# Miniaturized Arrow-Shaped Flexible Filter-Embedded Antenna for Industrial and Medical Applications
## 2 Filtering Antenna Design and Analysis
### Parametric Analysis  
Figure 6 shows the parametric analysis for length of the stub (ax) used to obtain the desired operational band at 2.45 GHz and suppress the other harmonic bands. It can be seen from the figure that the antenna obtained in Section 2.1 is transformed to filtenna by loading a rectangular-shaped stub having a length of 10.6 mm. At the optimal value of  
Figure 4: Filtering antenna design methodology.  
Figure 5: Effects on return loss with and without filtering stub.  
ax = 10.6 mm, the antenna offers wide harmonic suppression of upper bands, whereas the ISM band ranging from 2.3 to 2.75 GHz remains operational. If the value of ax is fixed at half of the optimal value (a\({}_{\rm x}\) = 5.3 mm), the upper band at 11 GHz is only suppressed, as evident in Figure 6. If the length of ax is further decreased to 0 mm, the antenna is no longer filtenna. It starts operating in tri-mode of 2.45 GHz, 5.2 GHz, and 9.1 GHz, as shown in Figure 6.  
Table 1 summarizes the antenna performance at various design stages considering the different parameters. It can be observed that antenna at step 1, with overall dimensions of \(30\,\mathrm{mm}\times 35\,\mathrm{mm}\), offers 2.35-2.47 ISM band with harmonic at 10.7-11.5 GHz. This initial stage did not offer any filtering band. The antenna design after step 2, with a geometrical size of 25 mm \(\times\) 33 mm, obtained an ISM band ranging from 2.4 to 2.8 GHz with 6.2-7.4 GHz harmonic. The antenna designed at the third stage, having a size of 23 mm \(\times\) 25 mm, offers a 2.3-2.75 GHz operational band and two harmonics ranging from 5.0 to 5.5 GHz and 8.2-10 GHz. The final proposed antenna, with an overall size of 23 mm \(\times\) 25 mm, offers an ISM band ranging from 2.276 to 2.75 GHz, with no harmonics, but with a filtering band ranging from 3 to 11 GHz.  
To further elaborate the operational behavior of proposed filtenna, the surface current distribution is provided at two filtered resonances of 5.2 GHz and 9 GHz, as illustrated in Figure 7. It can be seen that at 5.2 GHz, where there is no rectangular stub, the maximum current is on feedline and slots at the tail of an arrow-shaped patch. However, after the inclusion of the stub, the current at the upper part of the antenna is minimum, with the maximum amount of current at the edges of the rectangular-shaped stub, which is used to filter the resonance. Similarly for 9 GHz, the maximum amount of current is observed at the lower end of the antenna and feedline without a stub. On the other hand, when the rectangular stub is loaded, the maximum amount of current is noted at the edges of the loaded stub and feedline.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Prototype**} & **Size** & **Bandwidth** & **Harmonics** & **Filtering Bandwidth** \\  & **(mm \(\times\) mm)** & **(GHz)** & **(GHz)** & **(GHz)** \\ \hline Step 1 & \(30\times 35\) & 2.35–2.47 & 10.7–11.3 & No \\ Step 2 & \(25\times 33\) & 2.4–2.8 & 6.2–7.4 & No \\ Step 3 & \(23\times 30\) & 2.3–2.75 & 5–5.5/.82–10.2 & No \\ Proposed & \(23\times 30\) & 2.276–2.75 & No & 3–11 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance comparison among design methodology steps.  
Figure 6: Effects on return loss with and without filtering stub.  
Also, it is noticed that at 2.45 GHz the maximum number of current flows along the feedline and edges of the rectangular stubs as well as the arrow-shaped antenna, as depicted in Figure 8. Thus, it is verified that the rectangular stub leads towards the harmonic suppression of upper frequency bands, resulting in resonance at the desired ISM band.