Optimization and High Gain of a Microstrip Patch Antenna Excited by Coaxial Probe for RFID Reader Applications at 2.4 GHz

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1 Euopean Jounal o Scientiic Reseach ISSN X / X Vol. 14 No 3 June, 213, pp Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz Ali El Alami Univesity Sidi Mohamed Ben Abdellah, Highe School o Technology, Fez, Moocco Laboatoy o Inomation Pocessg and Tansmission Route D'imouzze, BP Fès, Maoc a.elalami1984@gmail.com Saad Dosse Bennani Univesity Sidi Mohamed Ben Abdellah, National School o Applied Sciences Fez, Moocco, Laboatoy o Inomation Pocessg and Tansmission Quatie Industiel A Chke, Route Ben Souda, BP 72, Fès Pcipale, 3, Maoc saad.d.bennani@gmail.com Moulhime El Bekkali Univesity Sidi Mohamed Ben Abdellah, Highe School o Technology Fez, Moocco Laboatoy o Inomation Pocessg and Tansmission Route D'imouzze, BP Fès, Maoc moulhime.elbekkali@usmba.ac.ma Ali Benbassou Univesity Sidi Mohamed Ben Abdellah, Highe School o Technology, Fez, Moocco Laboatoy o Inomation Pocessg and Tansmission Route D'imouzze, BP Fès, Maoc ali.benbassou@usmba.ac.ma Abstact In this pape, we pesent the design and optimization a ectangula patch antenna excited by coaxial pobe o RFID applications and which opeates at the cental equency o 2.4 GHz. The design o the poposed antenna was simulated usg the sotwae Ansot HFSS (High Fequency Stuctue Simulation). This design gives us a etun loss eached at db, a high ga o the ode o 7.7 db and a standg wave atio (VSWR) equal to 1.3. Keywods: Rectangula patch antenna, coaxial pobe, RFID Reade, etun loss, ga, VSWR, sotwae HFSS.

2 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz Intoduction Modem communication systems demand o low cost and low poile antennas. Micostip patch antenna is one o the candidate antennas meetg those equiements due to its conomal natue and capability to tegate with the est o the pted cicuity [1]. Radio Fequency Identiication o RFID ees to the set o technologies that use adio waves o identiyg objects o people. The RFID systems allow us to identiy dividual objects o thgs the envionment which can be monitoed though use o wieless technology. RFID is a geneic tem o technologies that use adio waves to emotely stoe and etieve data. In othe wods, it is a combed tem with RF and ID whee RF means a wieless communication technology and ID means identiication omation o tag. So it is said that RFID is theoetically a wieless netwokg technology to tansmit identiication omation stoed at an electonic memoy space [2]. Seveal authos [3, 4, 5, 6, 7, 8 & 9] have ocused on technical design patch antennas o RFID eade applications. In this pape, we pesent the design and analysis a micostip patch antenna excited by coaxial pobe o RFID eade applications and which opeates at the cental equency o 2.4 GHz. Ou objective is to optimize the chaacteistics o adiation the poposed antenna that is: the etun loss S 11, standg wave atio (VSWR), put impedance and the adiation patten (E-plane, H-plane). 2. Antenna Theoy 2.1. Histoy The concept o micostip antenna dates back to the 195 s, but it was not until the 197 s that geate emphasis was given to develop this technology. This is maly due to the availability o good substates. Sce then, extensive eseach and development o micostip antenna and aays, exploitg the numeous advantages such as light weight, low volume, low cost, plana coniguation, compatibility with tegated cicuits, have led to divesiied applications and to the establishment o the topic as a sepaate entity with the boad ield o micowave antennas [1]. The Micostip patch antenna is a esonant stuctue that consists o a dielectic substate sandwiched between a metallic conductg patch and a gand plane. The patch is geneally made o coppe o gold and can take any possible shape [11, 12]. Thee is a numbe o techniques available o analyzg micostip patch antennas. The analytical techniques clude tansmission le model [13, 14], and cavity model [15, 16]. The most common numeical techniques used ae moment method [17] and the ite dieence time doma method [18]. The late technique is time consumg while the ome method and the analytical techniques have been applied to egula shapes only like, ectangula, cicula, and elliptical shapes [13]. Howeve, the analysis o MSA is nomally diicult to handle which is pimaily due to the existence o a dielectic substate to suppot the conducto [19] Basic Chaacteistics An antenna is the tansitional adio between a guidg device [2, 21]. In ode to be ee-space and able to design a good antenna it s cucial to conside some o the basic but yet impotant paametes that chaacteize all antenna designs Radiation Patten The powe adiated o eceived by is a unction o the angula position and adial distance om the antenna. The adiation patten is best epesented the om o a thee dimensional gaph o powe vesus elevation and azimuth angles but moe commonly, epesented by E-plane o H-plane whee one angle is held ixed while the othe is vaied.

