# Dual-Band Monopole Antenna for RFID Applications
## 3 Results and Discussions  
The motive behind the presented design is to achieve a dual-band characteristic for use in RFID applications. This has been achieved by using the presented antenna design with a modified radiation patch. Return loss characteristics of the rectangular monopole antenna (Figure 2a), the antenna with a \(\mathrm{\Gamma}\)-shaped radiating patch (Figure 2b), and the antenna with a modified F-shaped radiator (Figure 2c) are illustrated compared in Figure 3.  
It is observed that the lower frequency bandwidth (2.2-2.6 GHz) with a resonance at 2.6 GHz is affected by \(\Gamma\)-shaped structure, and the upper-frequency bandwidth (5.2-6.8 GHz), resonating at 5.8 GHz, is created and improved by adding a short rectangular strip, which modifies the radiation patch to the F-shaped structure. The optimized length L\({}_{\text{resonance}}\) is set to resonate around 0.25\(\uplambda_{\text{resonance}}\), where L\({}_{\text{resonancel}}\) = L\({}_{3}\) + L\({}_{2}\) + L + L\({}_{4}\) + (W - W\({}_{3}\)) +L\({}_{1}\) and L\({}_{\text{resonance2}}\) = L\({}_{3}\) + W\({}_{2}\) + W\({}_{\text{f}}\)/2 + L\({}_{2}\). \(\uplambda_{\text{resonance1}}\) and \(\uplambda_{\text{resonance2}}\) correspond to the first and second resonance frequencies (2.4 GHz and 5.8 GHz), respectively. The voltage standing wave ratio (VSWR) and Smith-Chart results of the antenna are represented in Figure 4. It is seen that the antenna properly resonates at 2.4 and 5.8 GHz. As shown, it operates from 2.2 to 2.6 GHz (400 MHz impedance-bandwidth) and from 5.3 to 6.8 GHz (1500 MHz impedance-bandwidth). In addition, the reflected phase of the antenna S\({}_{11}\) and its real and imaginary parts are illustrated in Figure 5, respectively. As seen, the effects of the resonances at 2.4/5.8 GHz are evident in the provided results. In order to further understand the principle of the dual-band characteristic, simulated surface current densities for the resonant frequencies on the radiation patch and the ground plane are illustrated in Figure 6. As seen in Figure 6a, at 2.4 GHz, the current flow contrari  
Figure 4: (a) VSWR and (b) Smith-Chart result of the antenna. Figure 5: (a) Phase, (b) real and imaginary parts of the antenna. resonator (the main arm of the modified F-shaped radiation patch). At 5.8 GHz, the current flows are highly dominated around the second arm of the radiation patch, as illustrated in Figure 6b. Figure 6c,d show the current distributions in the ground plane of the proposed design at the resonant frequencies: the current flow concentrates around the top edge of the ground plane. In addition, it is observed that at 5.8 GHz, the direction of current flows changes at the edges of the ground plane in comparison to that at the first resonance (2.4 GHz). Results of varying fundamental design parameters, W\({}_{1}\), W\({}_{2}\), L, L\({}_{1}\), L\({}_{2}\), L\({}_{3}\), L\({}_{4}\), and L\({}_{\text{gnd}}\) are shown in Figure 7a-h. Figure 7a,b shows the effects of L and L\({}_{2}\) on the impedance matching; the operation frequency bands of the antenna are not affected significantly. In contrast, as shown in Figure 7c,d, the return loss characteristic of the antenna can be tuned at both of the resonant frequencies for different values of L\({}_{1}\) and W\({}_{1}\). Figure 7e,f illustrates the impact of different values for L\({}_{3}\) and W\({}_{2}\) on the antenna operation frequency; they have significant impacts on the second resonant frequency (5.8 GHz) with  
Figure 6: Current distributions at (**a**) 2.4 GHz and (**b**) 5.8 GHz on the radiation patch, and (**c**) 2.4 GHz and (**d**) 5.8 GHz in the ground plane. very little impact on the first resonant frequency (2.4 GHz). It is observed from Figure 7g,h that the isolation and impedance-matching characteristics of the antenna at both resonant frequencies are highly depended on values of L\({}_{\text{gnd}}\) and L\({}_{\text{4}}\). Figure 8 illustrates the antenna fundamental radiation characteristics over the operation bands.