnetic waves. Properly designed
and constructed copper RF shielding enclosures satisfy most RF shielding needs, from
computer and electrical switching rooms to hospital CAT-scan and MRI facilities.
Figure 1.2: Military Electronic
Equipment
Figure 1.3: Medical Electronic
Device
17. 4
Electromagnetic Interference (EMI) 2__
And EMI Shielding
Electromagnetic interference (EMI, also called radio-frequency interference or RFI
when in radio frequency) is disturbance that affects an electrical circuit due to either
electromagnetic induction or electromagnetic radiation emitted from an external source. The
disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of
the circuit. These effects can range from a simple degradation of data to a total loss of data.
The source may be any object, artificial or natural, that carries rapidly changing electrical
currents, such as an electrical circuit, the Sun or the Northern Lights.
Electromagnetic Interference (EMI) most commonly occurs in the 104 to 1012 Hertz frequency
range of the electromagnetic spectrum. A number of sources create this interference, including
radio transmitters, electric motors, power lines, fluorescent lights, and computer circuits. If
electrical equipment do not have suitable EMI shielding in place, device failure may result
from the interference due to the number of sensitive electronic components in most electronic
equipment produced today. Although there are many national regulations that restrict products
emissions today, taking into account EMI shielding for organically and non-organically created
EMI is still a fundamental part of the electronic design process.
Electromagnetic interference negatively impacts an electrical circuit due to direct interference
from RF transmissions or electromagnetic induction. This interference may degrade, interrupt,
obstruct, or otherwise limit the electronic circuit’s performance. Interference may occur
naturally or other electronic equipment may generate it. EMI can be generated purposely in
order to jam radios or radars as a form of electronic warfare.
2.1 Types of Electromagnetic Interference
EMI - Electromagnetic Interference can arise in many ways and from a number of sources. The
different types of EMI can be categorized in a number of ways.
2.1.1 Categorizing the type of EMI by the way it was created:
Man-made EMI: This type of EMI generally arises from other electronics circuits,
although some EMI can arise from switching of large currents, etc.
Naturally occurring EMI: This type of EMI can arise from many sources - cosmic
noise as well as lightning and other atmospheric types of noise all contribute.
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2.1.2 Categorizing the type of EMI is by its duration:
Continuous interference: This type of EMI generally arises from a source such as a
circuit that is emitting a continuous signal. However background noise, which is
continuous may be created in a number of ways, either manmade or naturally occurring.
Impulse noise: Again, this type of EMI may be man-made or naturally occurring.
Lightning, ESD, and switching systems all contribute to impulse noise which is a form
of EMI.
2.1.3 Categorizing the different types of EMI by their bandwidth:
Narrowband: Typically this form of EMI is likely to be a single carrier source -
possibly generated by an oscillator of some form. Another form of narrowband EMI is
the spurious signals caused by intermodulation and other forms of distortion in a
transmitter such as a mobile phone of Wi-Fi router. These spurious signals will appear
at different points in the spectrum and may cause interference to another user of the
radio spectrum. As such these spurious signals must be kept within tight limits.
Broadband: There are many forms of broadband noise which can be experienced. It
can arise from a great variety of sources. Man-made broadband interference can arise
from sources such as arc welders where a spark is continuously generated. Naturally
occurring broadband noise can be experienced from the Sun - it can cause sun-outs for
satellite television systems when the Sun appears behind the satellite and noise can
mask the wanted satellite signal. Fortunately these episodes only last for a few minutes.
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Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
2.2 Electromagnetic Interference (EMI) Shielding
2.2.1 What is EMI Shielding?
Electromagnetic shielding is designed to limit the influence of electromagnetic fields and
radiation on a device or object. The process uses a barrier made from conductive material
containing electric charges of either positive or negative properties at the subatomic particle
level. Usually, this material is used to separate the electrical components on the inside of the
device from the outside world. Cables also utilize the concept to separate wires from outside
environments. When used to block radio frequencies, it is known as RF shielding.
The exact purpose of this shielding is to protect devices from the coupling effect, the transfer
of one form of energy to a device that uses a different form. This is commonly caused by radio
waves, electrostatic fields, and the full spectrum of electromagnetic radiation. The full level of
protection is based on the amount of reduction to the electric and magnetic fields. This depends
on the size, shape and orientation of the shielding. No matter the standards in place, however,
shielding cannot protect against low-frequency magnetic fields.
2.2.2 How Does Electromagnetic Shielding Work?
Electromagnetic shielding provides “immunity” for electronic components that are susceptible
to EMI and prevents the same components from transmitting excessive interference to their
surrounding environment. The main method for doing this now entails circuit grounding and
design and placement of the critical components in the device architecture. EMI shielding
compounds and Faraday cages are also used in a housing technique to further shield a device’s
transmissions and protect against outside interference.
2.2.3 Need of EMI Shielding
Today’s electrical and electronic devices are
subject to mandatory EMC requirements
throughout the world. Many devices operate
at high frequencies and are very small. They
are placed in nonconductive plastic cases
providing no shielding. Essentially, all these
devices cannot meet these mandatory
requirements or they may cause interference
to other devices or receive interference
causing susceptibility problems without a
proper program of EMI control. This program consists of identifying the “suspect” components
Figure 2.1: Cables with the electromagnetic shielding to
separate the wires from outside environments
20. Electromagnetic Interference (EMI) and EMI Shielding [7]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
and circuits that may cause or be susceptible to EMI. This is completed early on in the program
to allow for an efficient design in keeping the cost of dealing with EMI as low as possible. A
complete EMC program consists of proper filtering, grounding and shielding.
2.2.4 Materials Used for Shielding
A variety of materials can be used as electromagnetic
shielding to protect an electrical device. Examples
include ionized gas in the form of plasma, metal foam
with gas-filled pores, or simply sheet metal. In order
for holes within the shielding to be present, they must
be considerably smaller than any wavelength from
the electromagnetic field. If the shielding contains
any openings larger than the wavelength, it cannot
effectively prevent the device from becoming
compromised.
Household devices often use a different shielding
method due to the likelihood of exposure to
electromagnetic fields. Plastic enclosures usually
use some sort of metallic ink consisting of copper or
nickel in a small particular state. This material can
be sprayed onto the enclosure, producing a
conductive layer of metal that acts as protection.
The main reason this layer works is due to its close
proximity to the grounding of the device.
Many common day-to-day items contain electromagnetic shielding. One of the most common
examples of this is the microwave oven found within most kitchens. With the metal housing
working in unison with the screen on the window, a Faraday cage is created. While some visible
light is able to pass through the window screen, waves of other frequencies cannot.
Figure 2.2: Sheet metal used to make
electromagnetic shielding
Figure 2.3: Microwave oven having
electromagnetic shielding.
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Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
2.3 Shielding Theory
There are two ways of approaching the theory of shielding. These are by the use of circuit
theory and by the use of field theory. The EMC industry uses a field theory approach to
shielding theory using abstract mathematical modeling techniques to yield a value of merit
classified as "shielding effectiveness". Shielding effectiveness is then used as a measurement
to gauge the attenuation of an EM field through shielding barrier material.
The problem with the use of shielding effectiveness is that there can be a significant differential
between the attenuation of the electric E fields, magnetic H fields, and power, where the
difference can exceed 100 dB. The actual difference will vary as a function of variables
associated with specific applications where the literature on the shielding of radiated EM fields
does not address these conditions. The result is a significant confusion factor in the selection
of shielding barrier material, facing design engineers who are required to meet EMC radiated
emission and susceptibility requirements.
The circuit theory approach (included herein) employs mathematical modeling techniques
consistent with college course work and yields a predicted field strength at any given distance
from the shielding barrier material. The results can also be used to predict the shielding of a
seam or gasketed joint in the barrier material (or enclosure). The circuit theory approach given
below examines the field as it penetrates a barrier and yields a value of the field as it exits the
barrier.
2.3.1 EM Field Generation and Shielding
A radiated electromagnetic (EM) force field is generated by the action of driving a current
through a wire. An example is shown in Figure 2.4.