3 379 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou Retun Loss Retun loss is a convenient way to chaacteize the put and output o signal souces. In othe wods, when the load is mismatched, not all the available powe om geneato is deliveed to the load. loss is temed as the etun loss (RL) and is this deed ( db) as: RL = -2 log Γ (db) (1) V Whee Γ, = V Z Z = L + Z L + Z Γ : Relection coeicient. V : The elected voltage. V + : The cident voltage. Z and Z ae load and chaacteistic impedances espectively. L Voltage Standg Wave Ratio (VSWR) Fo a adio (tansmitte o eceive) to delive powe to an antenna, the impedance o the adio and tansmission le must be well matched to the antenna's impedance. The paamete VSWR is a measue that numeically descibes how well the antenna is impedance matched to the adio o tansmission le it is connected to. VSWR stands o Voltage Standg Wave Ratio, and is also eeed to as Standg Wave Ratio (SWR). VSWR is a unction o the election coeicient, which descibes the powe elected om the antenna. I the election coeicient is given by Γ, then the VSWR is deed as: 1+ Γ VSWR = (2) 1 Γ Ga Antenna ga, usually expessed db, simply ees to the diection o maximum adiation. Mathematically the maximum ga G is obtaed by usg Equation (3): G = ŋ.d (3) Whee, ŋ = eiciency and D = diectivity Diectivity It is desiable to maximize the adiation patten o the antenna esponse a ixed diection to tansmit o eceive powe. Figue 1: Diectivity o an antenna

4 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz 38 Likewise, the diectivity is dependent only on the shape o adiation patten. It is always eeenced to an isotopic pot souce as shown i Figue 1. A quantitative measue o this esponse is the diective ga o the antenna o a given diection [22] Antenna Eiciency The antenna eiciency is deed as the atio o total powe adiated by the antenna to the put powe o the antenna. Just like any othe micowave components, an antenna may dissipate powe due to conducto loss o dielectic loss [23] Input Impedance The put impedance o an antenna is seen as the impedance pesented by the antenna at its put temal o the atio o the voltage to cuent at a pai o temals o the atio o the appopiate components o the electic to magnetic ields at a pot. Z = R + jx (4) Whee R, = eal pat, X = imagay pats. The desied eal pat o the impedance is made up o the put esistance, ( R ) which epesents powe dissipated though heat o adiation losses. The undesied imagay pat ( X ) epesents the eactance o the antenna and is the powe stoed the nea ield o the antenna Polaization The polaization o an antenna ees to the polaization o the electic ield vecto o the adiated wave. It can also be undestood as the oientation o the electic ields as obseved om the souce vesus time. The common and typical types o polaization clude the lea (hoizontal o vetical), cicula (ight hand polaization o the lee hand polaization). I the path o the electic ield vecto is back and oth along a le, it is said to be lealy polaized while cicula polaization has its electic ield vecto emag constant length but otates aound a cicula path. [24, 22] Mathematical Fomulations In geneal, patch antennas have the length o hal-wave stuctues at the equency o the undamental esonant mode. Sce the gg ield acts to extend the eect length o patch, the length o the halwave patch is slightly less than a hal wavelength the dielectic substate mateial. Appoximate value o the length o a esonant hal-wavelength path is given by [25]. λ L =.49 (5) ε Whee λ is the ee-space wavelength and ε the substate dielectic constant. Vaious appoaches may be used to meet the itial design equiements. In this wok, we used the tansmission le model. All the dimensions o the patch antenna have been calculated based on equations (6-9) [26]. The width is given by c 2 W = 2 ε + 1 Whee the esonant equency o the patch antenna, ε is the dielectic constant o the substate and c the ee-space velocity o light. The eective dielectic constant o ( W > 1) is given by ε e ε + 1 ε 1 1 = h W h (6) (7)

