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Electromagnetic Radiation

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Introduction Radiation : The process by which unstable nuclei of atom emit a quantity of energy, or a particle. This emitted particle or energy is known as radiation.

Classification of radiations:1. Particle radiation (tiny and fast moving material

having both energy and weight)2. Electro magnetic radiation (pure energy )

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Electromegnetic radiationElectromagnetic radiation (EM radiation or EMR) is a form of energy emitted by charged particles, which exhibits wave-like behavior as it travels through space. EMR has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation.

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Classification of E.M.R. :Non ionizing radiation : visible light, magnetic and radio waves .Ionizing radiation : infra red radiation and ultraviolet radiation.EMR in order to increasing freq. of waves :Radio waves < micro waves < IR < visible light< UV radiation< gamma rays

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Properties of EMR1. Electromagnetic waves are propagated by

oscillating electric and magnetic fields oscillating at right angles to each other.

2. Electromagnetic waves travel with a constant velocity of  3 x 108 ms-1 in vacuum.

3. Electromagnetic waves are not deflected by electric or magnetic field

4. Electromagnetic waves can show interference or diffraction.

5. Electromagnetic waves are transverse waves.6. Electromagnetic waves may be polarized.

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Sources of EMR

Natural sources Man made sources

cosmic exposure at public places

terrestrial occupational exposure

internal

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Major sources are : Broadcast antennas Cell towersDigital TV signals Doppler weather radar Electric trains Over telephone wires Microwave beacons Pagers Power lines Radio transmittersSatellite radiation Wi-Fi antennasWiring in airplanes Digital displayCell phone chargers Cell phonesCompact fluorescent light (CFL) Computers Cordless phones

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Impacts of EMRElectrical hazards : Strong radiation can induced current capable of delivering an electric shock to persons and animals. It can also overload and destroy electrical equipment.

Fire hazards : EMR can cause electric currents strong enough to create sparks and can ignite flammable material and gases , leading to an explosion.

Biological hazards : EM field can cause dielectric heating ex. Touching and antenna can cause severe burns.

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Health hazards of EMR :EMR can cause cancer , high blood pressure , brain tumors , headache, leukemia, brain damage etc.

WHO facts says that EMR can cause depression , cardiovascular disorder, reproductive function, developmental disorders, immunological modification etc. but these have lesser chances than for child hood leukemia.

Electromagnetic interference : it is an unwanted disturbance that effect the electrical circuit due to EMR emitted from an external source .

It May interrupt , obstruct or limit the effective performance of the circuit.

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Parameters affecting the impacts of EMREnergy of the incident radiation Depth of penetration Source of emissionDuration of exposureFrequency of EMR waves Type of EMR wavesDistance from the object

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Electro magnetic shieldingIt is a process by which a material is able to reduce the transmission of EMR that effects the humans or equipments.

EMS material are used to exclude the unwanted EMR or signals. It also provide protection against the EM pulses which can disrupt neighbouring computers .

It provides protection by reducing signals to level at which they no longer effect equipment or can no longer be received. This is achieved by reflecting and absorbing the radiation.

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How EMS worksThe primary mechanism of EMI shielding is usually reflection.For reflection of the radiation by the shield, the shield must have mobile charge carriers (electrons or holes) which interact with the electromagnetic fields in the radiation.

A secondary mechanism of EMI shielding is usually absorption. For significant absorption of the radiation by the shield, the shield should have electric and/or magnetic dipoles which interact with the electromagnetic fields in the radiation.

Other than reflection and absorption, a mechanism of shielding is multiple reflections, which refer to the reflections at various surfaces or interfaces in the shield.

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How EMS Works

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EMS ApplicationsElectromagnetic shielding is used in,Aerospace aviation: In protection of highly advanced avionic equipments.Anti-terrorism: Encryption of relevant secret codes related to national security.Communication: Safeguarding highly sensitive digital signal processing and microwave operating bandDefense: To protect equipment related to military, navy or air force.Information Technology: Protect users from radioactive effects of computers and peripherals.Medical: Prevents interference of electromagnetic waves given out by medical devices especially during critical operations.Transport: For improved performances of  automomotive electronics luxury devices, vehicle tracking system and  detection surveillance systems, electric cars etc.

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Material used for EMS1. Conductive materials 2. Nano and micro-carbon black 3. Sub micro- and micro-powders of Al, Cu, Ni etc.4. Stainless steel fibers5. Silver-coated polyamide fibres 6. Plastic fibre such as poly phenylene vinylene , poly

Acetylene(doped form)7. Cellulosic fibers.

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Required Properties of EMS materialReflection /absorption / multi reflection of EM wavesIt should be flexible & light weight.Conductivity Corrosion resistantDurability Low cost etc.

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Electromagnetic shielding by textile fabric

Material used for EMS include sheet metal mesh , metal form ,And plasma .Multi layer knitted fabric such as interlock structureTextile Multi layer structureComposite materials for shieldingConductive fabricShielding foil tapesEMI conductive adhesivesMostly conductive fabric are used prepared with different techniques.

