of the semi-circular slot at 2.45-GHz. While at 5.5-GHz, current distributions can be seen around the semi-circular slot and the horizontal slot, in other words, the horizontal slot will have a significant influence on antenna's upper band. For the antenna ASA2, by comparison, a prime difference is that the current distributions are mainly focused on the outline of the F-shaped slits at 3.5-GHz and a new resonant mode is excited as a result. Another clear difference is that additional current distributions are found at the bottom of the annular-ring slot at 2.45-GHz. These differences help to explain the different characteristics of operating bands for ASA1 and ASA2.Figures 7(a)-7(c) show the measured radiation patterns of ASA2 in the E-plane (y-z plane) and H-plane (x-z plane) at 2.45-, 3.5-, and 5.5-GHz. The radiation characteristics of ASA1 are consistent with the above results so they are omitted here. The measured radiation patterns in the H-plane show nearly omni-directional characteristic and bi-directional patterns can be seen in the E-plane, that is, monopole-like radiation patterns are presented. As observed, the radiation patterns of ASA2 have high stability in the entire operating bandwidth.The measured gains of ASA1 and ASA2 are shown in Figure 8. For the dual-band antenna ASA1, the ranges of the gain are 1.6-2.0 dBi and 2.9-4.1 dBi at lower and upper bands. While for the triple-band antenna ASA2, the values of the peak gains are 2.1 dBi, 2.6 dBi, and 4.1 dBi at lower, middle, and upper bands, respectively.
CONCLUSIONAn anchor-shaped slot dual-band antenna for 2.4/5.2/5.8-GHz WLAN and 2.5/5.5-GHz WiMAX applications is presented. By introducing a pair of F-shaped slits, a new 3.5-GHz resonant mode will be excited. From the perspective of extensibility and flexibility, the dual-band antenna can be easily converted to a triple-band antenna for 2.4/5.2/5.8-GHz WLAN and 2.5/3.5/5.5-GHz WiMAX applications, without the need to modify other original design parameters. With the same size of 35 mm 3 30 mm 3 1.6 mm, both antennas can be easily integrated into portable communication devices. Furthermore, stable monopolelike radiation characteristics and sufficient gains can be obtained over the entire operating bandwidth. Therefore, the proposed anchor-shaped slot antennas can be selected as promising internal antenna candidates suitable for WLAN and WiMAX applications.
