## 5 Conclusions  
For the first time, the power extraction capability of a double coil electromagnetic harvester was experimentally compared with that of a standard single coil electromagnetic harvester. The validity of the single degree of freedom model used for the comparison was verified by experimental tests on a commercial electromagnetic harvester. The comparison was performed by simulations and by experiments on the electrical circuits modeling the harvester's behavior. The circuits were implemented through electronic components mounted on a solderless breadboard. The experimental results showed that the double coil electromagnetic harvester is able to increase the power delivered to the optimal load for any vibration frequency, but the gain is particularly significant at frequencies lower than the harvester resonance frequency.  
The double coil electromagnetic harvester investigated in this paper is characterized by significant advantages with respect to the other techniques proposed in the literature to increase the energy efficiency of electromagnetic harvesters. In particular, the double coil harvester is able to significantly improve the extracted power as in the case of harvester arrays, non-linear harvesters and mechanical tuning techniques. However, its size is smaller than harvester arrays, its complexity is less than that of non-linear harvesters, and it does not need additional actuators as in the case of mechanical tuning techniques. Moreover, the double coil harvester exploits the electrical load matching like the electrical tuning  
Figure 14: Measured load average power for the double coil circuit in Figure 11a, in the four test cases. (**a**) Power as a function of \(R_{LOAD\_1}\) for the optimal values of \(C_{LOAD\_1}\), \(R_{LOAD\_2}\) and \(C_{LOAD\_2}\). (**b**) Power as a function of \(C_{LOAD\_1}\) for the optimal values of \(R_{LOAD\_1}\), \(R_{LOAD\_2}\) and \(C_{LOAD\_2}\). (**c**) Power as a function of \(R_{LOAD\_2}\) for the optimal values of \(C_{LOAD\_2}\), \(R_{LOAD\_1}\) and \(C_{LOAD\_1}\). (**d**) Power as a function of \(C_{LOAD\_2}\) for the optimal values of \(R_{LOAD\_2}\), \(R_{LOAD\_1}\) and \(C_{LOAD\_1}\).  
techniques, but it achieves higher performance than the standard application of an electrical tuning technique.  
Conceptualization, L.C., A.L.S. and M.V.; methodology, L.C., A.L.S. and M.V.; software, L.C., A.L.S. and M.V.; validation, L.C., A.L.S. and M.V.; formal analysis, L.C., A.L.S. and M.V.; investigation, L.C., A.L.S. and M.V.; resources, L.C., A.L.S. and M.V.; data curation, L.C., A.L.S. and M.V.; writing--review and editing, L.C., A.L.S. and M.V.; visualization, L.C., A.L.S. and M.V.; supervision, L.C., A.L.S. and M.V.; project administration, L.C., A.L.S. and M.V.; funding acquisition, L.C., A.L.S. and M.V. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded: by the research program \"VALERE: VAnviteLli pEr la RicErca\" by Universita degli Studi della Campania \"[PERSON]\", under grant PE-15/204/E-PASSION, and by the funding program \"Valorizzazione di Brevetti tramite il finanziamento di progetti Proof of Concept\" by the Italian Ministero per lo Sviluppo Economico (MISE), under grant #NOACRONYM-PoCn1-TEMELEV.  
The authors declare no conflict of interest.