## 4 Filter-Integrated Antenna Realized and Tested to Observe Channel Selectivity  
The wireless communication system has transmitting (TX) and receiving (RX) antennas. Likewise, the satellite transponder has two antennas that are tested to see the RF-signal transfer and its reception between the opposite sides. The TX antenna is directly connected to the signal generator, which is practiced on a lot for checking the antenna-only function. Horn antennas are adopted for satellite wireless equipment most of the time, and because they have the characteristics of wide bands, frequency channels having several hundred megahertz of bandwidths are not defined for them. As channels are important in any communication, the roles of bandpass filters are highly in demand. Therefore, the proposed bandpass filter is inserted into the TX antenna system as below. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \(f_{0}\) & **Insertion Loss** & **Attenuation** & **WG** & **Resonator** & **Meta-** \\  & **(GHz)** & **(dB)** & **(dB)** & **Flange** & **Length (\(\lambda_{g}\))** & **Material** \\ \hline
[5] & 11 & \(<\)1 & 55 & WR-90 & 0.68 & X \\
[7] & 11 & \(<\)1 & 50 & WR-90 & 0.51 & X \\
[15] & 30 & 2 & 4.6 & WR-28 & 0.57 & X \\
[16] & 9.45 & 0.08 & \(>\)20 & WR-90 & 0.51 & X \\
[17] & 8.175 & 0.35 & 7.9 & WR-112 & 0.41 Figure 10a is the scheme of the ordinary RF-link test setup, while Figure 10b depicts how the TX horn antenna is fed through the BPF from the signal generator. The shape of the horn antenna given in Figure 10c is commonly used for wireless connectivity observation. It is R&S\({}^{\otimes}\)HF907 made by Rohde-Schwarz. Figure 10d-g show its representative characteristics, in other words, S\({}_{11}\), \(\Phi=0^{\circ}\) plane beam-pattern and \(\Phi=90^{\circ}\) plane beam-pattern. These beam-patterns as the field-strength measured on the angular variation are side-data, and for the RF-link investigation, the RF transmission between the TX and RX sides on a straight line as in Figure 10h expresses the field-strength. Like Figure 10d, S\({}_{11}\) is below \(-10\) dB over a very broad band as in Figure 10i. S\({}_{21}\) as the RF transmission between the TX and RX antennas was measured over the distance ranging from 3 m (denoted as _do_) to 6 m (as _dr_), and has the feature of a broad band. Signals can be contaminated with noise and interference in that situation. Channel selectivity necessary for wireless communication is made possible by the following method foreseen in Figure 10b. Figure 10: _Cont._  
Figure 11a includes a gray rectangle denoting the proposed metamaterial BPF as part of the feed of the TX horn antenna. The real experiment was conducted as in Figure 11b. The field strength from the filter-incorporated antenna was measured by the RX horn antenna, which was plotted as Figure 11c. Channel selectivity is accomplished as revealed by S\({}_{11}\) as  
Figure 10: RF\(-\)link tests of the horn antennas without and with the BPF. (**a**) Signal transfer between the TX and RX horn antennas. (**b**) Case of the TX as the filtenna. (**c**) Horn antenna standing on either of the TX and RX sides. (**d,e**) S\({}_{11}\) of the horn antenna. (**f**) Far\(-\)field pattern on the \(\Phi=0^{\circ}\) plane for the horn antenna. (**g**) Far\(-\)field pattern on the \(\Phi=90^{\circ}\) plane for the horn antenna. (**h**) Over\(-\)the\(-\)air test in the anechoic chamber. (**i**) S\({}_{11}\) and S\({}_{21}\) when the setup is without the BPF with the antenna position adjusted back and forth in a small scale such as 1 cm to 4 cm. the input reflection coefficient of the TX horn and S\({}_{21}\) as the signal transfer between both the sides. The two s-parameters have clear distinction of the passband and stopband like in Figure 7. The antenna system is enabled to select the signals as required. Figure 11: RF\(-\)link and channel selectivity test. (**a**) Case of the TX as the filtenna that comprises the BPF and the antenna. (**b**) Over\(-\)the\(-\)air test.