5 381 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou The extension L o the patch length due to the gg eect can be obtaed by W ( ε +.3 ) h L =.412 h (8) W ( ε.258 ) +.8 h The eective length o the patch L e now becomes: L = L + 2 L (9) e Fo is given esonance equency, the eective length is C Le = (1) 2 ε. e C = m n 2 2. ε + e L W Whee m and n ae modes along L and W espectively Feed Pot The coaxial eed o pobe eed is a vey common technique used o eedg Micostip patch antenna.as seen om igue 2, the ne conducto o the coaxial connecto extends though the dielectic and is soldeed to the adiatg patch, while the oute conducto is connected to the gound plane. The eed W coodates wee calculated Y = and X = X L whee, 2 L 1 5 X = cos (12) π Z Z = 5. Z (13) : Chaacteistic impedance. Z Z : Input impedance o the antenna. The ma advantage o this type o eedg scheme is that the eed can be placed at any desied location side the patch ode to match with its put impedance. This eed method is easy to abicate and has low spuious adiation. Howeve, a majo disadvantage is that it povides naow bandwidth and is diicult to model sce a hole has to be dilled the substate and the connecto potudes outside the gound plane, thus not makg it completely plana o thick substates (h >.2λo). Also, o thicke substates, the ceased pobe length makes the put impedance moe ductive, leadg to matchg poblems [27]. It is seen above that o a thick dielectic substate, which povides boad bandwidth, the micostip le eed and the coaxial eed sue om numeous disadvantages. The non-contactg eed techniques which have been discussed below, solve these issues. (11) 3. Sotwae o Simulation The sotwae HFSS (High Fequency Stuctue Simulation) o Ansot Copoation is commecial sotwae which calculates the electomagnetic behavio o a stuctue the equency doma. It peoms electomagnetic modelg by solvg Maxwell's equations usg the ite element method. The simulation technique used to calculate the thee dimensional electomagnetic ield side a stuctue is based on the ite element method (FEM). The pciple o the method is to divide the study aea to many small egions (tetahedons), then calculate the local electomagnetic ield each element. The local ields E and H ae calculated each tetahedon om the ollowg equations [28-29]:

6 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz 382 Λ 1 1 = H ( ) E µ Λ ( ) 2 - K E ( ) ε jωω - Λ E ( ) = (14) ε and µ ae espectively the pemittivity and elative pemeability o mateials. K = 2π µ ε is the wave vecto vacuum, ω = 2π is the pulsation angula equency. HFSS uses an tepolation method combed with an iteative pocess which a mesh is ceated automatically and edeed the citical egions. The simulato geneates a solution based on the pedeed itial mesh. Then, it ees the mesh egions whee thee is a high density o eos, and geneates a new solution. The S ij micowave paametes ae calculated with the given ollowg steps: Division the stuctue to a ite numbe o elements. Excitation o each pot o the stuctue with a wave popagatg along a wave guide stuctue o a uniom tansmission le which has the same section as the pot. Calculation o the total coniguation o the electomagnetic ield side the stuctue. Calculation o matices S ij genealized om the elected and tansmitted powes. (15) 4. Design Analysis Figue 2 shows the poposed antenna excited by pobe coaxial. Figue 2: Geomety o the poposed antenna Figue 2: Geomety o the poposed antenna - contued