The wire (or PC card trace) acts as a transmitting antenna
as an emitter of EM interference and as a receptor with
regard to EM susceptibility. A common method of
reducing (or eliminating) the possibility of the PC trace
being an emitter or receptor is by the use of a shielding
barrier.
When an EM force field is impinged on a metallic
(conductive) shielding barrier, currents are caused to
flow in the barrier. As the field penetrates the barrier, the
current is attenuated (i.e., reduced in amplitude as illustrated in Figure 2.5) by a force called
skin effect.
The power of the field as it leaves the barrier is approximately equal to the current squared
times the impedance of the barrier, and is in watts per meter squared.
Figure 2.4: Current through a receiver
circuit on a PC card.
22. Electromagnetic Interference (EMI) and EMI Shielding [9]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
As we learned above, currents flow
in the shielding barrier as a function
of the radiated field being impinged
on the barrier. When the current
crosses a seam in the barrier (created
by maintenance covers, etc.), a
voltage is created across the seam,
where the value of the voltage is
equal to the current times the
impedance of the seam. The seam
then becomes a radiating antenna
where the impedance and pattern is
similar to that of a slot antenna. EMI gaskets are used to reduce the impedance of the seam and
subsequent power radiating from the seam.
2.3.2 Generation and Propagation of EM Fields
The undergraduate courses on EM theory introduce the concept of an EM field by driving a
pair of parallel plates with an AC voltage source as illustrated in Figure 26. The current that
flows through the wire comes from the top plate and
is stored in the bottom plate. The over-presence of the
electrons on the bottom plate is illustrated by + and
the absence of electrons on the top plate and is
illustrated by +. This creates an electromagnetic field
which is illustrated in Figure 4. The field consisting
of the straight lines is classified as a displacement
field and is in amperes per meter squared. The
magnitude of the E field is equal to the voltage
differential between the plates divided by the distance
between the plates in meters. The resultant E field is
in volts/meter
As is illustrated in Figure 2.7, the lines of flux in
the center of the plates are straight and flow from
the bottom to the top plate. At the edges they bow
out, where the fields or lines of flux repel each
other, forcing the bowing. The field that bows out
represents a radiated EM field. The radiated EM
field emanating from the trace of Figure 1 is similar
to the radiated EM field illustrated in Figure 4. The
electric "E" field is tangent to the lines of force as
illustrated in Figure 2.7. The magnetic "H" field is a
Figure 2.5: Field penetrating metallic shield barrier causing
current to get attenuated.
Figure 2.6: Applying AC voltage source into a
pair of parallel plates
Figure 2.7: Lines of flux with respect to E and H
field.
23. Electromagnetic Interference (EMI) and EMI Shielding [10]
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field perpendicular to the lines of force and points out of the paper.
The set of plates as illustrated in Figure 2.7 produce a field similar to that of the PC card trace
of Figure 2.4 (and of an electric dipole antenna). If the transmitted power is known, the field
strength can be calculated using the dipole antenna equation, i.e.,
PR ≈ 1.6Pt / 4𝜋𝑅2
Where, PR = Field strength at distance R (𝜔/𝑚2
)
Pt = Transmitted power (𝜔/2𝑚2
)
R = Distance from radiating source (m)
And power equation (pointing vector):
𝐸 x H = PR
E / H = 377λ/ 2𝜋R R <λ/2𝜋 (ohms)
= 377 R ≥λ/2𝜋
And, λ = 3x108
/f (m)
If the power is not known, the value of the electric field can be approximated using the
following equation:
E ≈ 𝑒/𝜋𝑅
E = Electric field strength (v/m)
e = Voltage across plates
H ≈ 2𝜋𝑅𝐸/377𝜆 R <λ/ 2𝜋(A / m)
= E / 377 R ≥ 𝜆/ 2𝜋
24. Electromagnetic Interference (EMI) and EMI Shielding [11]
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2.4 Introduction to Shielding
2.4.1 Suppression (Shielding) of EM Fields
When we place a shielding barrier in the path of the EM field, the force of the field causes
current to flow in the barrier. As is illustrated in Figure 2.8, the excess electrons in the bottom
plate create a force on the electrons to flow away from the point of contact. In a similar manner,
the lack of electrons on the upper plate will create an excess of electrons on the barrier at the
upper point of contact. This current flow in the barrier is called the “surface current density”
(Js) in amperes/meter, and is approximately equal to twice the H field incident on the barrier
when the field is perpendicular to the barrier. The current flowing in the barrier is attenuated
by the skin effect.
The current on the transmitted side is equal to Jsi 𝑒−𝑑/𝛿
(i.e, the current on the incident side
attenuated by skin effect). The impedance of the field emanating from the barrier is equal to
the impedance of the barrier. The values of Et and Ht are as illustrated in Figure5 and are as
follows.
Figure 2.8: Placing a shielding barrier in the path
of the EM field.
25. Electromagnetic Interference (EMI) and EMI Shielding [12]
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2.4.2 Field Strength Through Shield
From antenna theory we know that the
power from an antenna is reduced as the
square of the distance from its source.
Shielding theory purposes that the field as
it passes through a barrier is attenuated but
not changed with regard to direction. As
such, the loss of power is a function of the
distance from the original source of the
field as illustrated in Figure 2.9.
2.4.3 Shielding and Gasketing
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.
If the gasket is made of a material identical to the walls
of the shielded enclosure, the current distribution in
the gasket will also be the same assuming it could
perfectly fill the slot. (This is not possible due to
mechanical considerations.) The flow of current
through a shield including a gasket interface is
illustrated in Figure 2.10. Electromagnetic leakage
through the seam can occur in two ways. First, the
energy can leak through the material directly. The
gasket material shown in Figure 2.10 is assumed to have
lower conductivity than the material in the shield. The
rate of current decay, therefore, is also less in the gasket.
It is apparent that more current will appear on the far side of the shield.
Figure 2.9: Field penetrating barrier
Figure 2.10: The flow of current through a
shield including a gasket interface
26. Electromagnetic Interference (EMI) and EMI Shielding [13]
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This increased flow causes a larger leakage field to appear on the far side of the shield. Second,
leakage can occur at the interface between the gasket and the shield.
If an air gap exists in the seam, the flow of current will be diverted to those points or areas
which are in contact. A change in the direction of the flow of current alters the current
distribution in the shield as well as in the gasket. A high resistance joint does not behave much
differently than open seams. It simply alters the distribution of current somewhat. A current
distribution for a typical seam is shown in Figure 210. Lines of constant current flow spaced at
larger intervals indicate less flow of current. It is important in gasket design to make the
electrical properties of the gasket as similar to the shield as possible, maintain a high degree of
electrical conductivity at the interface, and avoid air, or high resistance gaps.
RF Gaskets
A gasket is a mechanical seal which fills the space between two or more mating surfaces,
generally to prevent leakage from or into the joined objects while under compression.
Although there are hundreds of gasket varieties based upon geometry and materials, there are
four principle categories of shielding gaskets: beryllium copper and other metal spring fingers,
knitted wire mesh, conductive particle filled elastomers and conductive fabric-over-foam. Each
of these materials has distinct advantages and disadvantages, depending upon the application.
Regardless of the gasket type, the important factors to be considered when choosing a gasket
are RF impedance (R + jX, where R = resistance, jX = inductive reactance), shielding
effectiveness, material compatibility corrosion control, compression forces, compressibility,
compression range, compression set, and environmental sealing.
Below is a comprehensive list of selection factors.
27. Electromagnetic Interference (EMI) and EMI Shielding [14]
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Gasketed joint shielding
When a radiated EM force field is impinged on a metallic shielding barrier, a current (surface
current density in amperes per meter) is generated in the material. When the current flows
across a gasketed maintenance cover as illustrated in Figure 2.11, a voltage e is generated
across the gasket. The value of e is equal to the current in amperes/meter times the impedance
of the joint (transfer impedance in ohm-meters).
