HANDSET PHONE-USER INTERACTIONS: POLARIZATION EFFECTSD. Picard, L. Ahlonsou, J. Ch. BolomeyService Electromagnettisme/SUPELEC

Plateau de Moulon/91192 Gif sur Yvette Cedex/FRANCETel: 33 01 69 85 15 55 / Fax: 33 01 69 41 30 60

E-MAIL: Dominique.Picard@supelec.fr

Abstract: The effect of the user on the radiation pattern of a cellular phone is investigatedexperimentally. More particularly, polarization aspects are considered. The user is simulated by abiological phantom to provide simple and repetable measurements. This allows to observe theinfluence of different parameters such as glasses, phone position... The use of near fieldtechnique permits easely to have the knowledge of the field in all direction. One result of thisstudy is the polarization analysis of the field radiated by a phone with and without the user.

I) INTRODUCTIONThe far field pattern of an handset phone is modified by the presence of the user. The size of theuser and the phone-user configuration are very variable. That is why it is necessary to study theglobal radiation pattern of the phone and the user considered as an equivalent antenna, as afunction of several parameter such as:- the phone-user relative position,- the size of the user,- the presence of some objects as glasses, clothes.- the kind of phone and antenna,- the frequency.To have repetable measurements, a biological phantom simulates the user. For obtaining the farfield pattems in all directions and polarization and for simplifying the experimental procedurenear-field techniques are used.

H) EXPERMIENTAL SETUPThe biological phantoms are made with polyurethane mannequin filled with a liquid whichsimulates the electromagnetic properties of the muscle. This liquid contains water, propane-dioland NaCl salt. Its relative dielectric constant is s'r=55 for the real part and '"r=24 for theimaginary part [1]. This biological phantoms have been validated in term of their effect on themodification of the reflection coefficient of the phone and on the radiated field. There is a goodagreement between the biological phantoms and human leaves. For an example the difference onthe radiated field is of the order of IdB, the same value than repetability on the same person orbetween two different persons [1].Near-field techniques consist in the measurement of the tangential components of the field on asurface containing the radiating object. The measurement allows the calculation of the coefficientof the modal expansion of the field. Then this modal expansion permits to calculate this fieldeverywhere and in the far zone. The surface can be: a plane, a cylinder or a sphere.The SUPELEC near-field range uses the cylindrical coordinates (figure 1). The choosen cylinderis about 4m high and his radius is 0,8m. The measurement cylinder is not infinite resulting introncature error on the far-field: the radiated power is not intercepted around the direction of thecylinder axis and for a given error on the far field there is a size limitation on the solid angle(following the elevation 0) in which the far-field is accurately known. Here for IdB peak errorthe elevation can vary from about 200 to 160 [2] [3].The vertical (Ez) and azimuthal (EO) near-field components are transformed in EO and E4 farfield components. The modulus and the phase of these two far field components allow thecalculation of the characteristics of the polarization ellipse. The polarization ratio t is defined asthe ratio of the small radius by the large radius of the ellipse (figure 2). The tilt angle a is theangle between the large radius and the eO axis. The rotation sense is given here by the sign ofthe ellipticity ratio.The effect of the measurement cylinder troncature on the polarization ratio rand the tilt angle a is respectively an error of a few percents and a few degrees [3].

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III) RESULTSThe operating frequency is 900MHz. The phone is located on the user in the direction of theazimuth angle 0=900. Its inclination from the vertical axis is 600.

1) User effectThe comparison between the field modulus with and without the user confirms a generaldecrease of the radiated power in the presence of the user (figure 3.a). The radiated power iscalculated by the mean of the radiated far field in all directions (o°<4K360', 00<0<1600). Theradiated power decreasing with the user is about 40% in agreement with previous numericalcalculations (e.g. [4],[5]). The polarization ratio r shows a decrease of the rectilinear polarization(t=0) in the presence of the user (figure 3.b). The tilt angle is also modified (figure 3.c). Theuser head mask effect is visible on the figure 4, around 4=270. More generally, it appears thatthe user head contributes to statistically disperse the polarisation of the radiated field.

2) Hand effectIn the precedent case the biological phantom has no hand. The radiated far-field with and withoutthe hand which held the phone is showed in figure 5. The main effect is the deceased level inpresence of the hand (about 1 or 2dB).

3) Phone axis inclination effectThree different phone axis inclinations have been studied: 30°, 600 (classical value) and 90° fromvertical axis. The main value of the total radiated far-field is quite the same in these differentcases (figure 6). The repartition of the radiated power between E4 and EO is variable:- for the 300 inclination the main component is ES, F4 is typically 6dB below,- for the 600 inclination the main component is E4, EO is typically 6dB below,- for the 600 inclination the main component is E4, EO is typically lOdB below.

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IV) CONCLUSIONThe use of biological phantoms and near-field techniques have been efficiently used tocharacterize the radiated far field of a cellular phone and the user effect. Expected extensions ofthis work are:- measurements in the other cellular frequency ranges,- statistical study and processing of the results,- defimition of an equivalent source model to the equivalent user-phone antenna for improving theprediction of power budget in in-door or urban environments.

Acknowledgment:This work was supported by France Telecom/CNET under grant n°94 lB 087.BiBLIOGRAPHIE[1] L. Ahlonsou, D. Picard, J. Ch. Bolomey, "Advanced mannequin phantoms for realistic assesment ofinteraction between a cellular telephone and its user via near-field techniques", Wireless 96, Vol2, pp.626-639,Calgary Canada, July 1996.[2] D. Picard, L. Ahlonsou, J. Ch. Bolomey, " Caractdrisation du rayonnement de t6ldphones portatifs en vue de lapr6diction des bilans de liaison dans des environnements complexes", Contract repport, contract CNET n"94 lB087, july 1996.[3] L. Ahlonsou, D. Picard, J. Ch. Bolomey, "Caract6risation du rayonnement tEl6phone cellulaire/usager par destechniques de chamnp proche cylindrique: Etude de l'erreur de troncature spatiale", JNM 97, Saint Malo, France,May 1997.[41 J. Toftgard, S. Hornsleth, J.B. Andersen, "Effects on portable antennas in the presence of a person", IEEETrans. on Ant. and Prop., AP-41, n°6,pp. 739-746, June 1993.[5] M. A. Jensen, Y. Rahmat-Samii, "EM interaction of handset antennas and a human in personnalcommunications", Proc. IEEE, Vol. 83, pp.7-17, 1995.

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