# Trade-Off Analysis for Array Configurations of Chipless RFID Sensor Tag Designs
## 6 Experimental Validation and Discussion
### Detectability of Planar Array Configurations  
#### 6.2.1 Linear Detectability  
First, to properly analyse the linear detection performance of various array configurations, it is crucial to ensure that the resonance frequency remains stable regardless of the linear distance between the chipless RFID tag and the reader. This stability can be confirmed by examining the frequency signature of the array. For example, in Figure 20a, it is shown that the resonance frequency of the unit cell measured in the monostatic configuration remains uniform up to a distance of 18 cm from the reader. However, beyond this distance, the resonance frequency begins to deviate from the intended frequency. However, the bistatic measured resonance frequency remains stable up to 26 cm and then begins fluctuating. In contrast, Figure 20b,c indicate that the resonance frequency of the 2 \(\times\) 2 and 3 \(\times\) 3 array configurations remains steady up to a distance of 30-32 cm, and variation is around 21 and 20 MHz, respectively. However, the bistatic measured resonance frequency tends to fluctuate at shorter distances, primarily because of the difficulty of focusing the antenna's main lobe into the chipless tag. Thus, it is removed from the measured data. Likewise, in the 4 \(\times\) 4 array configuration, as shown in Figure 20d, the resonance frequency remains stable up to 34 cm and shows a lower frequency variation of 18 MHz compared to the other two arrays. Similarly, the resonance frequency fluctuates at shorter distances; hence, it is neglected. Apart from the difficulty of focusing the antenna's main lobe into the chipless tag, the structural mode RCS is significant at a shorter distance. In order to investigate the extended linear reading range of a chipless RFID planar array, a comprehensive experimental investigation is conducted using a high-gain dual-polarised ultra-wideband array antenna [36]. This antenna has an impressive 26 dBi gain and operates in the 4.20 to 7.10 GHz frequency range. The study aims to determine the  
Figure 20: Variation in the resonance frequency of planar array configurations in monostatic and collocated bistatic antenna configurations. (**a**) Unit cell. (**b**) 2 \(\times\) 2 array. (**c**) 3 \(\times\) 3 array. (**d**) 4 \(\times\) 4 array. effectiveness of different array configurations in improving reading range as shown in Figure 21.  
According to Figure 22a, this experiment demonstrates that the resonance dip of the \(2\times 2\) array becomes noticeable at around 4.90 GHz when the distance is between 0.65 and 1.30 m. However, this resonance fades away when the reading distance increases to 2.00 m. On the other hand, the \(4\times 4\) array demonstrates a sustained resonance dip that is observable up to 3.00 m as shown in Figure 22b. This sustained resonance can be attributed to the higher RCS of the \(4\times 4\) array. However, as summarised in Table 5, there are minor resonance frequency variations when the distance increases. This variation is because the effect of the environmental noises becomes more prominent when the distance increases, and there is a slight variation in the test setup. This variation can be minimised when the measurements are conducted in the anechoic chamber, implementing a VNA averaging function, and manually triggering the measurements instead of continuous triggering. Figure 21: Extended reading range measurement setup using a high-gain antenna.