The EM force field illustrated in Figure 2.11 is generated by the voltage across the gap and has
the characteristics of a low impedance slot antenna.
The radiated field can be estimated from the example of Figure 2.11 as follows:
ET ≈ 2e / R = 2JS ZT /R (V/ m)
HT ≈ ET λ / 2 R (377) R< λ / 2 (A/ m)
HT = ET / 377 R > λ/2
Figure 2.11: The current flowing across a gasketed
maintenance cover
JS = Current due to Field Striking Barrier
e = Voltage across Gasket = JS ZT
ZT = Transfer Impedance of Gasketed Joint
(ohm-m)
29. Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
15
Introduction to 3__
Frequency Selective Surface (FSS)
FSS is a periodic structure of conductive elements or apertures in either one or two dimensions
that provide a filter operation when they are illuminated with EM wave. When illuminated by
an electromagnetic wave, FSS exhibits total transmission/reflection around the resonance
frequency. This spatial filter behavior of FSS is used in designing the FSS shield. The filter
behavior (low-pass, high-pass, band-pass and band-stop) of the FSS depends on the shape of
the element.
Figure 3.1: FSS types and response, (a) solid patch array — low pass, (b) slot array — high pass,
(c) patch looped array — band stop and (d) slot looped array — band pass.
A frequency-selective surface (FSS) is any thin, repetitive surface (such as the screen on a
microwave oven) designed to reflect, transmit or absorb electromagnetic fields based on
frequency. In this sense, an FSS is a type of optical filter or metal-mesh optical filters in which
the filtering is accomplished by virtue of the regular, periodic (usually metallic, but sometimes
dielectric) pattern on the surface of the FSS. Frequency-selective surfaces have been most
commonly used in the radio frequency region of the electromagnetic spectrum and find use in
applications as diverse as the aforementioned microwave oven, antenna radomes and
modern metamaterials. Sometimes frequency selective surfaces are referred to simply as
periodic surfaces and are a 2-dimensional analog of the new periodic volumes known as
photonic crystals.
30. Introduction to Frequency Selective Surface (FSS) [16]
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3.1 Background
Filters play a fundamental role in almost electronic or RF circuit. Once being incorporated into
a design, the filter acts as a device that controls the frequency content of the signal for
mitigating noise and unwanted interference. Filters are categorized, based on their function,
into three major groups: lowpass, bandpass, and highpass filter. A lowpass filter, for example,
allows for the lower frequencies to pass through the circuitry and blocks higher frequencies.
Frequency-selective surface (or dichroic) structures to space waves are the counterparts of filter
in transmission lines.
Once exposed to the electromagnetic radiation, a frequency- selective surface (FSS) acts like
a spatial filter; some frequency bands are transmitted and some are reflected. In a way, an FSS
can be a cover for hiding communication facilities.
This is probably the first potential application of FSS structures, as they have actually been
used as covers named radomes. Radomes are bandpass FSS filter that are used to reduce the
radar cross-section (RCS) of an antenna system outside its frequency band of operation.
Fig. 3.1 shows the radomes at the Cryptologic Operation Center in Japan.
Since the early 1960's, because of potential military applications, FSS structures have been the
subject of intensive study. Marconi and Franklin, however, are believed, to be the early
pioneers in this area for their contribution of a parabolic reflector made using half-wavelength
wire sections in 1919. FSSs as frequency-selective materials have been used traditionally in
stealth technology for reducing the RCS of communications systems.
The concept of stealth or being able to operate without the knowledge of the enemy has always
been a goal of military technology. In order to minimize the detection, FSS layers cover the
facilities to reduce the RCS.
Figure 3.2: Radomes at the Cryptologic Operations Center, Misawa, Japan
(photo courtesy of en. Wikipedia)
31. Introduction to Frequency Selective Surface (FSS) [17]
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FSSs most commonly take the form of planar, periodic metal-dielectric arrays in two-
dimensional space. Frequency behavior of an FSS is entirely determined by the geometry of
the surface in one period (unit cell) provided that the surface size is infinite. A periodic array
of patch elements is shown in Fig. 3.2. This array is shown to have a capacitive frequency
characteristic.
Although taking different shapes, conventional FSSs have similar operation mechanisms that
can be explained by the phenomenon of resonance. Consider an array of elements on a planar
surface. Upon contact with a plane-wave, the elements of the periodic surface resonate at
frequencies where the effective length of the elements is a multiple of the resonance length.
Corresponding to the phase front of the wave, these elements have a certain phase delay. As a
result, the scattered radiations of individual elements add up coherently. An example of such
arrangement of elements is Marconi and Franklin's reflector. This reflector is very much similar
to the most famous FSS design, an array of half-wave dipoles. A large reflector antenna
constructed using wire-grids is shown in Fig. 3.2.
The resonance characteristics of a resonance-length based FSS usually depend on the way the
surface is exposed to the electromagnetic wave. This includes the effective aperture size of the
FSS and the incidence angle of the wave. The dependence of the FSS frequency response with
respect to these factors could be a major drawback for some applications. As a result, over the
years new ideas have been sought to overcome the issue of the dependence on the size and
angle. Besides the two major problems mentioned above, harmonics are another effect that
influence the performance of an FSS. The situation becomes more involved given that the
harmonics of the intended frequency are themselves dependent upon the incidence angle.
3.2 History
Historically, the first approach to solving for fields reflected and transmitted by FSS was the
spectral domain method (SDM), and it's still a valuable tool even today. The spectral domain
method is known at Ohio State University as the periodic method of moments (PMM). The
SDM starts out with an assumed Floquet/Fourier series solution for all fields, currents and
potentials whereas the PMM starts out with a single scatterer, then adds in all of the scatterers
Figure 3.3: Two-dimensional periodic array of patch elements
32. Introduction to Frequency Selective Surface (FSS) [18]
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in the infinite plane (in the spatial domain), then uses a transformation to yield the spectral
domain representation of the fields. Both approaches are effectively the same approach, in the
sense that they both assume an infinite planar structure which gives rise to a discrete Fourier
series representation for the fields.
3.3 Advantages and disadvantages
The spectral domain method has one very important advantage over other – strictly numerical
- solutions to Maxwell's equations for FSS. And that is that it yields a matrix equation of very
small dimensionality, so it is amenable to solution on virtually any type of computer. The
dimension of the matrix is determined by the number of current basis functions on each
individual scatterer and can be as small as 1×1 for a dipole at or below resonance. The matrix
elements however take longer to compute than with volumetric approaches such as FEM.
Volumetric approaches require that a volume surrounding the unit cell be gridded accurately,
and can require many thousands of elements for an accurate solution, though the matrices are
usually sparse.
3.4 Floquet's principle
The spectral domain method is based on Floquet's principle, which says that if an infinite,
planar, periodic structure is illuminated by an infinite plane wave, then every unit cell in the
periodic plane will contain exactly the same currents and fields, except for a phase shift,
corresponding to the incident field phase. This principle allows all currents, fields and
potentials to be written in terms of a modified Fourier series, which consists of an ordinary
Fourier series multiplied by the incident field phase. If the periodic plane occupies the x-y
plane, then the Fourier series is a 2-dimensional Fourier series in x, y.
3.5 Field equations for 2D PEC frequency selective surfaces
Perfectly electrically conducting (PEC) periodic surfaces are not only the most common but
also the easiest to understand mathematically, as they admit only electric current sources J.
This section presents the spectral domain method for analyzing a free-standing (no substrate)
PEC FSS. The electric field E is related to the vector magnetic potential A via the well-known
relation:
------------------ (3.1)
33. Introduction to Frequency Selective Surface (FSS) [19]
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And the vector magnetic potential is in turn related to the source currents via:
Where,
3.6 Plane wave expansion of the fields in source-free media
Frequency-selective surfaces are frequently stratified in the direction normal to the plane of the
surface. That is, all dielectrics are stratified and all metallic conductors are considered stratified
as well, and they will be regarded as perfectly planar. As a result, we are excluding metallic
vias (wires perpendicular to the plane of the FSS) which could potentially connect currents
from different strata of the FSS structure. With this type of a stratified structure in mind, we
can then use a plane wave expansion for the fields in and around the FSS, since plane waves
are the eigenfunction solution to the vector wave equations in source-free media.
