# Design, Fabrication and Testing of a Multifrequency Microstrip RFID Tag Antenna on Si
## 6 Discussion and Conclusions  
Upon comparison of the simulated and measured results for the \(|S_{11}|\), as shown in Figures 10 and 15, it can be seen that the measured dip frequencies are close to the simulated ones, with the exception of the vicinity of 5.8 GHz where no dip is found, although the measured \(|S_{11}|\) value remains below 4 dB. On the other hand, the dip near 2.4 GHz is displaced to the right compared with the simulated results, although the measured \(|S_{11}|\) value at 2.4 GHz is about 3.6 dB. In future practical applications, a transmission line segment could be fabricated on the same substrate between the antenna pads and the input terminal, adjusting the parameters \(s\) and \(x\) (Figure 9) and possibly including matching components or stubs as appropriate to further improve reflection losses. The measured gain (Figure 16) appears to maintain considerable stability at relatively high values around both frequencies of interest (2.4 and 5.8 GHz), where a large beamwidth around the broadside direction has also been found (Figure 11a,b). As already noted, the measured gain values indicate significant losses on the antenna, which appear to be confirmed by the efficiency measurement obtained using the Wheeler cap technique. In all, the simulated and measured values were in good agreement, demonstrating the promising characteristics of the proposed RFID design on Si for a variety of applications.  
Figure 16: Measured gain \(G_{R}\) of the RFID antenna for frequencies of 1 to 6 GHz, with the antenna oriented toward the positive z direction (\(\theta=0^{\circ}\)), as defined in Figure 11a,b.  
Figure 15: Measured \(|S_{11}|\) values for the RFID tag antenna.  
**Author Contributions:** Conceptualization, T.K. and A.A.; Data curation, T.K., C.N.V. and C.B.; Formal analysis, T.K. and C.N.V.; Funding acquisition, A.S. and G.S.; Investigation, T.K., C.N.V. and C.B.; Methodology, T.K., C.N.V. and A.A.; Resources, C.N.V., A.S. and G.S.; Supervision, C.N.V.; Validation, C.N.V.; Visualization, T.K.; Writing--original draft, T.K.; Writing--review and editing, C.N.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** Parts of this research were co-financed by Greece and the European Union in the context of the project \"National Research Infrastructure on nanotechnology, advanced materials and micro/nanoelectronics\" (MIS 5002772) under the \"Action for the Strategic Development on the Research and Technological Sector\", and the project T1 EDK-00329 under the call RESEARCH--CREATE--INNOVATE under the Operational Programme \"Competitiveness, Entrepreneurship and Innovation\" (NSRF 2014-2020).  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.  
**Acknowledgments:** Specimen fabrication was carried out by Microelectronics Research Group, Institute of Electronic Structures and Lasers (IESL), Foundation for Research and Technology Hellas (FORTH), 70013 Heraklion, Greece.  
**Conflicts of Interest:** The authors declare no conflict of interest.