# Design of Miniaturized Dual-Band Microstrip Antenna for WLAN Application
## 4 Design of Dual-Band Microstrip Antenna
### The Choice of Antenna Slotting Form  
The microstrip antenna can be slotted to achieve multi band, and the slot form is varied. The key of the design is to adopt the appropriate slotting form. The effects of different slot forms on the performance of the antenna are discussed in the following.  
The initial parameters of the antenna are unchanged, and the antenna is not slotted, the return loss of the antenna is shown in Figure 15. The antenna has three resonant frequencies--2.74 GHz, 5.46 GHz, and 6.25 GHz, which are all not in the desired frequency band. Therefore, the structure of the antenna must be improved.  
When the antenna is not slotted, the resonant frequency of the antenna is too high. However, the resonant frequency can be moved to the left with the method of slotting, so that the size of the antenna can be reduced. Accordingly, the two edges of the antenna are opened a narrow rectangular groove with a width of 1 mm, a length of W1, as shown in Figure 16a. Figure 16b illustrates the parameter \(S_{11}\) of the antenna under this circumstance. The antenna resonates in the three frequency bands of 2.6 GHz, 5.45 GHz and 6.08 GHz. It can be seen that the frequency of the antenna, to some extent, moves left, but still does not resonate in the required frequency band. Increasing W1 can further reduce the operating frequency of the antenna, but the antenna gain will also be reduced, and the directional diagram will become worse to some extent. Hence, we can consider increasing the width of the gap.  
Figure 14: Antenna structure.  
Figure 15: Parameter \(S_{11}\) of the antenna without a slot.  
By adjusting the width W1 and length L1 of the slot, it can be seen from the parameter \(S_{11}\) of the antenna that the resonance frequency of the antenna will be reduced as the size of the antenna is increased, as shown in Figure 17. Figure 18 describes the \(S_{11}\) parameter curve of the optimized antenna. At this point, L1 = 9 mm, W1 = 3.7 mm, the resonance frequency of the antenna is 2.43 GHz, 5.25 GHz and 6.05 GHz, covering 2.4 GHz and 5.2 GHz operating frequency bands of WLAN. However, there is a useless resonance frequency (6.05 GHz), which will affect the normal operation. Thus, the following consideration is employing other forms to try to eliminate this effect.  
Figure 16: (a) Antenna structure with a rectangular slot; (b) The parameter \(S_{11}\) of the antenna with a rectangular slot.  
Figure 17: (a) The parameter \(S_{11}\) varies with L1 when opening the rectangular slot; (b) The parameter \(S_{11}\) varies with W1 when opening the rectangular slot.  
Figure 18: \(S_{11}\) parameter curve of the optimized antenna.  
The rectangular slot can obtain two frequency points that are required, but meanwhile, there will be unwanted frequency points that cannot be removed, so we switch to the ladder type slot as shown in Figure 14. By properly adjusting the size of the slot, the performance of the antenna can be obtained, as shown in Figure 19. The antenna has three resonant frequencies--2.44 GHz, 5.24 GHz and 6.2 GHz. The third frequency of the ladder type is larger than that of the rectangular slot, but it is still possible to have a passive effect on the performance of the antenna.