To solve equations (3.1) and (3.2) for a free-standing, doubly-periodic surface, we consider an
infinite 2D periodic surface occupying the entire x-y plane, and assume a discrete plane wave
expansion for all currents, fields and potentials .
where for mathematical simplicity, we assume a rectangular lattice in which α only depends
on m and β only depends on n. In the equations above,
and,
where lx, ly are the dimensions of the unit cell in the x,y directions respectively, λ is the free
space wavelength and θ0, φ0 are the directions of an assumed incident plane wave, with the
FSS regarded as lying in the x-y plane. In (3.5c), the root is taken which has a positive real part
and non-positive imaginary part).
------------------ (3.2)
------------------ (3.3)
------------------ (3.4a)
------------------ (3.4c)
------------------ (3.4b)
------------------ (3.5a)
------------------ (3.5c)
------------------ (3.5b)
------------------ (3.6)
34. Introduction to Frequency Selective Surface (FSS) [20]
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3.7 Elements of Design in Traditional FSS
As mentioned previously, FSS structures are periodic arrays of special elements printed on a
substrate. For numerical analysis, such arrays are assumed to be infinite in dimension as FSSs
usually consist of many elements. The infinite array approximation reduces the whole problem
of analysis to calculating the frequency response of a single element in the array given the
periodic nature of the FSSs. A brief overview of the available FSS elements is provided in this
section.
3.7.1 Element Geometries
In general, the FSS structures can be categorized into two major groups: patch-type elements
and aperture-type elements. As an introduction to FSS structures two complementary planar
arrays, array of patches and array of slots, are usually considered. A simple structure consisting
of periodic array of metallic patches (Fig. 3.3) has been shown to have a low-pass characteristic.
Once hit by a plane-wave, this surface transmits low-frequency content of the wave and reflects
the higher frequencies. Another observation is to consider such an array as a capacitive surface
given its frequency response provided in Fig. 3.3. The complementary structure (see Fig. 3.3)
has an inductive response, hence acting as a highpass filter.
Figure 3.4: Periodic structures comprising of complimentary elements, patches and slots (wire-
grid), and their surface impedance- The patch-array produces a capacitive response, whereas
the array of slots is inductive
35. Introduction to Frequency Selective Surface (FSS) [21]
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As it will be discussed later, inductive and capacitive surfaces can be put together to produce a
desired filter response. Over the years, a variety of FSS elements were introduced for bandpass
and band-stop applications.
Depending on the application requirements, different FSS designs can be chosen to fulfill the
demands. These requirements usually include level of dependence on the incidence angle of
the incoming wave; level of cross-polarization; bandwidth; and level of band separation.
A comparison between some of the most famous FSS designs is provided in Table. 3.1 based
on the above criteria. As shown, for instance, the dipole array is very sensitive to the angle.
3.7.2 Element Dimensions
As mentioned above, FSSs are traditionally designed based on the resonant elements. A planar
array of strip dipoles, for example, produces a frequency response consisting of multiple
notches at frequencies where the length of the dipoles is a multiple of half a wavelength.
A similar effect can explain the operation of other elements. The square loop element, for
example, can be imagined as two dipoles that are connected to one another at each end.
Using the same argument as that of the dipole, a loop resonates when the length of the two
sides equals the length of a resonant dipole, λ/2. In other words, each side of the loop is about
λ/4. Although the shape of the elements has the utmost importance effect in the frequency
response, the way these elements are arranged in the array format is also part of the design
work. Moreover, the response also depends on the characteristics of the substrate used. This in
Figure 3.5: A variety of FSS elements over past decades
36. Introduction to Frequency Selective Surface (FSS) [22]
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fact becomes very important as we will present a square loop element whose sides are as small
as ¸ λ /12. This is where the miniaturized-element FSS design comes to picture.
Element Angle
independence
Cross-
polarization
level
Larger
bandwidth
Smaller band
separation
Loaded dipole 1 2 1 1
Jerusalem cross 2 3 2 2
Rings 1 2 1 1
Tripole 3 3 3 2
Crossed dipole 3 3 3 3
Square loop 1 1 1 1
Dipole 4 1 4 1
Table 3.1: Comparison Between the Performance of the Common FSS Elements
This is where the miniaturized-element FSS design comes to picture. Elements that are much
smaller than the wavelength are designed to create capacitive gaps and inductive traces. By
thinning and miniaturizing the unit cell, capacitive junctions in the form of shunt or series
capacitors are achieved. Inductive traces are also held very close to one another to produce a
larger inductive effect as a result of mutual magnetic coupling.
37. Introduction to Frequency Selective Surface (FSS) [23]
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3.8 Traditional Frequency-Selective Surfaces: Design,
Characterization, and Applications
Periodic structures, or arrangements of equally spaced, identical elements, have been of interest
in many areas of physics and engineering. Although being very old from mathematical stand
view, periodic structures have also a long history of development from more practical
perspectives. In fact, these arrangements were considered in an engineering design problem
over 200 years ago by the physicist David Rittenhouse. He invented a grating with equally-
spaced hairs. A periodic structure can be optimized for a particular application which requires
certain characteristics. This process includes designing the elements as well as the way they
are placed in the periodic array.
3.8.1 Brief Design Concept
A frequency-selective surface (FSS) is a periodic, planar assembly of generally metallic
elements on a dielectric layer. It is built in conjunction with the electromagnetic waves in order
to tailor" an electromagnetic link in the free-space environment. Acting as a barrier for the
waves propagating along the link, the FSS controls the flow of the electromagnetic energy. The
transfer function of the FSS manipulates the spectral content of the wave. As a result, some of
the frequency constituents of the wave are blocked, and some pass through the
FSS fence. In another perspective, an FSS is analogous to a filter in circuit theory. For their
filtering effects, FSS structures are also called spatial filters in electromagnetic engineering.
There are many examples in real world which clearly confirm the importance of having
knowledge about periodic structures. Consider a reflective surface which is to be built by least
amount of metallization at a given frequency. Intuitively, the first observation is that the less
the metal is, the weaker the reflection becomes. Now, consider two possibilities: using an array
of long strips or an array of short dipoles. Having a larger metalized area, the long strip is
expected to produce a stronger reflection, according to our very first observation.
Rigorous analysis of the two candidates, however, reveals that the dipole array can actually
produce a total reflectivity at a certain frequency, whereas the array of strips never becomes
totally reflective. This problem, which was studied by Marconi and Franklin when they
proposed their reflector antenna, can be explained through modeling the two arrays using
circuit theory. This is shown in Fig. 3.5 where the model for the array of long strips is just an
inductor, while the array of dipoles is an L-C circuit. The dipole array, therefore, becomes total
reflective at the frequency of resonance of the L-C circuit.
38. Introduction to Frequency Selective Surface (FSS) [24]
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Following the brief discussions highlighted in Chapter 1, this chapter overviews some of the
past, famous FSS structures,
including the physical interpretation
of their operation.
The overview section is followed by
a discussion on the methods of
characterization and modeling of
FSS. Finally, later in this chapter, a
more detailed overview on the
applications of FSS is presented.
3.8.2 Overview of the Elements of Traditional FSS
The operation mechanism of traditional FSSs, as mentioned previously, is based on the
resonant elements. Simply, the idea is that a plane-wave illuminates an array of metallic
elements, thus exciting electric current on the elements. The amplitude of the generated current
depends on the strength of the coupling of energy between the wave and the elements.
The coupling reaches its highest level at the fundamental frequency where the length of
elements is a λ/2. As a result, the elements are shaped so that they are resonant near the
frequency of operation. Depending on its distribution, the current itself acts as an
electromagnetic source, thus producing a scattered field. The scattered field added to the
incident field constitutes the total field in the space surrounding the FSS. By controlling the
scattered field (designing elements), therefore, the required filter response is produced which
can be seen in the spectrum of the total field. As mentioned above, the distribution of the current
on the elements determines the frequency behavior of the FSS. The current itself depends on
the shape of the elements.
