A Simple and Reliable Method for the Evaluation of the Exposed Field Near the GSM Antenna

Transcription

1 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, A Smple and Relable Method for the Evaluaton of the Exposed Feld Near the GSM Antenna Algent Lala,Bexhet Kamo, Vlad Kolc, Shkelzen Cakaj, Faculty of Informaton Technology Polytechnc Unversty of Trana Trana, ALBANIA 2 Faculty of Electrcal and Computng Engneerng Prshtna Unversty Prshtna, KOSOVO Abstract The objectve of ths paper s to present a smple, accurate and very effcent method for the evaluaton of the feld n the vcnty of GSM antennas of the rado base-staton n urban areas. The method s based on the replacement of the antenna panel wth a group of dscrete source emtters. A geometrcal approxmaton s used for the evaluaton of the envronment s nfluence also. The calculated results are compared wth results taken from the use of NARDA SRM 3000 measurng equpment. The presented method could be successfully used for the exposure evaluaton of the electromagnetc feld emtted by GSM antennas of the basestaton n urban areas. Keywords evaluaton; electromagnetc feld;near feld; NARDA SRM 3000; base-staton I. INTRODUCTION The fast deployment of rado communcatng systems, especally n the moble technology, has rased concerns n regards to the effects of electromagnetc feld emtted by these systems on human health. Ths paper ams to evaluate the electromagnetc feld emtted by the antennas of the basestatons. The evaluaton of the feld n the vcnty of the BS can be done by usng smple calculatng methods and smulatons; both approaches are complementary and allow for the exploraton of the whole space around the antenna. The smulaton method s of great nterest as t can be used for verfyng the results of practcal measurements and for extrapolatng these results beyond the allowed space, where measurements cannot be done. Ths paper presents a bref examnaton of the methodologes used for predctng, n the context of verfyng reference levels of power densty near antenna panels of BS whch are usually used n moble telephony n GSM bands 900MHz and 800MHz. The exposure to BS antennas n free space has been studed n references [], [2], [3], [4] and n reference [5] for urban envronment untl now. We am to solve the problem of evaluatng the exposure to the near feld n the urban envronment. In the zone of far feld, whch s defned as the zone of antenna feld where the spread s n substance ndependent from the dstance from the antenna [6], the EM feld can be calculated relatvely easy snce all the requred nformaton s n the amplfyng model of the antenna and the emtted power. If the antenna has a maxmum dmenson D, the feld n the dstant zone, usually s taken as the radal dstance from the source R=2D²/λ, where λ s the wavelength. For dstances smaller than R, n the zone of the near feld, the EM feld has a complex form, whch s very dffcult to evaluate. For a GSM antenna panel of the BS (D=2m) the border between the near feld and the far feld s n the dstances of ~ 0 m and ~20 m for GSM 900 MHz and GSM 800 MHz respectvely. The condtons of exposure are completely dfferent n the urban envronment from the ones n the free space. The rgorous numerc modelng technques, such as the method of moments (MoM) or the fnte-dfference tme-doman method (FDTD), are used extensvely n the study of the feld n the proxmty of the antenna. The most common ssue related to the applcaton of these methods s to know the geometry of the BS antenna. In addton to the ssue mentoned above, when the exposure n the urban envronment s modeled, the dmensons of the area of study mght be very large compared wth the wavelength; as a result the tme needed for the analyss s unacceptably long. We defntvely need smpler and more relable calculatng methods for forecastng the exposure of EM feld n the urban envronment. Ths paper s organzed as follows. The frst secton goes over the need for a smple, and practcal method to evaluate the feld n the nearby of the rado base statons. Secton II descrbes the modelng and geometry of the antenna. Secton III s referred to proposed theoretcal model for the evaluaton of the far feld. Secton IV presents results and dscussons. Secton V draws the conclusons. II. THE MODELING AND GEOMETRY OF THE ANTENNA The numerc calculatons have been performed for a typcal BS antenna. We took nto consderatons the Katherne , and [7], type for a range of frequences between 870 MHz and 960 MHz, and 70 MHz and 880 Mhz. The emttng model of ths antenna s characterzed by a (G) 7.5 db amplfcaton, an openng (aperture) of 3 db 7 and 65 n the vertcal plan (plan E) horzontal plan (plan H), respectvely. The model s n the shape of group of dpoles postoned n front of a renforcng 96 P a g e

