# Towards an Efficient Chipless RFID System for Modern Applications in IoT Networks
## 3 The RFID Antenna Design  
Miniaturized ultra-wideband (UWB) antennas with high gain have been an area of interest for researchers in recent years, in particular, for RFID tags applications. Various techniques have been adopted to raise the bandwidth while maintaining a good gain-to-aperture ratio [19]. It includes meandered ground plane antenna design [20], monopole antenna with microstrip disc loading [21], dual-polarized microstrip patch antenna array [22], and UWB-printed circular monopole antenna [23]. It also includes patch antennas with gap-coupled feed [24] and an integrated band-pass filter [25]. However, the reported techniques come with various shortcomings, which include large size, low gain, reduced bandwidth, and copolarization effects [26, 27]. Therefore, to overcome the discrepancies in the reported techniques, we are proposing an ultra-wideband step-shape rectangular patch (USRP) antenna. The front side of the proposed antenna consists of a rectangular patch. The longer sides of the patch are cut in symmetric steps to increase the length for better bandwidth realization. A partial ground plane is adopted at the backside to maximize directivity and reduce back lobe radiation. The design parameters for the radiating patch, step cuts, feedline, and ground plane are listed in Table 1, while the fabricated antenna is shown in Figure 3.  
The S-parameter response of the USRP antenna is shown in Figure 4, which depicts the fact that the antenna can also effectively operate in the bandwidth from 5 to 12 GHz making its bandwidth compatible with our proposed chipless tag, which operates from 5 to 8 GHz. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Patch Design**}} & \multicolumn{3}{c}{**Partial Ground Plane**} & \multicolumn{3}{c}{**Step-Cut Design**} \\ \cline{2-7}  & & **and Feed Line Design** & & & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline Patch length (Lp) & 39.6 & Ground length (Lg) & 34.5 & 1\({}_{1}\) & 2.19 & w\({}_{1}\) & 1.89 \\ \hline Patch width (Wp) & 25 & Ground width (Wg) & 25 & 1\({}_{2}\) & 2.11 & w\({}_{2}\) & 1.99 \\ \hline Gap between the patch and ground plane & 0.52 & Feeder length (Lf) & 35 & 1\({}_{3}\) & 3.64 & w\({}_{3}\) & 1.97 \\ \cline{2-7}  & & Feeder width (Wf) & 2.4 & 1\({}_{4}\) & 23.72 & w\({}_{4}\) & 13.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical parameters of the proposed antenna. The seamless frequency operation of the USRP antenna can be examined from the VSWR graph of the antenna, as illustrated in Figure 5, which approaches an ideal value of one for a frequency range of 5-12 GHz. Figure 4: S\({}_{11}\) of the USRP antenna. Figure 3: The proposed USRP antenna: (**a**) Fabricated view; (**b**) front and back view. The simulated gain characteristic of the USRP antenna is depicted in Figure 6, which illustrates the fact that the proposed antenna can achieve a maximum gain of 8 dB for a frequency range of 13.5-18 GHz. This makes the proposed antenna an ideal candidate to be used for an extended reading range. The USRP antenna achieves radiation efficiency of more than 90% over a wideband of operating frequency, as shown in Figure 7.  
Figure 5: VSWR of the USRP antenna. Figure 6: The gain frequency variation of USRP antenna. Polarization sensitivity of an antenna is of paramount importance when it comes to designing an efficient RFID system. The co- and crosspolarization characteristics of the USRP antenna are shown in Figure 8. For an interference-free RFID system, there should be a high degree of copolarization between the antennas of different modules (tag and reader), such as communication between the reader's transmitting antenna and the tag's receiving antenna. Likewise, good crosspolarization should be exhibited between antennas of the same module, such as the tag's transmitting and receiving antennas.