Self-balanced and Wideband Characteristics of Improved Built-in Folded Dipole Antenna for Handsets

In conventional antennas, such as a monopole antenna and a planar inverted-F antenna (PIFA) for handsets, some gain degradation has been often observed when the operator holds the handsets at a talk position. This is caused by the variation of the current on the conducting box used in the handset due to human body effect. Because of this point, we have to consider reducing the current flow on the ground plane (GP). In the previous paper [1], Built-in Folded Dipole Antenna (BFDA) for handsets having a one-wavelength loop structure was introduced and analyzed. BFDA consists of a folded loop element placed horizontally on the top of GP so that the height of an antenna is reduced up to 20% in comparison with a folded loop antenna and it can place very close to the rectangular GP, which represents a shielding plate used in the handset unit. In addition, it has been shown that this antenna has a self-balanced structure not to produce unbalanced current on the feed line and is useful to reduce the current flow on the GP. Furthermore, the services (IMT-2000, Bluetooh, W-LAN, WiMAX) for the cellular phone and PHS are recently increasing. So the wider bandwidth antenna for handsets is required. The relative bandwidths of BFDA which is fed by coaxial cable was reported approximately 400 MHz (13.0%) at the center frequency 2,900 MHz. In this paper, the parameters related to the feed structure are optimized in order to obtain a wider bandwidth for the BFDA system, and the fundamental performances are analyzed in both theoretically and experimentally.