## 2 Structure of the Tri-Band Antenna
The rectangular patch in combination with the WCS antenna offered only a bidirectional pattern, covering the same dual-band resonance of 3.7-6.9 GHz and 13.2-17.6 GHz as [18]. A way to modify this structure to accomplish tri-band capability was by using an ILSS [18]. The narrow bandwidth for 5 GHz WLAN of the proposed antennas is specific to the required operating frequency and eliminates the adjacent bands, which is more advantageous for wireless communication compared to their counterparts. Supplementing this combination with an ILSS at the left (or right) side of the inner ring wall (see Figure 1a)induced the antenna to be sensitive to frequencies in another, new bandwidth--2.4 GHz WLAN, making it tri-band. In terms of current flow, it can be observed in Figure 2 that the ILSS addition changed the pattern of the flow--the flow was asymmetrical. Moreover, simulation of the current flow pattern in the proposed antenna, an antenna without an ILSS, and an antenna with two symmetrical ILSSs at 2.45 GHz, demonstrated a more asymmetrical flow in the proposed antenna. Compared to a tri-band antenna with two symmetrical ILSSs reported in [18], this proposed design provided 3.5/9.7% higher gain in the 2.4/5 GHz WLAN band, which was most likely because the current flow was more asymmetrical. In contrast with the highest band for Ku applications, the proposed antenna with a single-arm ILSS provided 6.7% lower gain, as shown in Figure 3b. Comparing the antenna gain of the proposed antenna with a single-arm ILSS for a three-transition band to the proposed antenna without ILSS for a dual-transition band, the gain of proposed antenna was worsened to 8.7/0.4% in the 5 GHz WLAN/Ku-band applications. With this new design, the proposed antenna offers a resonance frequency at 2.2 GHz, with 1511 of \(-\)17.8 dB, covering a 10-dB return loss frequency band of 2-2.56 GHz with 2.35 dBi gain, while a design with two symmetrical ILSSs offers a resonance frequency at 2.1 GHz with 1511 of \(-\)32 dB, covering the 10 dB return loss frequency band of 1.9-2.7 GHz, as shown in Figure 3. In detail, the proposed antenna provides three different resonance frequencies of 2.2 GHz, 5.3 GHz and 15.5 GHz, covering three bandwidths--2-2.56 GHz, 4.42-6.82 GHz and 13.26-17.70 GHz--over 2.4 GHz WLAN, 5 GHz WLAN and Ku-band, respectively. It has minimum/maximum gains of 1.74/2.35 dBi, 3.61/5.04 dBi and 3.93/4.79 dBi for 2.4 GHz WLAN, 5 GHz WLAN and Ku-band, respectively, as shown in Figure 4. Of note is that the gains of the proposed antenna in the middle band and the upper band were higher than its gain in the lower band. This difference in gain at the lower frequency band and the higher frequency bands can be due to the antenna structure consisting of the dielectric that affecting to the total efficiency of the antenna at the difference frequency--\(-1.35/-0.85/-1.37\) dB at the center frequency for the 2.4/5 WLAN/Ku band--as well as it can be attributed to the difference in the components of the radiating part of the antenna, i.e., the radiating rectangular patch and an induced ILSS. Incidentally, the middle and upper bands were mostly influenced by the radiating rectangular patch, as depicted by the current flows in Figure 5b,c, while the lower band was mostly influenced by the ILSS attaching at the side wall of the ring, as shown in Figure 5a. In addition, the length \(l_{3}\) of ILSS affected the resonance frequency of the lower band strongly, as shown in Figure 1:0 the longer the length \(l_{3}\), the lower the achieved resonance frequency. Lastly, \(l_{3}\) of 13 mm was selected, as it was the length that provided the best coverage of 2.4 GHz band for WLAN applications. Note that the change of the width, \(l_{1}\), and thickness, \(l_{2}\), of the ILSS was very faintly influenced to 1511, as depicted in Figures 7 and 8. The width \(l_{1}\) of 2 mm and thickness \(l_{2}\) of 1 mm were chosen, as the best return loss was achieved at the 2.4 GHz band.