7 383 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou 4.1. Design Speciication The thee essential paametes o the design o a ectangula micos tip Antenna ae [22]: Fequency o opeation ( ): The esonant equency o the antenna must be selected appopiately. The esonant equency selected o ou design is 2.4 GHz. Dielectic constant o the substate ( ε ): the dielectic mateial selected o ou design is Roges RT/duiod 588 (tm) which has a dielectic constant o 2.2. A substate with a high dielectic constant has been selected sce it educes the dimensions o the antenna. Height o dielectic substate (h): Fo the micostip Patch antenna to be used wieless applications, it is essential that the antenna is not bulky. Hence, the height o the dielectic substate is selected as.32 cm. So, the essential paametes o the design ae: = 2.4 GHz. ε = 2.2. h =.32 cm Design Pocedue The tansmission le model descibed Section 2 will be used to design the antenna. Step 1: Calculation o the Width (W): The width o the Micostip patch antenna is given by equation (6) with substitutg ε = 2.2, we get W = 4.94 cm. Step 2: Calculation o Eective dielectic constant ( ε e ): Equation (7) gives the eective dielectic constant, with substitutg h =.32 cm and W = 4.94 cm, we get ε e = 2.5. Step 3: Calculation o the Eective length ( L e ): Equation (1) gives the eective length, with substitutg ε e = 2.5, we get Step 4: Calculation o the length extension ( L): Equation (8) gives the length extension o antenna, with substitutg =.16 cm. Step 5: Calculation o actual length o patch (L): Equation (9) gives the actual length o patch, with substitutg L e L e = 4.36 cm. = 4.36 cm, we get L L e = 4.36 cm and L =.16 cm, we get L = 4.3 cm. Step 6: Calculation o the gound plane dimensions (L g and W g ): The tansmission le model is applicable to ite gound planes only. Howeve, o pactical consideations, it is essential to have a ite gound plane. Fite and ite gound plane can be obtaed i the size o the gound plane is geate than the patch dimensions by appoximately six times the substate thickness all aound the peiphey. Hence, o this design, the gound plane dimensions would be given as: L = 6h + L = 6. (.32) = 5.95 cm. g W = 6h + W = 6. (.32) = 6.86 cm. g Step 7: Calculation o the put impedance (Z ): The typical impedance at the edge o a esonant ectangula patch can be appoximated as Z 2 ε L = 9 ε 1 W 2 By equation (16), we get Z = Ω which does not match well with a 5 Ω standad micostip. By equation (13), the chaacteistic impedance o the tansition section should be 19.9 Ω. (16)

8 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz 384 Step 8: Detemation o the Feed Pot Locations (X, Y ): A coaxial pobe type eed is to be used this design. As shown Figue 2, the cente o the patch is taken as the oig and the eed pot location is given by the coodates ( X, Y ) om the oig. The eed pot must be located at that pot on the patch, whee the put impedance is 5 Ω o the esonant equency. The calculated eed coodates o the given ectangula patch opeatg at 2.4 GHz ae Y = 2.47 cm and X = 1.47 cm. The esults o calculations ae summaized Table 1. Table 1: Paametes o patch antenna Paametes Dimension (cm) W 6.86 g L g 5.95 W 4.91 L 4.3 ( X, Y ) (1.47, 2.47) Pobe adius p.7 Coaxial adius c Analysis and Optimization Ate detemg the paametes o the ectangula patch antenna with the aid o a Matlab pogam (table 1). The use o these paametes the design gives us the ollowg esult (Figue 3). Figue 3: Retun loss vs. equency o the RMPA We d that the etun loss is not eached at a mimum level and the esonance equency is uthe than simulation (2.4 GHz). Fo this it is necessay to change the size o patch antenna until his widthw = 4 cm and length L = 3cm. Next we have vaied the dieent eed pot locations (X,Y ) up to d a bette adaptation o the put impedance o the antenna. Figues 4, 5, 6, 7, 8, 9, 1 & 11 espectively show the vaiations o etun loss, voltage standg wave atio, put impedance (eal and imagay pats) and the adiation patten (E-plane and H-plane) o dieent eed locations.

9 385 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou Figue 4: Retun loss vs. equency o the RMPA Figue 5: Voltage standg wave atio o the RMPA Figue 6: Real o Input Impedance o the RMPA Figue 7: Imagay o Input Impedance o the RMPA Figue 8: Ga o the RMPA o phi = Figue 9: Ga o the RMPA o phi = 9

10 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz 386 Figue 1: Diectivity o the RMPA o phi = Figue 11: Diectivity o the RMPA o phi = 9 Ate the miatuization o the size o patch antenna and change the dieent eed pot locations (Table 2), we d that: Ga and diectivity o the antenna ae optimized successively ceased. The put impedance changes and becomes moe adapted to the pot o excitation. Table 2: Results o the optimization Feed location Fequency Retun Loss Input Impedance Diectivity (, ) (cm) VSWR Ga (db) (GHz) (db) (Ω) (db) (1.8, 2) j (1, 1.8) j (.7,.2) j (.5,.1) j Results o Optimization Table 3 contas the optimal dimensions o the patch antenna poposed. The etun loss, voltage standg wave atio, put impedance (eal and imagay pats) and the adiation patten (E-plane and H-plane) ae espectively shown Figues 12, 13, 14, 15 and 16. Table 3: Dimensions o the Antenna optimized Paametes Dimension (cm) W 6.86 g L g 5.95 W 4 L 3 ( X, Y ) (.5,.1) Pobe adius p.7 Coaxial adius c Retun Loss The ollowg cuve shows the put etun loss o the ectangula micostip patch antenna (RMPA) as a unction o the equency:

