A Dual-Band Unidirectional Coplanar Antenna for 2.4-5-GHz Wireless Applications

In recent years, a large number of dual-band antennas have been developed for wireless applications [1]-[6]. Most of these dual-band antennas are used for mobile terminals, such as cellular phones or laptop computers. For such applications, the desirable radiation patterns are usually omnidirectional. A unidirectional radiation pattern is sometime required for wireless base station and/or access point applications. For a dual-band antenna, a higher-order mode can be excited at the upper frequency band, which causes a radiation pattern with large ripples. In mobile communications environments, a large ripple may not be a problem. But for base station or access point applications, the large ripples can cause interference and reduce the antenna gain. A lot of techniques have proposed to suppress the higher-order mode, such as slotted patches [7]-[9] or notched patches [10]. Patch antennas normally have a narrow bandwidth. To enhance the bandwidth, it usually needs a stacked configuration [11][13]. A stacked dual-band patch antenna has high cross-polarization (>15 dB) and large ripples (>10 dB) at a higher frequency of the upper band. A printed dual-band dipole antenna was presented in [14]. The dipoles for the lower band and for the upper band have different heights above a ground plane and the total height of dual-band antenna is approximately a quarter wavelength (λLo/4) of the lower band. In this paper, we propose a coplanar dual-band unidirectional dipole antenna. The antenna height is only 0.1λLo. There is no ripple in the radiation patterns for the low band and fro the upper band. The antenna gain is about 9-10 dBi in the upper band and 7.5 dBi in the lower band.