## 4 Prototypes and Validation  
The Rogers RO4003 substrate with thickness of 12 mil was chosen for design and fabrication of the proposed FSS filters. A commercial chemical etching (or PCB) process, which is a cost-effective process, was used for the fabrication. Required high-performance of the FSSs can be attained with careful optimization of dimension parameters even in the presence of fabrication tolerances involved with the low-cost commercial etching process. The commercial chemical etching process for printed circuits is typically performed as following. Firstly, the substrate is cleaned for lamination. Then, a photosensitive resist is laminated on the top and bottom sides of the substrate using hot-roller lamination process. A stencil film for square- and circular-loop structures is coated on the both sides of substrate and is exposed to the ultra-violet light, which hardens the structure patterns into acid resistance surfaces. Then, an etchant is sprayed on the substrate dissolving the exposed metal, which results in the etching the square- and circular-loop structures. Figure 7 shows the fabricated single-layer double-sided square and circular-loop FSS filters. The front and back configurations of the FSS filters are identical. The total size of the fabricated FSS panel is 19.7 \(\times\) 6.3 inches (500 \(\times\) 160 mm) for both cases, which is the required size to protect the detector array box in the ECE imaging system. From Figure 7, it is observed that the fabricated square loops do not maintain their sharp corner edges and also the width of the loops is slightly irregular due to fabrication inaccuracy. It is also seen that the square-loops are not perfectly aligned: that is, some loops are tilted to left side and others are slanted to right side. Whereas, in the circular-loop array, there are no sharp corner edges and even if the loops are tilted, they remained aligned due to the circular property. The total number of elements in the fabricated square-loop or circular-loop FSSs are 128,905 (635 \(\times\) 203). Therefore, for the measurements, arbitrary portions from the multiple fabricated FSS samples were selected and measured for the dimensions of the fabricated loops using an automatic CNC Vision Measurement machine. The comparison between the designed and fabricated values of the dimension parameters is shown in Table 1. It is obviously observed that the errors between the designed and fabricated parameter values in the case of the square-loop structure are much higher than those in the circular-loop structure: that is, the percentage errors of \(d_{s}\) and \(d_{c}\) are 1.46% and 0.07%, respectively. Under the same fabrication process with given fabrication tolerances, it can be seen that much smaller errors occurs in fabricated dimension parameters in the case of the circular-loop structures as compared with the square-loop structures, thereby expecting more predictable and accurate FSS performance with the circular-loop structures. Figure 6: Influence of the design parameters on the FSS frequency response: (**a**) loop length “\(d\),” (**b**) unit cell period “\(p\),” (**c**) gap between the loops “\(g\).”Based on the measured parameter values, the influences of deviated parameter values due to the fabrication tolerances on the frequency responses of square and circular-loop FSSs are evaluated. Figure 8 shows the simulated results of the square-loop and circular-loop FSSs without and with consideration of fabrication tolerances. In Figure 8, in order to predict the possible shifts in the resonant frequencies of square- and circular-loop FSSs, the amount of averaged dimensional deviations due to fabrication tolerances for the square and circular loops shown in Table 1 are added to the optimized dimension parameters: that is, \(d_{sa}\) = 23.65, \(d_{ca}\) = 27.04, \(w_{sa}\) = 3.75, \(w_{ca}\) = 4.21, \(p_{sa}\) = 60.06, \(p_{\alpha}\) = 60.1, \(g_{sa}\) = 36.41, \(g_{\alpha}\) = 33.06 (units: mil).