Given the dependence of the traditional FSS on the length, excitation of the higher-order
modes, in addition to the first fundamental mode, becomes inevitable. As a result, the frequency
response of the traditional FSS usually has a high harmonic content. The first and the second
possible modes are shown in Fig. 3.6. The issue of harmonics not only affects the frequency
characteristics of the FSS but also degrades its scan performance because some of the
Figure 3.6: A periodic array of infinitely long metallic strips (top)
produces an inductive characteristic, whereas an infinite array of closely-
spaced dipoles (bottom) produces a notch frequency behavior which in
turn results in a total reflectivity at the resonance frequency of the notch.
In this way, although the dipole array uses less metallic area, it produces
a complete reflective state
39. Introduction to Frequency Selective Surface (FSS) [25]
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harmonics may be excited only when the incidence angle changes from normal to the FSS
plane.
Although the geometry of the elements has a critical influence on the filtering behavior, there
are other parameters that can affect the frequency response. This could be the choice of the
parameters of the substrate supporting the elements of the FSS and also its inter- element
spacing. The substrate is shown to affect both the frequency of operation and the bandwidth of
the response. The spacing between the elements, on the other hand, points out the issue of the
grating lobe which is inherent in any array radiator; the larger the spacing, the earlier the onset
of the grating lobes. As a result, a smaller inter-element spacing is usually preferred. However,
the spacing can also change the bandwidth; a larger spacing in general produces a narrower
bandwidth.
To develop a better understanding of operation of FSS structures, a comparative study over
different types of FSS is provided here. But, first, we need to somehow categorize FSS
structures. There are a number of constraints based upon which FSSs are classified. Here, we
use Munk's approach in classifying frequency-selective surfaces which is based on the shape
of the elements.
In Fig. 3.7, four major categories of FSS arrangements are demonstrated which are:
The center connected or N-poles, such as dipole, three-legged element, the Jerusalem
cross, and the square spiral.
The loop types such as the three and four-legged loaded elements, the circular loops,
and the square and hexagonal loops.
Solid interiors or plate types.
Combinations.
Figure 3.7: The first and the second resonant modes-(top) shows the fundamental mode which
is excited for any element shape irrespective as the incidence angle. (Bottom) shows the first
odd mode at about 2ff which may be excited only at oblique incidence. The frequency of this
mode may change slightly depending on the element shape
40. Introduction to Frequency Selective Surface (FSS) [26]
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Each class along with simulated examples are presented next. All the simulation results belong
to the book by Munk on FSS, and use a substrate with dielectric constant of ϵr= 2.2 with the
thickness of 0.5 mm. The examples were intended for operation at 10 GHz.
The simulated results show the performance for both TE and TM polarizations at 0° and
45°.
Figure 3.8: Typical FSS Elements classified in four major groups based on their shapes
Our design constructions includes group II shape structures, and here is the detail description
about those shapes.
Group I: The center connected or N-poles
Group II: The loop types
Group III: Solid interiors or plate types
Group IV: Combinations
41. Introduction to Frequency Selective Surface (FSS) [27]
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3.8.3 Group II- loop type structures
I. Three-legged Loaded Element:
A surface of three-legged loaded elements is shown in Figs. 3.8 to 3.10. It was developed as a
direct consequence of the four-legged loaded element, although it is not as easy to explain.
However, the fact remains that both of these elements resonate when their circumferences are
approximately one full wavelength, and they show a load null at the frequency where the legs
are approximately h/4 long.
The bandwidth variation with polarization is suited quite well for use in a hybrid radome (better
than the three-legged unloaded element). It is considerably more broadbanded than the
Cm
Cm
Frequency (GHz)
Reflection(dB)
Dx=0 .3457 cm
Dz=0 .3792cm
Monopole Length = 0.2784 cm
Monopole Width = 12 cm
Wire Width = 0.0171 cm
Surface profile-
Ɛr= 2.2 D=20mil
Figure 3.9: Reflection coefficient curve for closely packed FSS of three-legged loaded
elements. Angle of incidence equals 45", orthogonal and parallel polarization. Note the load
null at about 19 GHz for both polarizations.
Ɛr= 2.2 D=20mil
42. Introduction to Frequency Selective Surface (FSS) [28]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Four-legged cases, the primary reason being that the inter-element spacings Dx,Dz are
considerably smaller; in other words, an increase in Dx,Dz could reduce the bandwidth
considerably. We finally show in Fig. 3.9 the reflection curves of the same three-legged loaded
element in the larger frequency range 0 to 40 GHz. Here the plane of incidence is along the
vertical leg, α = 90°. It is worth mentioning that we have also run the reflection curves in the
horizontal plane (α = 0°).
However, since these two curves are practically indistinguishable, we do not show the latter
case.
Reflection(dB)
Frequency (GHz)
Cm
Cm
Dx=0 .3457 cm
Dz=0 .3792cm
Monopole Length = 0.2784 cm
Monopole Width = 12 cm
Wire Width = 0.0171 cm
Surface profile-
Ɛr= 2.2 D=20mil
Ɛr= 2.2 D=20mil
Figure 3.10: Same as in Fig. 3.7 but frequency range 0 to 40 GHz.
43. Introduction to Frequency Selective Surface (FSS) [29]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Reflection(dB)
Frequency (GHz)
Cm
Cm
Dx=0 .3457 cm
Dz=0 .3792cm
Monopole Length = 0.2784 cm
Monopole Width = 12 cm
Wire Width = 0.0171 cm
Surface profile-
Ɛr= 2.2 D=20mil
Ɛr= 2.2 D=20mil
Figure 3.11: Same FSS as in Fig. 3.7 but normal angle of incidence
and frequency range 0 to 40 GHz.
Angles of incidence and parallel polarization. Inspection of Fig. 3.8 shows this to happen at 25
GHz, while the second even resonance is now being pushed up to 40 GHz (as usual with a null
between). Finally note that grating lobes are not excited before around 40 GHz for 45"
incidence angles. If this feature is important, the three-legged loaded element is hard to beat (a
competitor would be a Gangbuster of higher order and an interlaced four-legged loaded
element).
44. Introduction to Frequency Selective Surface (FSS) [30]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
II. Hexagon Element
The physical appearance of the hexagon array is shown in Fig. 3.11 for close inter-element
spacing and in Fig. 3.12 for wider spacing, where we also show the reflection curves at 45°
angles of incidence for orthogonal and parallel polarization, respectively. Note that the
frequency range in both cases is 0 to 40 GHz. not just 0 to 20 GHz. This is done to fully show
the superior behavior of the hexagon array, namely the location of the first modal
Interaction null at about 29 GHz; this is about twice as high as anything seen earlier with the
exception of the square spiral element. Also the difference in inter-element spacing is seen to
lead to a rather significant change of bandwidth and also a difference in element circumference:
1.5 cm compared to 1.98 cm.
Note how the voltage distribution for the fundamental mode leads to a strong voltage difference
across the “capacitor,” that is, the fundamental frequency will be greatly pulled downward.
However, inspection of the voltage distribution at the second harmonic shows no voltage
difference across the capacitor, that is, no downward pulling of that frequency. This explains
First Even Resonance ~ 11 GHz
(a)
Second Even Resonance ~ 30-40 GHz
(b)
First Odd Resonance ~ 25 GHz
(c)
Figure 3.12: Current distribution on a three-legged loaded element at the (a) first even
resonance at about 11 GHz; (b) second even resonance from about 30 to 40 GHz; (c) first
odd resonance at about 25 GHz.
45. Introduction to Frequency Selective Surface (FSS) [31]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
how the second resonance of the hexagon element may be approximately three times the
fundamental.
It should be noted that other loop elements act similar, but none are superior to the hexagon.
