Contents I. Introduction
The DTB (Digital Terrestrial Broadcasting) service for mobile devices has been widespread. A built-in antenna for DTB reception is demanded for durable and neat mobile devices. However, such an antenna usually has the demerits of narrower operation bandwidth and lower radiation efficiency because the wavelength of the DTB band, 470 MHz through 710 MHz in Japan, is much longer than the size of mobile devices. As a technique generally useful for wideband characteristics, tunable antennas using variable capacitors have been developed [1]–[3]. But as the value of the capacitor becomes larger, the operation frequency becomes lower and the efficiency seriously degrades because the power consumption in the parasitic resistance of the capacitor becomes larger due to the increase of the current in the capacitor. Some papers report that the RF current on the printed circuit boards of the mobile device can be employed to enhance the efficiency [2], [3].