2 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, reflector wth dmensons: 2m long and 40 cm wde, wth angles for lmtng the man ray. Accordng to the reflectng theory[8], the dvdng dstance between the reflector and the group of dpoles s set at λ/4. The number of dpoles, whch depends from the desred amplfcaton, and dvdng dstance between them s calculated accordng to [8] h 2m h 2 90 cos () 2d N Where Θ h s half the power of the ray wdth, θ m s the maxmum value of the group factor whch s set at 90, θ h s the pont wth half power, λ s the wavelength (for f = MHz whch s the the mddle frequency of the downlnk band n GSM), N s the number of elements, β s the shft of the phase between the elements and d s the dstance between them. In the vertcal plan (plan E), the dstance between the poles (center-center) whch are along the Z axs and wth the same dstance, can be calculated by (). Consderng the effect of the reflector, by puttng the dvdng dstance between elements at d z 3λ/4, whch s gven at 22 cm, the number of N z elements can be calculated n order to acheve a half power of the ray wdth 7 by consderng β=0 and the pont of half power at θ h = 3.5 ; ths corresponds wth eght elements. In the horzontal plan (plan H), the dvdng dstance between d y dpoles whch are encountered along the Y axs can be calculated n the same way from () by puttng d y λ/2 whch s gven at cm. The number of N y elements can be calculated n order to gan a ray wdth of 65 wth half power. Consderng the pont of half power at θ h = 32.5, whch coresponds wth two elements. As a result the group of dpoles s formed by 8 2 dpoles half wavelength (λ/2) along the Z axs and Y axs, as shown n Fg cm 8 cm d z = 22 cm d y = cm 40 cm Fg.. The front and sde vew of the projected antenna l = 5 cm By usng the sub-antenna descrpton, the antenna model s vald for the near and dstant felds for the whole antenna. Because of the dstrbuton and reflectons, the envronment has some nfluence n the power of the feld. In order to take nto account the possble dstrbuton, the evaluaton of the x z y feld near the antenna s done by usng the reflectons theory. The ray tracng method can be used n the dstant feld of these sub-antennas III. THE METHOD FOR EVALUATING THE NEAR FIELD OF AN ANTENNA Modelng of the electromagnetc feld near the rado base statons s a way to evaluate and defne the excluded zones near these statons. The selecton of an approprate model s mportant n order to have a good estmaton of the levels of the radaton. In the references [8], [9], [0], there are many models for the defnton of the zones of the near feld as well as the far feld. Attenton must be pad to the fact that n the zones of the near feld, the levels of the radaton depends not only on the dstance from the antenna but also on the movement along the vertcal axs, whereas n the case of the far feld the levels depend only on the dstance not on the movement along the vertcal axes. The models of the far feld am towards smple formulatons and based on them numercal methods can be appled whch make possble the estmaton of the electromagnetc feld n a short tme and wth modest computer processng power. The proposed model s based on the model Far-feld Gan-based [] and eq. (2). Ths model provdes a smple and effcent method for the evaluaton of the levels of the electromagnetc feld radated by the antennas of the rado base statons wth unform groups of cells n the zone of the near feld and the ones of the far feld. The above s acheved n two steps: In the frst step, electrcal ntensty of the antenna s calculated by combnng the radaton of the far feld of the antenna elements, and the group factors, by acceptng that the antennas of the rado base statons are a unform group of cells. Modelng the antenna wth N source cells s shown n Fg. 2. N E( d,, ) Second step as eq. (2): 30PnGe (, ) ) e d j u(, ), d 3 (2) GM DVe ( ) DHe ( ) Ge (, ) (3) N 2d ( ) (4) Where N s the number of radatng cells, (d, θ, φ ) are sphercal coordnates of the -th element up to the N-th one, P n s total radated power by a gven group, G e (θ,φ ) s amplfcaton of the radatng element, d s dstance from the - th element, u (θ,φ) s unt vector of the -th element, λ s wave length,gm maxmal gan of the antenna, D Ve (θ),d He (φ) are the models of the radatng element n the vertcal and horzontal plan and Φ s the dfferences of the phases between the coeffcents of the radatng element P a g e

