Shorted, folded planar monopole antenna for dual-band mobile phone

Abstract

The geometry of a dual-band DTHA for EGSM/DCS applications is shown in Figure 1. It consists of two tailed helices mounted concentrically on a bottom piece for 50⍀ coaxial-line feeding. The radius, tail length, and height of the exterior helix are 3, 25, and 6 mm, respectively. For the interior helix, the radius, tail length, and height are 1.5, 10, and 6 mm, respectively. In addition, the wire diameters are 0.6 and 0.4 mm for the exterior and the interior helices, respectively. The thickness of the bottom piece is 0.5 mm. The bottom piece of the DTHA is mounted on a grounded FR4 substrate that is 76 ϫ 40 mm, which represents a mobile-handset circuit board. The FR4 substrate has a thickness of 0.8 mm and a dielectric constant of 4.4.The aim of employing the exterior and interior helices is clear: the exterior helix is designed to generate the lower resonant mode, while the interior helix is designed to generate the higher resonant mode. The total lengths of the exterior and interior helices are about a quarter-wavelength of the lower and higher resonant frequencies, respectively. Making the exterior and interior helices helical serves the purpose of obtaining a broader impedance bandwidth and reducing the size. However, the pitches of the helices cannot be too short to sacrifice the broadband performance. In addition, the spacing between the two helices should be large enough to avoid excessive coupling between them. It is worth mentioning that the DTHA is not sensitive to the parasitic effect induced by the handset ground. Therefore, its electrical performance does not depend on an antenna mounting position at the top edge of the ground. EXPERIMENTAL RESULTSA prototype of a dual-band DTHA was constructed, measured, and studied. This antenna was designed for EGSM (880 -960-MHz)/ DCS (1710 -1880-MHz) dual-band applications. To optimize the design parameters, a 3D full-wave simulator was used. After the simulated optimum was obtained, the prototype was constructed. The reflection loss and radiation patterns were measured and discussed. Figure 2 shows the simulated and measured reflection-loss results. The resonant frequencies are indeed located at the EGSM and DCS bands. The measured bandwidths (determined by 2.5:1 VSWR) are about 110 and 220 MHz at the EGSM and DCS bands, respectively. The bandwidths are very sufficient to meet the bandwidth requirements of the EGSM/DCS systems.The measured radiation patterns are presented in Figures 3 and 4 for the resonant frequencies at the EGSM and DCS bands, respectively. For both resonant frequencies, good omnidirectional radiation is observed. The maximum antenna gain for both the vertical and horizontal patterns is estimated to be about 1.97 and 2.43 dBi across the EGSM and DCS bands, respectively. CONCLUSIONA novel DTHA design has been proposed and a dual-band DTHA prototype has been shown as an example. It has been demonstrated that the resonant modes at the EGSM and DCS bands can be excited. As the measured and simulated results show, not only the radiation characteris...

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