# Trade-Off Analysis for Array Configurations of Chipless RFID Sensor Tag Designs
## 6 Experimental Validation and Discussion  
### RCS of Planar Array Configurations  
After investigating the simulation results of the linear and planar array configurations, chipless RFID tags of the planar array configuration are fabricated to validate the simulation results because the planar arrays show better RCS improvement and detectability than the linear configurations. We fabricate four planar array configurations using a milling machine, as illustrated in Figure 16, with a plastic ruler for the actual size comparison of the tags. Figure 15: Simulated Q-factor variations of linear and planar array configurations with respect to RCS. Figure 17 shows the collocated bistatic measurement setup to measure the chipless planar array tags using the Keysight E8361A vector network analyser (VNA). Vertically polarised patch antennas with 9 dBi gain are connected to the VNA using ports 1 and 2, and more technical information about the antenna can be found in [35]. We use monostatic and collocated bistatic antenna configurations in this experiment to obtain the measurements. In the monostatic configuration, only one antenna is connected to VNA using port 1. This is one of the most common measurement arrangements for copolar chipless RFID tags since it only transmits the signal in the same direction as received (i.e., the E plane is vertical in both TX and RX). The VNA output power is adjusted to 0 dBm across the whole frequency band, with an averaging function active for 20 measurements to eliminate minor variations. For the RCS measurements, background subtraction is necessary to cancel out the antenna's effect and environmental noises on the received signal. In addition, a reference object, such as a metal plate of the same size as the tag, is used to scale the subtracted signal. Therefore, as shown in Figure 18, the S21 parameter is first captured in the empty background (e.g., without the tag). Tags holding forms should be included in the empty backdrop measurement because we only require the RCS of the tag. Secondly, we measure the S21 of a reference object with a known RCS. For example, aluminium plates with the exact sizes of the array tags are used as reference objects. Finally, the RCS is calculated using (3). Furthermore, the expression for the RCS of reference object \(\sigma_{\text{ref}}\) is generated from the simulation of CST Microwave Studio (academic version 2022) rather than using an empirical formula to obtain accurate results as shown in Figure 18.  
Figure 16: Fabricated planar array configurations of Pi-shaped resonator unit cell, \(2\times 2\) array, \(3\times 3\) array and \(4\times 4\) array. Figure 17: Collocated bistatic measurement setup using a VNA with dual-polarised patch antennas. The results of the collocated bistatic RCS experiment are presented in Figure 19 and Table 4. We find that the measured RCS values are very close to the simulated values, so the tags can be easily detected and identified using a radar system. However, we do notice some minor differences between the resonance frequencies of the measured and simulated results, which could be due to various factors, such as the fabrication process or measurement conditions. In this scenario, the unit cell tag has a resonance of 4.80 GHz instead of the expected resonance of 4.78 GHz. This results in a slight difference of \(\Delta\)f = 0.02 GHz. Additionally, the other array configurations, including 2 \(\times\) 2, 3 \(\times\) 3, and 4 \(\times\) 4 arrays, also show deviations from the simulated values of 0.10, 0.11, and 0.08 GHz, respectively as shown in Table 4.  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Resonator**} & \multicolumn{3}{c}{**Simulated**} & \multicolumn{3}{c}{**Measured**} & \multicolumn{3}{c}{**\(\Delta\)RCS**} & \multicolumn{1}{c}{\(\Delta\)f (GHz)**} & \multirow{2}{*}{**AQ**} \\ \cline{2-2} \cline{5-10}
**Configuration** & **RCS** & **Resonance** & **Q** & **RCS** & **Resonance** & **Q** & **(dBsm)** & **(GHz)** & \\  & **Level** & **Freq.