For example, a circular loop has the second resonance about twice the fundamental, unless the
elements are almost touching each other. One may finally wonder: Why is it that the end
Reflection(dB)
Frequency (GHz)
Cm
Cm
Dx=0 .3457 cm
Dz=0 .3792cm
Monopole Length = 0.3225 cm
Monopole Width = 0.06 cm
Wire Width = 0.0171 cm
Surface profile-
Ɛr= 2.2 D=20mil
Ɛr= 2.2 D=20mil
Figure 3.13: By reducing the transmission line spacing we obtain a significant reduction
in bandwidth compared to Fig. 3.8. With the same Dx and Dz, we preserve the same onset
frequency of the grating lobes.
46. Introduction to Frequency Selective Surface (FSS) [32]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
capacity of the anchor element does not similarly produce a large difference in frequency
pulling of the fundamental and the second harmonic as we just saw for the hexagon element
(see Figs. 3.8 and 3.9) The answer to this question is obtained by inspection of the current and
voltage modes for the anchor element. Note that a significant voltage difference between the
element tips and the center of the adjacent elements are present for both the first and second
harmonics; that is, they are both pulled significantly downward.
Frequency (GHz)
Reflection(dB)
Cm
Cm
Dx=0 .4171 cm
Dz=0 .4751cm
Gap= .025cm
Hexagon perimeter= 1.5cm
Line Width = 0.0171 cm
Surface profile-
Ɛr= 2.2 D=20mil
Ɛr= 2.2 D=20mil
Figure 3.14: Reflection coefficient curves for an FSS of closely packed hexagon elements.
Angle of incidence equals 45", orthogonal and parallel polarization. Note that the location
of first null is unusually high (around 28 GHz).
47. Introduction to Frequency Selective Surface (FSS) [33]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
3.8.4 Most Significant Traditional Applications of FSS
Common electromagnetic design problems that can benefit from frequency-selective surfaces
is presented here. As briefly discussed in Chapter 1, conventional applications include radomes
(bandpass spatial filters) and multi-frequency reflector antennas. Because of their frequency-
dependent behavior, FSSs have also been looked at as reactive surfaces and been used in a
variety of beam forming applications as well as high-performance antenna designs.
In this section, some of the current applications are presented.
I. Radomes:
Frequency-selective surfaces have been employed primarily in design of special covers called
radomes that are capable of reducing the radar cross-section (RCS) of antennas under cover. In
this application, FSS is transparent to a desired frequency band and reflecting at other
frequencies.
In practice, bandpass, spatial covers of this type are used in certain applications, for example,
in the case of an aircraft. A typical assembly is shown in Fig. 3.13. As shown, the radar
(antenna) mounted at the front tip of the airplane is covered by a shaped radome. Radome has
two modes of operation: In the transparent mode the signal passes through the radome and is
collected by the antenna. In the reflecting mode, however, the radome behaves like a metallic
surface which reflects the signal in the specular direction. The advantage of this method is that
the radome can be shaped particularly in order to reach the lowest levels of RCS. This can be
done by designing the shape of the radome so that the direction of the strong reflection
(specular) in reflecting mode is out of the sight of the transmitter of the signal, i.e. it is not
directed toward the incoming signal.
Figure 3.15: Application of frequency-selective surfaces as radome
covers in the aircraft technology for reducing the antenna RCS.
48. Introduction to Frequency Selective Surface (FSS) [34]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
II. Multi-Frequency Reflectors
As mentioned earlier, frequency-selective surfaces have been utilized in multiband reflector
antenna applications. A common approach uses an FSS as a subreflector in addition to the large
reflecting dish in a reflector antenna, as shown in Fig. 3.14. The subreflector, which is an FSS,
is designed to be reflective at a frequency band and be simultaneously transparent at another
desired band. The frequency-dependent subreflector allows for application of multiple feeds in
this systems, thus enabling the multi- frequency state of the antenna. Different frequency feeds
are optimized and positioned at the real and virtual foci of the subreflector. Hence, only one
main reflector can act as a multiband antenna.
A practical application of this idea is the high-gain antenna (HGA) of the Voyager space- craft
which was designed, to diplex S and X bands. An FSS forms the subreflector which is reflecting
at X-band whereas is transmitting at S-band. In this antenna, the S-band feed is placed at the
prime focus of the reflector, whereas the X-band feed is located at the Cassegrain focal point
(see Fig. 3.14). As a result, loaded with an FSS, a single reflector acts as a dual-band antenna,
thus reducing the overall mass, volume, and most importantly the fabrication cost. A similar
approach was proposed later by Wu to build a four-band reflector system. In this work, a four-
frequency FSS was utilized as the subreflector.
III. Beam Control Arrays
Frequency-selective surfaces have also been used in creation of impedance surfaces with tuning
capability. This problem is believed, that first appeared in where scattering properties of a
corrugated surface loaded with microwave solid-state amplifiers were studied. In this surface,
connecting each slot region to an amplifier, the authors have been able to not only control the
phase of the reflected wave but also to manipulate its power gain. It was envisioned that this
loaded array would be able to change the phase front (shape) and intensity of an incident plane-
wave.
Figure 3.16: Dual-frequency reflector antenna using an FSS
as the subreflector.
49. Introduction to Frequency Selective Surface (FSS) [35]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
FSSs considered for the beam-shaping application are commonly of the type series resonators,
L-C. A possible way for constructing an L-C resonating surface is to use an array of metallic
strips that are cut at periodic intervals in order to create capacitive junctions along the strips.
This array, which is simply an array of small strips, is shown in
Fig. 3.15. The basic idea can be explained using a quasi-optical approach in which the beam
is assumed to be a plane-wave over an infinitely long, periodic array in two dimensions on the
surface.
The metallic traces are inductive which together with the gap capacitances create a series
combination of inductors with capacitors. By mounting lumped reactive elements such as
varactors diodes, one can tune the resonance characteristics of the array. It should be pointed
out that this is only an approximate method since if the elements of the array are long, the wave
variations along the elements must be taken into account. Nevertheless, this approach builds
up a good understanding on the behavior of active arrays. Two modes of operation can be
specified: 1) Transmission mode (shown in Fig. 3.16(a)) for controlling the beam amplitude,
and 2) reflection mode (shown in Fig. 3.16(b)) for changing the beam phase. As shown in Fig.
3.16(b), in the reflection mode, the array is backed by a metallic surface to assure the total
reflection.
Figure 3.17: An active L-C array comprising metallic strips interrupted by gaps in
a periodic fashion. The gaps further are loaded with varactor diodes to alter the
gap capacitance.
50. Introduction to Frequency Selective Surface (FSS) [36]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
The first experimental version of L-C active arrays in reflection mode was demonstrated
previously. This reflective array was designed to work at 93 GHz and was able to produce a
70° phase shift with about 6.5 dB loss. This research was further expanded and lead to
demonstration of more involved active arrays such as multiplier arrays, oscillator arrays, and
amplifier arrays. Transmission mode operation has also been tested and demonstrated.
3.8.5 Chapter Conclusions
A numerical, comparative study over the conventional elements in design of frequency-
selective surfaces is presented in this chapter. Traditional surfaces are categorized based on
their geometries, into four major groups: I) the center connected or N-poles, such as dipole,
three-legged element, the Jerusalem cross, and the square spiral. II) The loop types such as the
three- and four-legged loaded elements, the circular loops, and the square and hexagonal loops.
III) Solid interiors or plate types. IV) Combinations which are a mixer of previous groups.
Through a physical discussion, it is shown that the operation mechanism of the traditional FSS
is dependent on the structural-based resonances. The overview study of this section is followed
by introducing some of the conventional applications involving frequency-selective surfaces,
demonstrating the versatility of these surfaces in microwave engineering.
