# Design of Multi-Mode Antenna Array for Use in Next-Generation Mobile Handsets
## 3 Characteristics of the Single-Element/Multi-Band PIFA Resonator  
The PIFA is a compact size antenna radiator that provides omnidirectional radiation patterns and can be used in hand-held devices [46; 47; 48]. The conventional PIFAs exhibit single-band operation. However, the modified designs can cover multi-frequency bands for multi-mode operation [49]. The configuration of the PIFA element is depicted in Figure 2a. Its structure is composed of an open-loop resonator with an L-shaped strip protruding from the ground plane. As shown, it has a low profile with the dimension of W \(\times\) L. A 50 \(\Omega\) discrete feeding port is employed to excite the antenna. The computer simulation technology (CST) software is used to investigate the properties of the designed mobile-handset antenna [50]. Figure 1: (**a**) Side and (**b**) back views of the multi-mode antenna array. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Parameter** & \(\mathbf{W_{X}}\) & \(\mathbf{L_{X}}\) & \(\mathbf{W_{XI}}\) & \(\mathbf{L_{XI}}\) & \(\mathbf{W_{X2}}\) & \(\mathbf{W}\) & \(\mathbf{L}\) & \(\mathbf{W_{f}}\) \\ \hline
**Value (mm)** & 150 & 75 & 83 & 18 & 5 & 20.5 & 6.5 & 1 \\
**Parameter** & L\({}_{\text{f}}\) & W\({}_{1}\) & L\({}_{1}\) & W\({}_{2}\) & L\({}_{2}\) & W\({}_{3}\) & L\({}_{3}\) & L\({}_{4}\) \\
**Value (mm)** & 1 & 1 & 15.3 & 3.5 & 14.3 & 16 & 5.5 & 4.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The dimension values of the presented multi-mode array. The main motive behind the modified PIFA is to obtain a compact antenna element which can support different frequencies and could be integrated with a mainboard circuit while occupying a small clearance. Figure 2b illustrates the simulated reflection coefficient (S\({}_{11}\)) characteristic of the PIFA element. As shown, the antenna operates at 2.6, 3.6, and 5.8 GHz and provides wide impedance bandwidths at these frequencies. In order to justify the tri-band function of the design, the simulated current densities of the modified PIFA element at different operation frequencies are illustrated in Figure 3. It is worth mentioning that the maximum scaling for all figures is the same. At 2.6 GHz (first resonance), as can be seen, the L-shaped strip has high current densities with the maximum distribution. Additionally, the current flow reverses on the interior edge of the surrounded open loop [51]. It is evident that the second resonance of the antenna S\({}_{11}\) has been achieved using the open-loop resonator as it appears very active at 3.6 GHz. The third resonance can be considered as the second harmonic of the first resonance [52]. As shown in Figure 3c, the current distribution is almost equal around the L-shaped strip and the open-loop resonator. Nevertheless, some coupling and interactions between the employed parasitic strip of the modified PIFA can be discovered which could affect the frequency response of the antenna [53]. The main motive behind the PIFA design is to obtain a low-profile and multi-mode radiator with the possibility of integration in the mobile-handset mainboard. The S\({}_{11}\) characteristics of the modified PIFA antenna can be adjusted by changing the values of the fundamental antenna parameters [54; 55]. The first resonance (at 2.6 GHz for the low-band) is mainly determined by the L-shaped strip. The second and third resonances (at 3.6 GHz and 5.8 GHz) depends on the main resonator (open-loop). Figure 2: (a) Transparent view of the planar-inverted F antenna (PIFA) and (b) its S\({}_{11}\) performance. Therefore, the circumference lengths of the resonators can satisfy the dielectric wavelength at the corresponding frequency points [56]. Figure 4 illustrates the antenna S\({}_{11}\) characteristic of varying design parameters including W\({}_{3}\), L\({}_{4}\), W\({}_{2}\), and W. In the simulation of the designed antenna, when one parameter changes, the rest of the parameters are kept the same as listed in Table 1. The antenna S\({}_{11}\) results for different values of W\({}_{3}\) are illustrated in Figure 4a.