## 4 Conclusions and Future Works  
An analysis of a ring topology as physical support for a PLC system specially intended for PV strings has been presented, proposing that pushing this loop to resonance optimises the reception levels along the cable. Previous works on this subject had proposed resonant circuits for coupling to the line; we added here to this feature the possibility of working with the whole loop under resonance, improving even more the signal levels along the line. In addition, the conditions needed for the levels to be reasonably equalised has been derived. Control electronics have been designed to keep the communications signal path on resonance, making the system flexible and able to self-adapt to different specific installations or changes within the same setup. As this check and adaptation to resonance is a task that does not need to be accomplished continuously (it would be enough to execute it once per day), the MCU that controls all the communications in a final application could be the same, allowing a lower-cost system.  
The insertion of transmission (TX) and reception (RX) circuits along the loop shows reactive impedances (inductive or capacitive coupled to the line) or even small resistances, consequently modifying the resonance condition on the loop; nonetheless, our tests have shown that the addition of the compensation reactance supplied by the circuit described before is able to return the loop to the optimal working point of resonance. The approach here presented is useful as a starting point for further research that could determine the precise influence of lumped impedances along the loop in the communications performance, but for moderate reactances in the TX/RX circuits (enough for signal injection and recovery), the compensation circuit is enough, with only small deviations compared to the behaviour of the raw cable loop.  
\begin{table}
\begin{tabular}{c c} \hline
**Receiver** & **Voltage Amplitude (V)** \\ \hline
1 & 32.3 \\ \hline
2 & 32.1 \\ \hline
3 & 32.3 \\ \hline
4 & 33.5 \\ \hline
5 & 34.6 \\ \hline
6 & 34.0 \\ \hline
7 & 33.5 \\ \hline
8 & 32.7 \\ \hline
9 & 32.5 \\ \hline
10 & 32.8 \\ \hline \end{tabular}
\end{table}
Table 2: Voltage amplitude measurements of the \(\%\) wavelength configuration.  
Two options regarding the relationship between the loop length and the wavelength have been presented: a loop with one wavelength perimeter and a loop with \(\
icefrac{{1}}{{4}}\) wavelength perimeter or less. The measurement levels in the loop have shown the possibility of working with relatively low signal levels without compromising the SNR in both options, allowing the design of cheaper and simpler transceivers. The main line of research from our results must be to explore and test for future work the ways proposed to excite the travelling wave mode in the one wavelength loop, which in essence could suppose a great leap forward for equalising the signal levels working at carrier frequencies over 5 MHz, leading to baud rates close to or even over 1 Mbps (depending on the spectral efficiency of the modulation scheme chosen), keeping all the advantages of working in resonance.  
Conceptualisation, J.M.-A. and L.H.-C.; methodology, J.I.M.-A., V.A.G., M.S.M.W. and L.H.-C.; validation, J.I.M.-A., V.A.G., M.S.M.W., J.G.F.C. and L.H.-C.; formal analysis, J.I.M.-A. and M.S.M.W.; investigation, J.I.M.-A., V.A.G., M.S.M.W., A.R.-P., D.F.-M., F.J.S.-P. and L.H.-C.; writing--original draft preparation, J.I.M.-A., M.S.M.W., S.G.-S. and L.H.-C.; writing--review and editing, J.I.M.-A., J.G.F.C., S.G.-S. and M.S.M.W; supervision, L.H.-C.; project administration, L.H.-C.; funding acquisition, J.I.M.-A. and L.H.-C. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Universidad of Valladolid with the predoctoral contracts of 2020 cofunded by Santander Bank. This study was supported by the Universidad of Valladolid with ERASMUS+ KA-107.  
Not applicable.  
The authors declare no conflict of interest.