3 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, E Rfrequence - the electromagnetc feld radated by the antenna on the calculated pont [3]. E Rfrequence N 2 N 2 N 2 Ex Ey Ez ( Gven that the behavor of the near feld s much more complex than the dstant feld, t would be easer to nclude the space of the near feld as forbdden zone for people. In practce, the space of the zone of near feld would nclude an area wth a radus of m for a typcal base-staton antenna 2m long (for ths work we have lmted the dstance up to 4 m from the antenna panel center) s shown n Fg. 3. (7) Fg. 2. Modelng the antenna wth N source cells In ths paper, reference s made to Kathren antennas specfcally to the models, , and The Kathren model [7] s used for the spectrums 900/800/200 MHZ. The ntensty of the electrc feld calculated for each of the elements (cells) of the antenna s vectoral and t can be projected accordng to the axs x, y, z, and obtan the respectve components for each of the axes. The electrc and magnetc felds n the Cartesan coordnates are composed of each of the three components Ex, Ey and Ez for each and every frequency the same s vald for the magnetc feld wth respectve Hx, Hy and Hz components [8] as (5): E 2 E 2 E 2 E (5) Re sultan t ( x y z We propose for the modelng of the base antennas the followng: - The use of the Far-feld Gan-based model n whch the ntensty of the electrc feld s calculated by the equaton (2) j wth the approxmaton that e u, ) =. Ths ( approxmaton nfluents the accuracy of the model Far-feld Gan-based for the near felds up to 5 λ. Ths falls wthn the safety dstances as defned by the standards. The acqured equaton s (6): N PnGe E d 30 (, ) ) (,, ) (6) d 2- The statstcal study consdered s The worst case scenaro the vectors E x E x2 E xn n the same phase as the E y and E z. Ths defnton wll lead to an overestmaton of the electromagnetc feld on the gven pont. The ntensty of the electrcal feld n a gven pont (weght per frequency) near the antenna of a rado base staton when the antenna s Three- Band (900/800/200) and by consderng the vectors Ex, Ex2...Exn n the same phase as the Ey and Ez s as per the (7). Fg. 3. The shape and dmenson of a volume for a sector of the base staton transmtter IV. ANALYSIS OF THE RESULTS Ths secton shows the results of our work. In order to check the accuracy of the calculatng method presented, the approxmate results are compared wth the results from the measurement done wth the NARDA SRM 3000 [2] measurng equpment. The value for a sample s acheved by an average of measurements that last 360 seconds. The samplng step n the Z axs s every 0 cm n the nterval ±m the mddle of the antenna; the samplng step n the X axs s every 20 cm n the nterval 0 to 4m. The samplng step n the Y axs s every 0 cm n the nterval ±m front and back. The results taken are shown n Fg.4 and Fg.5. The results of the electrc feld n the vcnty of the antenna Katherne , are presented n Fg. 4. As t can be easly noted, the approxmated results are n accordance wth the results receved from the analyss usng a full wavelength. The contnuous lnes represent the smulated values. The dashed lnes represent the measured values P a g e

4 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, Fg. 4. The electrc feld n the vcnty as per Z drecton To smplfy the comparson of the results between the computed values and the measured ones, we have prepared a table. Coordnates of the ponts A, A2, B, B2, C, C2, D, D2, E, and E2 as per the X, and Z axs are presented n Table I. In the last two columns are the results computed by applyng the proposed method smulated n Matlab, and the measured values. TABLE I. THE MEASURED AND ESTIMATED VALUES IN THE SELECTED POINTS OF INTEREST AS IN FIG. 4 Ponts X (m) Z (m) E estmated(v/m) E measured(v/m) A A B B C C D D E E In Fg. 5 are selected the ponts A0, A, A2, A3, B0, B, B2, B3, C0, C, D0, D, E0 and E. The respectve coordnates as per X and Y drectons are presented n the Table II. In the last two columns of the table are ntroduced the results of the electromagnetc feld, computed by usng the proposed method smulated n Matlab, and the measured values of the electromagnetc feld. Fg. 5. The electrc feld n vcnty as per Y drecton TABLE II. THE MEASURED AND ESTIMATED VALUES IN THE SELECTED POINTS OF INTEREST AS IN FIG. 5 Ponts X (m) Y (m) E estmated(v/m) E measured(v/m) A A A A B B B B C C D D E E As t can be noted by the above tables (TableI, and Table II) we can see a consderable approxmaton of the calculated values usng the proposed method, and the measured ones n the same ponts. The error between the two s less than 0% P a g e

