# Electric-Field-Coupled Resonator Antenna for 5G Applications
## 4 Performance Analysis of the ELC Antenna  
From the above analysis, it is clear that when the resonator is used as the radiating element of an antenna, it will exhibit the same resonance characteristics provided the substrate material and its thickness, as well as their orientation, are the same for both cases. The same orientation can be achieved when the structure is excited by a microstrip feed line in a way so that the polarization of the electric field will be in the same direction. Since Setup 1 causes low-frequency resonance, the antenna will be miniaturized if the ELC is excited in the way shown in Figure 6a because then the electric field will be polarized along the capacitive gap as in Setup 1. However, antenna simulation shows that when the ELC is excited as shown in Figure 6a, then two resonances occur at almost the same frequencies (2.88 GHz and 7.96 GHz) as in Setup 1, but they are matched with different ground plane sizes as depicted in Figure 6b. For compactness, the ground plane size L6 = 7.5 is chosen. However, this antenna results in a narrow bandwidth (10.41%) and poor gain (0.07 dBi). The performance of Antenna 1 is improved at the desired frequency region by some evolutionary steps as shown in Figure 7.  
Since the antenna is designed for the 5G C band applications, the resonant frequency needs to be shifted to the higher frequency region. It can be achieved by reducing the length and width, reducing the electrical length, or by decreasing the gap capacitance of Antenna 1. The first method is avoided since it can cause an even less efficient radiator. However, the second method is implemented in two steps. Figure 5: Distribution of (**a**) electric field and (**b**) current densities at 4.23 GHz for Setup 2. Figure 6: (**a**) Electric field distribution of Antenna 1. (**b**) Simulated resonance characteristics of Antenna 1 for different ground plane sizes. Firstly, the capacitive gap is moved near to the left side, which changes the current path inside the structure, and, secondly, an inductive strip is connected in parallel near to the right side of the structure (Antenna 2 in Figure 7b). As a result, the electrical length, responsible for the low-frequency resonance, is reduced, which shifts the resonance to the higher frequency region as shown in Figure 8a. However, the second method has a negligible effect on bandwidth, but the gain is increased considerably as shown in Figure 8b. The third method is applied by increasing the capacitive gap width of the resonator (Antenna 3 in Figure 7c), which causes the gap capacitance to decrease. For this reason, the resonance is shifted to the higher frequency region (Figure 8a). A parametric study on the gap width is shown in Figure 9a to demonstrate the effect of gap capacitance on reflection  
Figure 8: (a) Comparison of S-parameters of the antennas. (b) Comparison of gains of the antennas. Figure 7: Patch of (a) Antenna 1; (b) Antenna 2; (c) Antenna 3; (d) Antenna 4.  
coefficients. The study shows that increasing the capacitive gap shifts the resonance to the higher frequency region and increases the bandwidth as well. However, the effect of gap width on antenna bandwidth can be easily realized by the parametric studies of various gap widths with real and imaginary values of the input impedance (see Figure 9b,c). As the gap width increases, the real impedance value decreases and the imaginary value flatters at high frequency, which causes a wide bandwidth. This happens because the electric field between the microstrip feed line and the ground plane decreases as the gap width increases, which in turn decreases the coupling capacitance between the feed and the ground, which is shown in Figure 9d. The optimized value of the gap width (L6) is chosen to be 3 mm. Further increment of the gap width results in poor impedance matching and the bandwidth enhancement is also negligible. Finally, the gain of the antenna is improved by adding two rectangular metallic blocks of the same size on both sides of the microstrip feed line (Antenna 4 in Figure 7d).