## 2 Antenna Geometry and Design  
The geometry of the proposed antenna is shown in Figure 1. The presented antenna consists of a coupling microstrip line, two F-shaped strips, two rectangular strips and a defected ground plane. The coupling microstrip line is printed on the top side of the substrate. The two F-shaped strips, two rectangular strips and the defected ground plane are printed on the bottom side of the substrate. The coupling microstrip line is fed by a coaxial SubMiniature version A (SMA) connector through the ground plane. The proposed antenna is fabricated on a 50 mm \(\times\) 69 mm FR4 substrate with relative permittivity \(\varepsilon\)r of 4.4, loss tan \(\delta\) of 0.02, and thickness of 1.6 mm.  
The defected ground plane is connected to two symmetrical rectangular strips and two symmetrical F-shaped strips. A rectangular slit is etched in the middle of the ground plane  
Figure 1: Geometry of the proposed antenna. (**a**) Top view; (**b**) Bottom view.  
to enhance the impedance matching and realize a high gain of the directional antenna. In addition, the coupling between the rectangular microstrip line and rectangular slit results in good directional radiation characteristics. In order to achieve the dual-band characteristics of the presented antenna, two symmetrical F-shaped strips are introduced into the ground plane near the upper part of the rectangular slit. A rectangular strip with a length of L7 and a width of W8 is added to the F-shaped structure to provide impedance matching at the desired frequency bands. The two rectangular strips and F-shaped strips are used to obtain good impedance matching in the lower and higher frequency bands, respectively.  
A photograph of the manufactured antenna is shown in Figure 2.  
The design and optimization of the entire antenna are completed with the 3D full-wave high frequency structural simulator (HFSS). In order to further study the influence of the ground plane on the proposed antenna, the current distribution of the antenna at different frequency bands has been investigated. The simulated current distribution at 2.4 GHz and 5.5 GHz is shown in Figure 3. As shown in Figure 3, the surface currents are concentrated on the edge of the rectangular strips at 2.4 GHz. The current mainly flows along the two F-shaped strips at 5.5 GHz, as shown in Figure 3. Therefore, we can adjust the dimension of the rectangular strips and the F-shaped strips to generate a lower and higher resonant frequency, respectively. In Figure 3, it can be seen that most of the current is concentrated around the rectangular slit. The current in the vertical direction is in the opposite direction, which can cancel part of the radiation. The current in the horizontal direction is almost in the same direction, which can enhance part of the radiation. Therefore, the characteristics of directional radiation can be realized.  
According to the results of the current distribution diagram, the antenna can be optimized by adjusting the dimension of the F-shaped strips and the rectangular strips. During the optimization process, one geometric parameter is adjusted, and the other parameters remain fixed. Figure 4,b illustrate the reflection coefficients of the presented antenna with different values of L7 and L4, respectively. As shown in Figure 4, the value of L7 mainly affects the lower frequency band. It can be seen that when L7 is 18 mm, the proposed antenna operates at the desired lower frequency band. Figure 4 shows the effect of different values of L4 on the higher frequency band. When L4 is 6.5 mm, the desired  
Figure 2: Photograph of the manufactured antenna. (**a**) Top view; (**b**) Bottom view.