11 387 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou Figue 12: Retun loss vs. equency o the RMPA We obseve that the etun loss at the put o the patch antenna eaches the level o db (at a esonance equency o 2.38 GHz) with a bandwidth o 2.6% (the dieence between 2.34 GHz and 2.4 GHz). This means that the elected powe is mimal and theeoe it will have a positive impact on ceasg the level o the powe tansmitted by the RFID eade antenna Voltage Standg Wave Ratio Figue 13 shows the voltage standg wave atio (VSWR) o the ectangula patch antenna usg coaxial eed method at ou design equency o 2.4 GHz. We mentioned section 2 o this pape that a VSWR is a measue o impedance mismatch. As can be obseved om the gaph, the VSWR obtaed is 1.3 o a equency o 2.38 GHz. This is consideed a good value as the level o mismatch is not vey high. A high VSWR means the pot is not popely matched. Thus, this value poves that the pot o the antenna is popely matched. Figue 13: Voltage standg wave atio as a unction o the equency 5.3. Input Impedance o the Patch Antenna The impedance matchg would optimize the tanse o electic powe between a souce and a load. The ollowg igue shows the layout o the put impedance o ou geomety (Figue 14).

12 Optimization and High Ga o a Micostip Patch Antenna Excited by Coaxial Pobe o RFID Reade Applications at 2.4 GHz 388 Figue 14: Input impedance o the patch antenna as a Function o equency In ode to match the pot to the antenna, it must meet the 5Ω matchg impedance. Theeoe once the pot is placed onto the patch, simulation is done. The Z paamete is a measue o the matchg impedance value ate simulation. Fom Figue 14, the matchg impedance value is ( j1.72) Ω In. This esult is nea to 5O Ω (which is the oigal value) with an imagay pat almost nil, so it shows that the pot is well matched Ga o the Patch Antenna Figues 15 and 16 show the adiation patten o the antenna ectangula patch coodated Catesian the planes E and H. The simulated ga o the ectangula micostip patch antenna at 2.4 GHz is shown the igue 15. The peak ga the 2.4 GHz is about 7.7 db o phi = (H-plane) and phi = 9 (E-plane). Figue 15: Ga o the RMPA o phi = and phi = 9 at 2.4 GHz

13 389 Ali El Alami, Saad Dosse Bennani, Moulhime El Bekkali and Ali Benbassou 5.5. Diectivity o the Patch Antenna The simulated diectivity o the poposed patch antenna at vaious equencies is shown Figue 16. The maximum achieved diectivity is 7.9 db o phi = and phi = 9 at the equency o 2.4 GHz. Figue 16: Diectivity o the RMPA o phi = and phi = 9 at 2.4 GHz 5.6. Antenna Eiciency The adiation eiciency η is deed as the atio o the powe adiated to the powe eceived by the put to the element. Fom equation (3) is ound that η = 95 % :. The esults o this wok ae summaized Table 4. Table 4: Radiation chaacteistics o the poposed antenna Antenna Antenna Feed Input Antenna Width Length location Retun VSWR Impedance Ga Diectivity Bandwidh Eiciency Loss (db) (db) (db) (%) (cm) (cm) (, ) (cm) (Ω) (%) 4 3 (.5,.1) j Conclusion This pape pesents an optimization methodology the dieent adiation chaacteistic o a ectangula patch antenna. This method is based on the miatuization o the antenna dimensions and also the change o the dieent eed pot locations. The poposed antenna has a etun loss o db, a VSWR o 1.3, a high ga o 7.7 db, diectivity much good to 7.9 db, a bandwidth o 2.6 % and a total eiciency o 95 %. The poposed design has simple design stuctue and can easily be abicated at low cost thus beg a good solution o many RFID eade applications. Reeences [1] Ramesh, M., and Yip K., Design Fomula o the Inset Fed Patch Antenna, Jounal o Micowave and Optoelectonics, Vol. 3, Decembe 23, pp.5-1. [2] ITU-T Wokshop on "Netwoked RFID: Systems and Sevices" Geneva, Febuay 26.

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