# Compact Dual-Band Tapered Open-Ended Slot-Loop Antenna For Energy Harvesting Systems
## 2 Antenna Design Architecture
### Parametric Sweep Analysis  
In order to understand the proper operation of the proposed antenna, a parametric study is examined, as shown in Figure 2. As mentioned earlier, a parallel L strip line is inserted alongside the main radiator, so the antenna configuration can produce a dual-band feature. The length of such transmission line is \(L_{p1}\) and is optimized for the best performance. This parameter is varied from 25 to 45 mm with a step of 5 mm. The corresponding S11 is shown in Figure 2a. It is noted from the figure that, the lower resonance frequency is significantly affected by changing \(L_{p1}\). In the same manner, the higher resonance frequency has no change but the matching is improved. The value of \(L_{p1}\) is optimized to be 41 mm, which produces the best matching condition at 0.92 and 2.4 GHz.  
Similarly, the parameters of the OES antenna are studied. The length of the open stub \(L_{p6}\) and slot separation width \(S_{3}\) are tuned, as given in Figure 2b,c. Both of the parameters affect the impedance matching level at the higher frequency. As expected, there is a minimal change of S11 at the lower frequency where both parameters significantly affect the antenna performance at the higher frequency band. The optimized values of \(L_{p6}\) and \(S_{3}\) are given as 26 and 2 mm, respectively. For further clarification, two antennas were developed, so that we can simply understand the antenna operation. The S11 of the Ant.1 and Ant.2 is illustrated in Figure 3a. The blue curve represents Ant.1. As can be observed, this antenna does not utilize an L-section, so it produces a single resonance at 2.4 GHz. In order to allow this configuration exhibit a second resonance at 0.92 GHz, rectangle L metal strip is inserted to form the OEL antenna (Ant. 2). This L-strip is extended alongside with the main radiator and ended with a separation gap of 0.3 mm. This gap acts as a capacitive reactance, so the total length of the radiator at the resonance frequency can be reduced by approximately 45%.  
Figure 3: Performance of the antenna structure, (**a**) Reflection coefficients of Ant. 1 and Ant. 2, where Ant.1 represents the nominal case without the side arm that is parallel to the main radiator, and Ant. 2 represents the developed antenna structure to achieve dual-band operation by exploiting a parallel segment to the main trapezoidal radiator; (**b**) surface current of the antenna at 0.92 GHz and 2.45 GHz.  
Figure 2: Parametric study of the open-ended slot antenna, (**a**) parametric study of the side-arm length, \(L_{p1}\) is the length of the arm; (**b**) variation of the open stub length, \(L_{p6}\) which adjust the level of impedance matching; and, (**c**) changing of the slot width \(S_{3}\), this parameter doesn’t alter the resonance frequency, but it controls the impedance matching.  
The surface current distribution is indicated Figure 3b to investigate the radiation pattern. The current is basically concentrated inside the loop antenna and almost no current flow in the ground plane. This explains that the proposed antenna behaves as a loop antenna at the lower band frequency. On the other hand, the surface current distribution at the higher frequency is illustrated in the right side of Figure 3b. It is observed that the current is flowing through the main radiator and ground plane. The L-arm of the main radiator acts as a director that concentrates beam in the end-fire direction at the higher band.  
The radiation efficiency of the antenna is demonstrated using the following method. The proposed antenna is considered as a small antenna because it achieves the condition of small antennas ka < 1 (where \(k=\frac{2\times\pi}{A}\) and a is the half of maximum antenna dimensions). Therefore, the Wheeler cap method has been selected to calculate the measured radiation efficiency for the proposed antenna [25; 26; 27]. The radiation efficiency of the proposed antenna is shown in Figure 4c. In the lower band, the average peak efficiency is less than 70%, while the higher band efficiency exceeds 70%.