FREQUENCY-TUNABLE INTERNAL ANTENNA FOR MOBILE PHONES Outi Kivekäs, Jani Ollikainen, and Pertti Vainikainen Helsinki University of Technology, Institute of Digital Communications, Radio Laboratory, P.O. Box 3000, FIN-02015 HUT, Finland e-mail: outi.kivekas@hut.fi Abstract – New approaches are needed for improving the performance of small antennas to fulfill the multiband operation requirements of future wireless mobile communications terminals. In this paper, a novel low-loss frequency- tuning circuit for mobile handset antennas is proposed. The presented design takes into account several factors that affect the practical mobile handset antenna design, such as the biasing limitations and distortion of the switching component as well as the effect of the mobile handset -sized ground plane. An antenna prototype that is capable of switching between the US cellular and GSM systems at 800-900 MHz frequency range was designed and measured. The antenna was positioned on a metallized printed circuit board (PCB) having size equal to that of a typical mobile phone. The tuning circuit, consisting of transmission line sections and an SPDT (single-pole, double-throw) FET switch, was fabricated directly on the substrate of the PCB. The designed antenna has high radiation efficiency and low distortion in both system bands. 1 INTRODUCTION Microstrip patch antennas are commonly used in mobile communications terminals due to their many attractive features, such as simple structure, low production cost, light weight, and robustness. Dual-frequency antenna elements are required, as today’s standard mobile terminals operate in two frequency bands, e.g. GSM900/GSM1800 in Europe. It is desirable to have more universal phones that operate in multiple systems around the world, but the inherently narrow impedance bandwidth of patch antennas combined with the restricted volume for the antenna element limit their applicability in multiband phones. One way to overcome the bandwidth limitation is to increase the effective bandwidth of the antenna element by tuning its resonant frequency, and thus the operation band, between different communication systems. This can be realized by loading the antenna with a reactive tuning component, which can be either an electrically controlled reactance or a passive reactance with a switching component. One common method is to connect a tunable reactance or switch between the patch and the ground plane, as first proposed in [1]. Another approach is to use the tuning component for connecting separate parts of the patch [2, 3]. In these cases the tuning components are typically placed at high RF voltage [1] or current [2, 3] locations to maximize the obtained frequency shift. This may result in significant losses in the tuning component [4]. Also, many important factors, which may restrict the use and performance of certain tuning circuits in mobile terminal antennas, have been given less attention in previously published designs. These limiting factors are e.g. the available dc-bias and distortion of the switching component as well as the effect of the mobile handset -sized ground plane on bandwidth [5-7]. Recently, a theoretical analysis on the minimization of power loss in frequency tuning circuits of small resonant antennas has been presented [8]. To support the theory, the design of frequency-tunable patch antennas mounted on large ground planes was demonstrated. In this paper, the same basic principles were adapted to the design of a frequency-tuning circuit for practical mobile handset antenna. A tuning circuit was added to a previously published dual-band antenna element [9] for the European GSM bands (880-960 MHz, 1710-1880 MHz) to cover also the US cellular system band (824-894 MHz) with the same antenna. The tuning circuit consisted of low-loss transmission line sections and an SPDT FET switch, which has suitable characteristics for use in real mobile phones. The antenna was positioned on a metallized PCB having dimensions 110 mm × 40 mm, thus representing the PCB of a typical mobile phone. Both simulated and measured results for the design are presented. 2 ANTENNA STRUCTURE AND DESIGN The electrical properties of a handset antenna, especially the bandwidth, are known to depend strongly on the size of the ground plane of the device on which the antenna is mounted (phone chassis) and the position of the antenna on it [5-7]. Based on this, the total radiation bandwidth of the antenna-chassis combination is partly defined by the dipole-type radiation of the chassis currents, whose level further depends on whether the chassis is at resonance or not [6, 7]. If the chassis resonates at the operating frequency of the antenna element, the effective unloaded quality factor of the antenna- chassis combination (Q 0,eff ) decreases, and the bandwidth increases strongly. When an antenna, which is positioned on a chassis having size equal to that of a typical mobile phone, is tuned from the GSM900 band down to the US cellular © 2002 SEE-GRéCA. Reprinted with permission from Proceedings of the 12èmes Journées Internationales de Nice sur les Antennes, 12th International Symposium on Antennas (JINA 2002). Nice, France, 12-14 November 2002, volume 2, pages 53-56 (CD-ROM, paper 124.pdf).