(a) (b)
Figure 3.18: Modes of operation for an active L-C array. (a) Transmission mode
for adjusting the wave intensity. (b) Reflection mode for changing the phase
51. CHAPTER – 4
Some Significant Works on
Frequency Selective Surface (FSS)
52. Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
37
Some Significant Works on 4__
Frequency Selective Surface (FSS)
Frequency Selective Surface (FSS) has find lot of applications in Electromagnetic Shielding
and it has become the base for many works on Electromagnetic Shielding. Some of that works
are as follows:
4.1 A Single-Layer Frequency Selective Surface for Ultra-Wideband Electromagnetic
Shielding:-
An efficient approach to achieve the shielding effectiveness (SE) by using a frequency-
selective surface (FSS) is presented. This FSS, which consists of cross dipoles and rings printed
on the opposite sides of a single-layer FR-4 substrate, exhibits a wide, 7.5-GHz stop band to
provide simultaneous shielding in both X- and Ka-bands. Experimental results confirm SE of
the prototype over an ultra-wide band with more than 20-dB measured attenuation. The design
is compact and suitable to provide shielding against the radiation interference caused by
license-free and other radio systems.
Figure 4.1: (a) Top surface: cross dipole element. (b) Bottom surface:
ring patch Element. (c) Perspective view: both (all dimensions are in
mm).
53. Some Significant Works on Frequency Selective Surface [38]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
This paper presents a single-layer FSS that exhibits an attenuation of more than 20 dB with an
ultra-wide 7.5-GHz stop band from 6.5 to 14 GHz range.
Figure 4.2: Measurement setup showing the FSS prototype, two ridge horn
antennas and network analyzer connected via 50-Ω SMA cables.
Figure 4.3: Comparison of the numerically computed transmission through
the FSS, with only the cross dipoles, with only the rings and with both cross
dipoles and rings.
54. Some Significant Works on Frequency Selective Surface [39]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
The FSS uses cross dipoles and rings printed on the opposite surfaces of a thin dielectric
substrate. This design, which provides a wide 7.5-GHz stopband from 6.5 to 14 GHz, has been
successfully tested to provide effective shielding with a minimum attenuation from around 20–
35dB. Due to symmetry, the designed FSS is stable to both TE and TM linear waves
polarizations as well as all other polarizations of normally incident waves and its oblique
incidence performance is satisfactory for smaller angles of incidence. The FSS can be easily
tuned to a desirable stopband by properly choosing the dimensions of resonators and other key
parameters. A complete parametric study is presented to demonstrate the possible control of
stopband.
4.2 An EMI Shielding FSS for Ku-Band Applications:-
A FSS consisting of cross-slot elements coated on a flat glass was used to study its impact on
transmission and reflection at Ku-band frequencies of 12.2~12.7 GHz. Based on the simulation
experience of the first partial study, the FSS element with geometrical parameters W=1.0 mm,
L=7.2 mm, P =9 mm, and T=5 mm was proposed for signals to transmit the flat glass coated
with the FSS at Ku-band frequencies of 12.2~12.7 GHz and to be blocked by the flat glass
coated with the FSS outside the Ku-band frequencies. From simulation results and
measurement data, it is shown that the resonant frequency of the flat glass coated with the
proposed FSS is found to be around 12.5 GHz which is in the Ku-band (12.2~12.7 GHz) for
the reflection with better than -10 dB. It is also shown that high transmission in the useful
frequency band (12.2~12.7 GHz) can be achieved by using the proposed FSS coated on the flat
glass.
Figure 4.4: An element of the FSS coated on a flat
glass
55. Some Significant Works on Frequency Selective Surface [40]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Dimensions. Parameters shown in the figure are W=0.4~1.6 mm L=6~7.4 mm, P=9~12 mm,
and T=2~7 mm.
Based on simulation studies, a cross-slot FSS element with geometrical parameters W=1.0 mm,
L=7.2 mm, P =9 mm, and T=5 mm was proposed for signals to transmit the flat glass coated
with the FSS at Ku-band frequencies of 12.2~12.7 GHz and to be blocked by the flat glass
coated with the FSS outside the Ku-band frequencies. From simulation results and
measurement data, it is demonstrated that the flat glass coated with the FSS having geometrical
parameters W=1.0 mm, L=7.2 mm, P =9 mm, and T=5 mm can successfully be used to finish
the above purpose.
4.3 A novel shield for GSM 1800MHz band using frequency selective surface
This paper describes a novel FSS which functions as band stop filter to shield the GSM
1800MHz downlink band. The FSS is designed to operate with the resonant frequency of
1820MHz which is the center frequency for the GSM 1800MHz downlink band. The novelty
is attributed to its unique geometry and the circular apertures endowed with it. The proposed
geometry provides shielding effectiveness of 20 dB alongside with 133MHz bandwidth.
The structure holds identical response for both TE and TM Modes of polarization. In addition,
the geometry with its circular apertures, a hitherto unexplored feasibility serves the purpose of
ventilation and heat dissipation. The simulated results are validated using experimental
measurements.
Figure 4.5: Measurement data of reflection (S11) for the square flat Glass
coated with and without the proposed FSS.
56. Some Significant Works on Frequency Selective Surface [41]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
4.3.1 Proposed FSS geometry:
The unit cell contour of the proposed FSS is portrayed in Fig. 4.6. The structure is obtained by
rotating the alphabet V one to the other by 90± to get a cross like design (CLD) and is printed
on either side of the dielectric substrate. The circular slots penetrating deep into the substrate
in each of its notches cater to the ventilation needs.
Figure 4.6: Unit cell geometry.
Figure 4.7 (A): TE and TM mode characteristics
Figure 4.7 (B): Comparison of the transmission
characteristics
57. Some Significant Works on Frequency Selective Surface [42]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
4.4 Electromagnetic Shielding Glass of Frequency Selective Surface:-
A Frequency Selective Surface (FSS) consists of a cluster of thin wires that act as dipole
antennas. The cross-section of the reflecting aperture is much wider than that of the wire dipole
antenna. Since an FSS provides a similar response to a dipole antenna, its reflection
corresponds with the dipole resonant frequency. We have developed FSS Glass, which has star
elements on its surface. For 0.5mm diameter silver wire, the typical peak attenuation of FSS
operating at 1.9GHz is 35dB or more. Since FSS Glass has a wide reflecting aperture, the
window glass provides a reflector panel, like a metal plate. There are noncontact gaps between
the frame and the FSS element, but the Electro-magnetic wave cannot penetrate the window
glass. The FSS Glass enables the PHS resources to be utilized efficiently.
4.4.1 Outline of FSS Glass
FSS Glass is an Electro-magnetic wave reflector, and it has a cluster of thin antennas printed
on it the two antennas are short circuited (resistance Rt=O), making half wavelength dipole
antenna. This antenna thus makes an area called a “Reflecting aperture”, which corresponds to
the dipole resonant frequency. The reflecting aperture is calculated as hX 1/2 h. When these
apertures are arranged correctly on the window glass, they form a frequency selective shield.
As shown in Fg-5 the FSS Glass has a particular attenuation peak at 1.9GHz. It attenuates 35dB
or more, and the FSS Glass has a 35MHz period of band around l.9GHz, where it attenuates
more than 30dB. The result shows that the FSS Glass has satisfactory qualities as a PHS shield
Figure 4.8: Attenuation Peak
Frequency (GHz)
Attenuation(dB)
59. Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
43
Design and Simulation of 5__
Multi-Band EMI Rejection Shield
5.1 Introduction
In this ultra-thin and multi-band EMI shield, the design of the FSS structure is made circular
loop type. To achieve multi-band rejection capability with a single layer structure, a ring
structure is proposed, as multiple resonances can be created using concentric rings of different
radii. The simulation is done on a unit cell and result is obtained based on that cell.
5.2 Design of Multiple Band Stop EMI Shield
We have inspired from a journal of Electromagnetic analysis and applications and try to work
on that type of structural arrangements mentioned in that journal.
The objective of the screen is to act as a spatial band stop filter. Hence, the appropriate
configuration would be a patch design. This configuration allows the screen to act as a band-
stop filter at specific intended resonant frequency. To meet the other objectives of providing
optical transmission, concentric rings would be the most appropriate design. The use of hollow
rings rather than patches makes it almost ideal for application where high degree of optical
transparency is needed.
