A Novel Wideband Built-in Antenna with Resonance Frequency Variability for Digital Terrestrial Broadcasting (2006)

1. Introduction terrestrial broadcasting service for mobile users which is to launch in Japan by 2006 will use the UHF band from 470MHz to 770MHz. From the viewpoint of external appearance of a mobile phone terminal, a built-in antenna has recently become a requirement in Japan and is also expected to apply as the antenna to receive the digital terrestrial broadcasting service. However, it is difficult to realize a built-in antenna for the UHF band because the wavelength of the UHF band is much longer than the length of a mobile phone terminal and a frequency bandwidth of an antenna generally becomes narrow if the antenna is small and built into a mobile phone terminal. Therefore, it is necessary to make the built-in antenna wideband. As a technique for reducing the volume of an antenna, the usage of the boards of the mobile phone terminal has been developed [1]. In this paper, we propose a novel antenna which is small due to the usage of the boards of the mobile phone terminal and the usage of a variable capacitance applied to the antenna element. Applying the variable capacitance, the resonant frequency of this antenna changes, so the antenna is expected to have a wideband performance. We calculated and measured the properties of the proposed antenna, and verified the availability of this antenna. 2. Proposed antenna Figure 1 shows the overall view of the antenna model used in this work and Fig. 2 shows the enlarged view of this antenna. This antenna is located at the top of the upper board of a clamshell mobile phone, and the antenna element shapes a meander structure for a wideband property and a built-in structure, and the midpoint of the meander structure connects to the upper board through a variable capacitance. This antenna resonates at the frequency corresponding to the total length of the antenna elements and upper and lower boards. The resonant frequency of the antenna changes with the capacity of the variable capacitance. 3. Simulation results First, we calculated the impedance characteristics of this proposed antenna with the variable capacitance. The calculation model of the antenna is shown in Fig. 3. The upper and lower boards of this antenna are composed of linear elements. The simulation tool is the NEC2 (Numerical Electromagnetic Code 2) based on the moment method. Impedance characteristics of the antenna were calculated in the frequency from 400MHz to 800MHz. VSWRs of the antenna with the variation of the variable capacitance from 0.55pF to 2.3pF are shown in Fig. 4. Figure 4 shows that the frequency bandwidth of 310MHz (from 450MHz to 760MHz) can be obtained by the changes of the capacity of the variable capacitance although the frequency bandwidth of an antenna with fixed capacity 1.5pF is only 130MHz, where the frequency bandwidth is specified as VSWR<3. 4. Experimental results Second, the proposed antenna was fabricated and measured in order to verify the properties of this antenna. A diode (Toshiba JDV2S17S) for the variable capacitance of the antenna was used. The experimental model antenna is shown in Fig. 5-(a) and the circuit around the variable capacitance diode is shown in Fig. 5-(b). In Fig. 5-(b), the symbol VD indicates a variable capacitance diode, the