5 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, V. CONCLUSION AND FUTURE WORK Ths paper ntroduced a smple but very effectve method n terms of calculatons, for the evaluaton of the electrc feld n the vcnty of the GSM antennas of a base staton. The method replaces the antenna panel wth a group of dscrete sources of emsson.the models represent a very smple tool for the estmaton of the electrc feld. The proposed method for the calculaton, requres pror knowledge about the antennas, usually provded by the manufacturer (emttng space n the horzontal plan and vertcal plan) Analyss and comparson between the calculated values and the measured ones concludes that the proposed method provdes accurate results for the feld close and far from the rado base statons n a gven urban area. The error between calculated and measured values s less than 0%. In an envronment wth presence of several rado base statons, the ntensty of the electrcal feld, magnetc feld and the densty of power for dfferent dstances from the antenna can be calculated n a short perod of tme provdng confdent and accurate results. Our work n the future wll focus n development of an algorthm for the evaluaton of the radaton of the feld n the presence of several rado base statons by usng the proposed theoretcal method. Ths algorthm for the calculaton of the electromagnetc feld wll requres, pror knowledge about the antennas, usually provded by the manufacturer (emttng space n the horzontal plan and vertcal plan). REFERENCES [] Z. Altman, A. Karwowsk, M. Wong, J. Wart and L. Gattouf, Dosmetrc analyss of base staton antennas va smulaton and measurements, 5th Internatonal Wroclaw Symposum of Electromagnetc Compablty EMC 2000, Part-, pp [2] A. Karwowsk, Z. Altman, Comparson of smple models for predctng radofquency felds n vcnty of base staton antennas, Vol. 36, no. 0, pp , th May 202. [3] A. Karwowsk and D. Wojck, Near feld of the GSM base staton antennas, Proc. Natonal Telecomuncatons Symp. KST 2004, Bydgoszcz, vol. 8, pp , Sept [4] P. Bemard, M. Cavagnaro, S. Psa and E. Puzz, Human exposure to rado base-staton antennas n urban envronment, IEEE Transactons on Mcrowave Theory and Technques, vol. 48, no., pp , Nov [5] C. Balans, Antenna theory analyss and desgn, pp , 998. [6] A. Marttnez-Gonzalez, A. Fenandez-Pascual, E. Delos, H. Sanchez and D. Hernandez, Practcal procedure for verfcaton of complance of dgtal moble base staton to lmtatons of exposure of the general publc to electromagnetc felds, IEEE Proc-McroWave and Antenna Propagaton, vo.49, no.4, Aug [7] KATHREIN-Werke KG, Techncal Informaton and New Products MHz Base Staton Antennas for Moble Communcatons, Catalogue Issue 04/08/0. [8] M. Barbrol, C. Carcof, V. Degl-Espost and G. Facasecca, Evaluaton of exposure levels generated by cellular systems: methodology and results, IEEE Transactons on Vehcular Technology, vol. 5, No. 6, pp , [9] Z. Altman, B.Begasse, C. Dale, A. Karwowsk, J. Wart and L. Gattouf, Effcent models for base staton antennas for human exposure assessment, IEEE Transactons on Electromagnetc Compatblty, vol. 44, No. 4, pp , Nov [0] I. Correa, C. Fernandes, G. Carpntero and C. Olvera, A procedure for estmaton and measurement of electromagnetc radaton n the presence of multple base statons, Insttuto Superor Técnco, Lsbon, Portugal, Sept [] M. Bzz and P. Ganola, Electromagnetc felds radated by GSM antennas, IEEE Transactons on Electromagnetc Compatblty, Electronc Letters, vol. 35, No., pp , May [2] Avalable at AUTHOR PROFILES Algent LALA receved hs Bachelor of Engneerng degree n 999 and a Masters n Telecommuncaton Engneerng from the Polytechnc Unversty of Trana (PUT) n He obtaned hs Ph.D. from PUT n 203. From 2008 he has been a Lecturer at the Department of Electroncs and Telecommuncatons, Faculty of Informaton Technology, at PUT. Hs research nterests le n the area of electromagnetc feld, wave propagaton, transmsson and antennas. In the latest years he has been focused on methods of calculaton of the electromagnetc feld n the vcnty of the cellular base statons. He has publshed several papers n referred Journals and Internatonal conference proceedngs. He s currently lecturer of several subjects ncludng Measurements of the rado frequences, Antennas and wave propagaton etc. He s scentfc tutor of master students, whle contnung hs scentfc research P a g e