Characteristics of a balance-fed loop antenna system for handsets in the vicinity of human head or hand

Abstract

ntroductionAntennas used in the handset must follow down-sizing of the handset unit. Even though the antenna size iecomes small, the antenna performance should not be degraded. Antennas for handset are mostly of the type laving unbalanced terminals and being fed by unbalanced lines, so that in almost all cases radiation currents ire induced on the conducting materials existing in the handset unit, as well as on the antenna element [I].rhese currents on such conducting materials contribute to improvement of the antenna performance, Jarticularly when the antenna element is very small. A design concept up to now has been to use these current iositively. However, since these currents are varied by the influence of the operator's hand and/or head, it vould cause significant variation that will result in degradation of the antenna performance. This implies that he antenna performance may be degraded sometimes seriously when placed at a talk position.In the previous paper [2], a loop antenna system, which has the balance structure and is fed with the 2alanced line in order to reduce the effect of currents on the conducting box, has been introduced and it is ,hown that the reduction of the currents on the ground plane is remarkable by using balanced system. In the malysis, a simple model consisting of a small loop antenna mounted on a ground plane is used and antenna :haracteristics are analyzed both theoretically and experimentally.In this paper, in order to confirm the effectiveness of using the balanced antenna structure, the performance Df the balance-fed antenna system for handset under the influence of a human head or hand is analyzed numerically and experimentally. As a human head model, COST (European Cooperation in the field of Scientific and Technical Research) 244 model [3], which has a spherical or cubic form, is used. The hand is simply simulated as the combined rectangular prism on the basis of the average size of human hands [4]. The zlectromagnetic IE3D simulator by Zeland Sofhvare is used to obtain the antenna characteristics such as current distribution and radiation pattern. Antenna structureAn antenna model is shown in Fig. 1. Fig. 1 illustrates a small rectangular loop on a rectangular ground plane and a feeding system of both unbalanced and balanced structures. The rectangular and L-shaped loops are taken into consideration. The rectangular and L-shaped loops have a perimeter of about a half wavelength and are placed very close to the rectangular ground plane, which has the perimeter of about two wavelengths. Both balanced and unbalanced types of feed are considered as Fig.1 (a) and (b). The center frequency& is I860MHz. Fig. 2 shows simulation models of the antenna placed near the human head or hand. As the human head model, COST 244 model is used and is the one of the normative human head model for the international standard [3]. The model has a spherical form with a diameter of 200 mm. The head model has the dielectric properties of the relative permittivity of 43.37 and the conductivity of 1.204...

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