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Different approaches to produce conductive fabric

Incorporation of conductive fibres or yarns in to the fabricLamination of conductive layer on to the surface of the fabric such as conductive coating , spray ,ionic plating .Addition to the conductive fillers such as conductive carbon black, carbon fibre ,metal fibre (stainless steal AL, CU),metal powder and flex to the insulating materialGrafting a conductive polymer such as poly anilines , poly pyrolle , poly vinyl alchol,poly amide on to the fabricCoating of individual fibre by conductive polymer

Few examples : Filosano fabric , Flectron , Phantom fabric, High performance silver mesh fabric etc.

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EMS Effectiveness Test

Shielding effectiveness:- It is the ratio of electromagnetic field strength measured without (E0) & with (E1) the tested material when it separates the field source and the receptor. It I s given by

SE = Eo /E1 Insertion loss (A) : It is a measure of losses in a transmission signal cause by the tested material being inserted in to the measuring channel . It is given by A = Uo/ U1Where Uo = channel out put voltage without tested material U1 = output voltage with tested material

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Effect of various parameter on EMS of textile fabric

For woven fabric :1. Effect of type of material: metals have

significantly higher EMS value compare to polymer or cellulosic material Metal > cellulose > polyester (synthetic polymer)

2. Effect of no. of apertures : EMS value decreases with increase in no. of holes . However for metals no significance difference is there in shielding.

3. Effect of yarn count and thread density : with increase in yarn count the effectiveness increases.

4. Effect of no. of fabric layers : the shielding of the material is directly proportional to the thickness of the material.

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For knitted fabric :

1. Effect of metal wire diameter: With increase in dia. the shielding effectiveness decreases .

2. Effect of knitted structure : Interlock and rib structure have more shielding effectiveness then the plain structure.

3. Effect of thickness: Thickness of the knitted fabric shows negligible influence on EMS effectiveness at low to higher frequency.

4. Effect of tightness factor : Fabric with higher tightness factor have good shielding effectiveness then lower.

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Mesurement standards Mil –STD 285 : 1. Introduced in 19562. This standard has an upper frequency limit of 400

MHz3. Developed for large-enclosure and shelter

assessment.

IEEE-STD 299: 4. Developed by institute of electrical and electronics

engg.5. Upper freq. limit 100 GHz.6. This standard is only applicable to an enclosure

whose smallest linear dimension is > 2mt. 7. It does not apply on small and medium size

enclosure .

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Mesurement standards ASTM-E1851 : 1. Developed by the American society for testing and

materials .2. It requires freq. range between 140 and 160 KHz and

between 14 to 16 MHz.3. Far field shielding measurement between 300 to 500 MHz,

900 to 1000 MHz , and 8.5 and 10.5 GHz are required .4. Used for large enclosure and shelters.

VG 95373 part 15 : 5. German military standard.6. The freq. range above 30 to 200 MHz 7. Minimum antenna to enclosure separation distance of 2.5

mt.8. Only standard applicable to small and medium sized

enclosures.

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Mesurement standards ASTM-D 4935 : 1. Developed for evaluation of flat thin samples .2. This standard is used for plastic materials .3. Freq. range from 30 MHz to 100 MHz.

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Conclusion The EMR are increasing day by day due the increasing use of electronics equipment and other sources of EMR . Since the EMR are harmful for us, so it’s a big issue of concern. Due to the new technologies and advancement such as use of conductive polymer ,multi layer fabrics and composites etc. the EMR hazards can be reduce to some extent. After considering the different parameters of construction for EMS , the effectiveness can be increased . In recent year conductive fabric considered mainly due to their characteristics of flexibility , lightness and competitive price.

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References 1. R Perumalraj, B.S. Dasaradan and are Nagaswarana, Electrically

conductive polymer materials for EMI Shielding, Asian Textile Journal, Jan. 2009 (p- 49 to 57).

2. R Perumalraj, B.S. Dasaradan, Electromagnetic shielding fabric, Asian Textile Journal , Oct. 2008 (p-62to 68).

3. R Perumalraj , B.S. Dasaradan , Electromagnetic shielding effectiveness of copper core yarn knitted fabric , Indian Journal of Fibre and Textile Research, vol-34 , June 2009 (p-149-154).

4. A.Das, V.K. Kothari, A. Kothari and A.Kumar, effect of various parameter on EMS effectiveness of textile fabric , Indian Journal of Fibre and Textile Research, vol-34 , June 2009 (p-144-148).

5. K.K.Gupta , S.M. Abbas and A.Srivastava, Microwave interactive fabric: A review , Man Made Textiles in India , Feb 2012 (p-41t0 48).

6. P.R.Surwase, EMI shielding Material and measuring methods : A Review , Man Made Textiles in India , Sep. 2011 (p-327 to 330).

7. Department of defense, United States of America,MIL – STD 285 .8. V.K. Kothari ,Progress in Textile Science & Technology, vol. 3, (p-396).