Figure 5.1: Geometry of a unit cell and periodic array of single-ring and multiple concentric rings.
60. Design and Simulation of Multi-Band EMI Rejection Shield [44]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
As mentioned earlier, the ring structure is selected as it allows multiple rings to be placed on
the same layer. Figure 5.1 shows the geometry of a unit.
The fundamental resonance for a concentric ring can be estimated with the following equation:
λ = c/f = 2𝜋𝑟
where, λ is the resonant wavelength
c is the speed of light in vacuum,
f is the resonant frequency and
r is the radius of the ring.
For a single ring, its inductance is given by:
L = 39.37 (
𝑟2
8𝑟+11𝑤
) k
Where, K = 0.57-0.145 ln(𝜔/ℎ)
k is a correction factor to account for the effect of the ground plane
h is the substrate thickness and
w is the width of the ring.
With the known inductance and the resonant frequency of the ring structure, its equivalent
capacitance can be obtained by:
C = (1/𝐿) × (1/2𝜋𝑓)2
With the calculated equivalent inductance and capacitance of the ring, the ABCD matrix
representation of a ring is shown in Figure 5.2
The reflection and transmission coefficients can be determined as follows:
𝑆11 =
(𝐴+𝐵−𝐶−𝐷)
(𝐴+𝐵+𝐶+𝐷)
𝑆21 =
2(𝐴𝐷−𝐵𝐶)
(𝐴+𝐵+𝐶+𝐷)
61. Design and Simulation of Multi-Band EMI Rejection Shield [45]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Figure 5.2: Geometry of a unit cell of multiple concentric rings
Figure 5.3: Equivalent circuit and two-port ABCD network representations of single ring
62. Design and Simulation of Multi-Band EMI Rejection Shield [46]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Where A = D = 1, for a shunt network, B = 0 and C is the normalized admittance Y. For a
series network, B is the normalized impedance and C = 0. The values of 𝑆11 and 𝑆12 allow
efficient computation of the transmission and reflection coefficients.
5.3 Parameters and Geometrical Dimensions for Triple Band Stop EMI Shield
Here, Teflon is used as the dielectric substrate. The thickness of the substrate is 0.1 mm. around
the substrate two air boxes are created. The height of those air boxes is 20 mm.
Now, on the substrate the FSS structure is created. The ring structures are made of PEC
material.
There are three concentric rings on the substrate. The radius of the innermost circle is 19.5 mm.
and the thickness is 3 mm. The radius of the second circle is 25.5 mm which has a width of
25.5 mm. And the outermost circle’s radius is 50 mm which has a thickness of 3 mm.
Figure 5.4: 3D Model of a unit cell of multiple concentric rings
63. Design and Simulation of Multi-Band EMI Rejection Shield [47]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
5.4 Setup of Full Wave Model
Once the initial geometrical dimensions of the ring structure is obtained from the equivalent
circuit model, 3D full wave simulation of the structure can be carried out using a commercial
3D EM solver software with the necessary boundary conditions being considered.
To perform the simulation for a unit cell using finite difference time domain (FDTD), special
boundary conditions have to be applied. A unit cell is constructed with two wave-guide ports,
placed in the -z and +z direction, being used as the excitation and receiving source. For
simulating periodic boundary conditions in time domain, a perfect electric wall is placed in the
-x and +x direction while a perfect magnetic wall is placed in the -y and +y direction.
Figure 5.5: Parameters and geometrical dimensions concentric ring
64. Design and Simulation of Multi-Band EMI Rejection Shield [48]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
5.5 Result and Analysis
5.5.1 Analysis of Full Wave Model
Figure 5.6: Experimental and simulation result of tri-band rejection EMI shield
Here reflection coefficient S(1,1) and transmission coefficient S(1,2) are taken in the range of
0 GHz – 3.50 GHz. There are three bands shielded.
Figure 5.7: First stop band measured at -3 dB
65. Design and Simulation of Multi-Band EMI Rejection Shield [49]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Stop Band Frequency Range
(−3 𝑑𝐵)
Bandwidth
(−3 𝑑𝐵)
Centre Frequency
1nd stop band 0.52 GHz – 1.32 GHz 0.8080 GHz 0.917 GHz
Stop Band Frequency Range
(−10 𝑑𝐵)
Bandwidth
(−10 𝑑𝐵)
Centre Frequency
1st
Stop Band 0.76 GHz – 1.07 GHz 0.3185 GHz 0.917 GHz
Figure 5.8: First stop band measured at -10 dB
Figure 5.9: Second stop band measured at -3 dB
66. Design and Simulation of Multi-Band EMI Rejection Shield [50]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Stop Band Frequency Range
(−3 𝑑𝐵)
Bandwidth
(−3 𝑑𝐵)
Centre Frequency
2nd
Stop Band 1.74 GHz – 2.04 GHz 0.3026 GHz 1.84 GHz
Stop Band Frequency Range
(−10 𝑑𝐵)
Bandwidth
(−10 𝑑𝐵)
Centre Frequency
2nd
stop band 1.79 GHz – 1.90 GHz 0.1079 GHz 1.84 GHz
Figure 5.11: Third stop band measured at -3 dB
Figure 5.10: Second stop band measured at -10 dB
67. Design and Simulation of Multi-Band EMI Rejection Shield [51]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
Stop Band Frequency Range
(−3 𝑑𝐵)
Bandwidth
(−3 𝑑𝐵)
Centre Frequency
3rd
Stop band 2.36 GHz – 2.93
GHz
0.5717 GHz 2.47 GHz
Stop Band Frequency Range
(−10 𝑑𝐵)
Bandwidth
(−10 𝑑𝐵)
Centre Frequency
3rd
Stop band 2.42 GHz – 2.56 GHz 0.1425 GHz 2.47 GHz
A triple band-stop EMI shield is designed using the results obtained from the earlier study.
Based on the previous study, it was observed that in order to achieve a shield with resonant
frequencies at 917 MHz, 1.846 GHz, and 2.47 GHz, the radii of the rings are approximately
19.5 mm, 25.5 mm, and 50 mm, respectively. Using the initial findings, further fine-tuning is
carried out to achieve the desired dimensions for all resonances to be met on a single design.
Figure 5.12: Third stop band measured at -10 dB
68. Design and Simulation of Multi-Band EMI Rejection Shield [52]
Multi-Band Rejection EMI Shield Using FSS Dept. of ECE, SKFGI
5.5.2 Single Ring with Different Radii
Figure 5.13 shows the full-wave simulated and estimated transmission coefficients for different
radii, respectively. It can be seen that the results estimated based on the equivalent circuit model
(ECM) resemble well with those simulated using full-wave simulation tool except very slight
shift in resonant frequency. However, the ECM approach is highly efficient to synthesize the
design quickly without heavy computational effort. This allows parametric analysis to be
carried out efficiently as compared to full-wave simulation.
Figure 5.13: full-wave simulated and estimated transmission coefficients for different radii
![A
Project report on
Multi-Band Rejection EMI Shield Using
Frequency Selective Surface
Submitted by
Sourav Rakshit [Roll: 25300311094 Reg. No. : 112530110157]
Souradeep Sinha [Roll: 25300311092 Reg. No. : 112530110155]
Souradip Mukherjee [Roll: 25300311093 Reg. No. : 112530110156]
Sumanta Chakraborty [Roll: 25300311107 Reg. No. : 112530110170]
As a partial fulfillment for the award of the degree of Bachelor of
Technology in Electronics and Communication Engineering of
West Bengal University of Technology
Under supervision of
Mr. Syed Majdur Rahim
Department of Electronics and Communication Engineering
Supreme Knowledge Foundation Group of Institutions
Mankundu, Hooghly, W.B.-712139, India
(Affiliated to West Bengal University of Technology)
May, 2015](https://image.slidesharecdn.com/a436f895-a36f-4881-ad53-50d599ba168d-150715151139-lva1-app6892/85/Multi-Band-Rejection-EMI-Shielding-1-320.jpg)
