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Int. J. Elec&Electr.Eng&Telecoms. 2014 J Thirumal Murugan and T R Suresh Kumar, 2014

FREQUENCY SELECTIVE TRANSPARENT FRONTDOOR FOR MICROWAVE-OVEN

J Thirumal Murugan1* and T R Suresh Kumar1

*Corresponding Author: J Thirumal Murugan, thirumalmuruganj@gmail.com

Microwave oven generates a harmful Electromagnetic Wave at 2.4 GHz of 1000 Watts. Thegenerated microwave has to be confined within the cavity of the oven for efficient heating andsecured operation. To prevent the microwave leakage through the front glass door, a specialconstruction of Faraday Cage being involved. In this paper, Faraday Cage is replaced withTransparent Frequency Selective Surface Front Door is proposed, this provide better visibilityand to avoid microwave energy to escape from the oven. Band pass response has been achievedfor 10 GHz by printing array of Greek cross aperture (FSS) on the front glass door. Design ofFSS array and the simulation results were discussed.

Keywords: Frequency selective surface, Greek cross, Electromagnetic shielding

INTRODUCTIONA Microwave oven is a Microwave Generatorused for heating food, which works by passingnon-ionizing microwave radiation, at afrequency of 2.45 GHz through the food.Microwave is generated by the Magnetron andfed to the metal cavity (cooking compartment)through waveguide. The metal cavity (size≈33.6 x 22.5 x 34.9 cm3) is closed with solidmetal plates on five sides and Faraday Cageincorporated glass plate on the sixth side.Since all the six faces are reflecting themicrowave, the energy is confined inside theoven for heating.

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1 Department of ECE, Muthayammal Engineering College, Rasipuram 637408,

The Front Glass window design isimportant, because it should see through andshould not allow the microwave to passthrough. To achieve this Faraday Cage isplaced inside the glass plate. Faraday Cageis conducting mesh screen which blocks outexternal static and non-static electric fields. Itsoperation depends on the fact that an externalstatic electrical field will cause the electricalcharges within the cage’s conducting materialto redistribute themselves in order to cancelthe field’s effects in the cage’s interior. The holesize the in the mesh should smaller than thewavelength of the microwave signal so that it

Research Paper

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can’t penetrate. In this paper Faraday Cageis replaced with Frequency Selective Surfaceto provide improved visibility and Microwaveradiation blocking capability.

FREQUENCY SELECTIVESURFACE (FSS)FSS is the periodic structure of conductiveelements or apertures in either one or twodimensions that provide a filter operation whenthey are illuminated with EM wave. Whenilluminated by an electromagnetic wave, FSSexhibits total transmission/reflection around theresonance frequency. This spatial filterbehavior of FSS is used in designing the FSSShield. The filter behavior (low-pass, high-pass, band-pass and band-stop) of the FSSdepends on the shape of the element (Munk,2000). In this paper Greek Cross element isconsidered for FSS design. Figure 1 showsvarious arrays of FSS and the frequencyresponse.

The major application of FSS is selectiveshielding. By carefully choosing the elementshape and size, desired frequency response

can be obtained. FSS has already applied toblock/allow Wi-Fi band 2.4 GHz.

In Cheng-Nan Chiu et al. (2008) Band passShielding Enclosure has been designed toallow the 2.4 GHz band and block theremaining frequencies. Here the size of theenclosure considered in 10x16x2.1 cm3, andarray of Jerusalem cross apertures has beenmade. The design objectives of a practicalBPSE are - high transmittance in the specifiedwireless-signal band and high shieldingoutside that band. Also several works hasbeen proposed to block the Wi-Fi inside thebuilding. In Taylor (2011) a Frequency SelectiveSurface (FSS) of square loops printed on bothsides of a dielectric substrate is used to allow/block 2.4 GHz band inside a room. This FSSstructure is printed on the walls to control theWi-Fi band.

In Raspopoulos (2011) also FSS wall hasbeen designed to stop penetration of Wi-Fiinside the building. Here square loop patch isprinted on the walls to achieve the band stopresponse. In Mias (2006) transparent FSS boxhas been designed to place Microwave oven.The microwave oven measurements in thepresence of the FSS box demonstrated thatthere is a satisfactory attenuation, around 20dB. In all the above literatures FSS is printedon substrate which is either transparent or nottransparent for blocking or allowing the Wi-Fisignal. But so far FSS design approach hasnot been applied to design the front doors ofmicrowave oven.

DESIGN OF FSS FRONTWINDOWDesign Objective: To design a FrequencySelective Surface (i) blocking the radiation

Figure 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

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Int. J. Elec&Electr.Eng&Telecoms. 2014 J Thirumal Murugan and T R Suresh Kumar, 2014

of 2.4 GHz and (ii) providing the transparencyin FSS wall so that light pass through thewindow and user can see through the oven.So FSS has to be designed to block 2.4 GHz,i.e., a band stop response at 2.4 GHz. But forsafety reasons, in this paper FSS has beendesigned for allow 10 GHz band. This designwill block till 5 GHz strongly. Array of GreekCross apertures will be made on theconductor and this FSS will be placed abovethe glass. The model is simulated with Feko5.5 Lite Version. Microwave oven with seethrough FSS window has been shown inFigure.2.

