## 3 Results  
Figure 7 shows the action of the compensation described, where a 20-m-long loop was kept in resonance at 9.5 MHz initially with the corresponding control voltage of 5 V. Then, the length of the loop was shortened to 15.10 m, and with the resonance control electronics off, the resonance frequency was displaced to 11.1 MHz. If we then switched on the electronics, the control voltage was readjusted by the MCU to get the resonance back to 9.5 MHz. All the electronics described above could work to keep the resonance condition on any of the two modes described in Section 2. The S21 modulus was measured with a VNA connected in series with the loop, the tuning circuit and a 50-ohm load. Maximums show the frequencies of minimal attenuation of the propagation along the (loop)-(tuning circuit) chain. The black trace corresponds to the 20 m cable with a control voltage of 5 V in the tuning circuit, where the maximum is over the chosen carrier frequency (9.5 MHz). The blue solid trace corresponds to a variation in the loop length from 20.00 m to 15.10 m with no change in control voltage, and the new maximum is over 11.1 MHz and the carrier (9.5 MHz) is attenuated from \(\mid\)S21 \(\mid\) = 0.228 to 0.100. The blue dashed trace corresponds to the new loop length (15.10 m) after the tuning circuit action for compensation, leading to a control voltage of 0.9 V, and the maximum is back over the carrier at 9.5 MHz.  
During the current distribution measurements in the one-wavelength loop presented before (Figure 2), with an exciting signal of 10 volts in amplitude, the absolute voltages measured across the current sensor were between 70 and 330 mV, which, considering that the sensor sensibility is 10 Volts/Amp, indicates averaged current amplitude values in the order of 200 mA. This suggests that with much lower exciting voltages, communication with reasonable SNRs is possible due to loop resonance, making simpler and low-cost transceivers conceivable.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Model** & **Cells** & **Power/W** & **VOC/V** & **ISC/A** & **VMP/V** & **IMP/A** \\ \hline
**Eoply** & 72 cells & 175 & 44.35 & 5.45 & 36.26 & 4.83 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Technical characteristics of the PV module.  
Figure 6: PV plant on Duques de Soria campus of the University of Valladolid.  
Figure 7: Effect of microcontroller action to retune the loop, in response to variations in cable distance.  
As a validation of the current levels present in the short-loop configuration (\(\%\) wavelength), measurements were made over an experimental setup composed of a cable loop 20 m long with ten capacitors and ten small toroidal transformers distributed evenly in series simulating the bypass capacitors and the transceiver inductively coupled and installed within each PV module in a real installation. The toroidal transformers had a transformation ratio of 30:1 with the secondary connected to the loop, and their inductances were adjusted to form with the bypass capacitors LC resonators at the carrier frequency, in such a way that the transceivers were LC resonators at the same frequency as the loop resonance. The control electronics were also placed in series with the loop. A carrier frequency of 1 MHz was chosen, fulfilling the condition of a loop shorter than \(\%\) wavelength, and was injected in the primary of one of the transformers with an amplitude of 5 volts, simulating a transmission from one of the transceivers towards the others. The voltage amplitude measurements at the primaries of the ten transformers receiving the signal are shown in Table 2.