Figure 2: Microwave Oven with FSS Wall(FSS Slots are Embedded on the Front

Door)

Table 1: Literatures on FSS Shelding

[1]

[2]

[3]

[4]

[5]

Center connected,Loop type and allpossiblecombinations

Slot arrays,Tripole-slot arrays,and Cross-slotarrays

Ring element

Cross slots withand withoutloading

Circular patches,Tripole elementand Tripoleelement withintriangular element

Band pass,Band stop,High passand Lowpass

Band pass

Band pass

Selectiveradio bandcontrol, bandstop

Band stop

As per thenecessity basedupon stopping orpassing thewavelength

IEEE 802.11 b/g(2400-2484 MHz)

2.45 GHz WLANband

Wi-Fi, 2.4 GHz

10 GHz, 1.9 GHzand 2.4 GHz

Hybrid Radome, Band-stop filters, Dichroic subreflectors, Dichroic main reflectors, Circuit Analogabsorbers, Meander line Polarizers, etc.

High transmittance in the IEEE 802.11 b/g bandand High shielding effectiveness outside thisband, together with minimal effect on the radiationcharacteristics of the internal antenna.

Controlling the electromagnetic architecture ofbuildings, Offering good performance in terms ofall polarizations affected and good angularstability. The two states offered by the surfaceenable it to be either transparent or reflective atthe frequency of interest.

FSS in indoor wireless environments andinvestigates their effect on radio wavepropagation. FSS can be deployed to selectivelyconfine radio propagation in indoor areas, byartificially increasing the radio transmission lossnaturally caused by building walls.

Designing transparent films for windows that canshield at some desired frequencies. Either 2.45GHz for wireless Local Area Network (LAN)applications or 1.9 GHz for Personal Hand-phoneSystem (PHS) applications. They also indicatethat their product does not disturb mobile phonecommunication bands at 900 MHz and televisionfrequency bands.

Ref.Paper FSS Element Frequency

Response Frequency Band Application

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DESIGN OF FSS UNITELEMENTFirst the single unit cell of the FSS array shouldbe decided. The length of the Greek Crossaperture should be of approximately/2 whichthe resonance length of the aperture pole.Since band pass response is expected the atf = 10 GHz (= 3 cm), the half wavelength is 1.5cm. The single element is shown in Figure 3.

For Simulation Feko 5.5 Lite version isused. As the FSS is infinite periodic array,Periodic Boundary condition is applied. Unitcell meshed view is shown in Figure 4. ThisGreek Cross aperture shaped unit cell is place

above the Glass. The white part of the crosswill be the transparent part and remaining aremetal. As the array is backed by glass theresonance frequency curve is shifted to fewhundred megahertz.

RESULTS AND CONCLUSIONFrom the simulation we observed that, thisFSS screen possess band pass response at10 GHz and high attenuation at 2.4 GHz band.

Figure 3: FSS Unit Element

Figure 4: Unit Cell Meshed View

Figure 5: Response Curve BetweenElectric Field vs Frequency

In this paper Frequency Selective Surfaceis designed to block the Microwave energyfrom the Microwave oven front glass window.For this purpose a FSS layer which consist ofarray of greek cross shaped apertures weremade on the conducting surface and this isprinted on the glass front window of themicrowave oven.

To provide further attenuation one morelayer of FSS has been inserted. This provides

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improved transparent area over theconventional Faraday Cage based oven door.So simultaneously transparency andelectromagnetic shielding were achieved.

REFERENCES1. Cheng-Nan Chiu, Chang-Hsin Kuo and

Ming-Shing Lin (2008), “BandpassShielding Enclosure Design UsingMultipole-Slot Arrays for Modern PortableDigital Devices”, IEEE Transactions onElectromagnetic Compatibility, Vol. 50,No. 4.

2. Mias C (2006), “An Investigation into theFeasibility of Designing Frequency

Selective Windows Employing PeriodicStructures (Ref. AY3922)”, NottinghamTrent University, Tech. Rep.,

3. Munk B A (2000), Frequency SelectiveSurfaces-Theory and Design, p. 227,John Wiley.

4. Raspopoulos M (2011), “FrequencySelective Buildings Through FrequencySelective Surfaces”, Antennas andPropagation, IEEE Transactions, Vol. 59,No. 8, pp. 2998-3005.

5. Taylor P S (2011), “An Active Annular RingFrequency Selective Surface”, Antennasand Propagation, IEEE Transactions,Vol. 59, No. 9, pp. 3265-3271.