## 5 Conclusions  
Improving the gain measurement accuracy on HF antennas is critical in over-the-horizon communication systems for smart grid energy facilities. A good control of the in situ radiation patterns is needed since transmitted powers are low, and most of the HF spectrum must be exploited to constantly provide reliable links. In this work, we proposed to use an electrically small, folded dipole as a probe for in situ drone-borne gain measurements on HF antennas. We presented a calibration procedure taking into account the effect of the field zone, ground reflection, and drone body proximity. Compared to existing solutions based on a drone-borne transmitter driving a full-wave antenna, our approach provides a wider frequency band of operation (typically, five times wider), and better mechanical stability. Our system can also be a cheaper alternative to drone-borne, active antenna configurations, as the receiver consists of a simple peak detector and consequently a cheaper, low-payload drone is needed. We compared the characteristics of our probe to those of other electrically small antennas utilized for a similar purpose. The magnitude of the input reactance for an electrically short, folded dipole is twenty times lower than for a straight dipole with the same length. The electrically small loop seemingly yields a less variable pattern diagram; in fact, the folded dipole provides a combination between loop-mode and dipole-mode radiation, which is stable enough over the frequency range of interest. Conversely, the  
Figure 14: On-drone versus off-drone gain. Figure 13: Gain variation over the HF range: (**a**) without field-zone correction; (**b**) with field-zone correction. loop must be hanged vertically below the drone for effective operation, i.e., in a less stable mechanical position. Regarding the design, we found that losses in the antenna wire and balun rather have a positive impact, as the VSWR is smaller than five over the entire frequency band of interest, i.e., between, 7 and 30 MHz. The realized gain exhibits a smooth variation between \(-25\) and \(-15\) dBi; transmission measurements between two identical folded dipoles show that such low gain figures provide stable and noiseless results at distances of up to \(2.5\,\mathrm{m}\) between antennas even at an input power in the order of \(1\,\mathrm{mW}\). The dipole-type radiation mode dominates over the loop-type mode over the frequency range of interest, and the impact of the ground reflection is mostly less than \(1\,\mathrm{dB}\) for a face-to-face orientation. Our calibration procedure helps to improve the accuracy of gain measurements of up to \(4\,\mathrm{dB}\) for field-zone related discrepancies, and by around \(2\,\mathrm{dB}\) for the drone body impact. Future work on improving the accuracy of our approach will focus on including in the calibration procedure the effect of the peak detector non-linearity and thermal drift, as well as on calibrating the probes in a more controlled environment, i.e., a low-frequency anechoic chamber. Our calibration approach can be further adapted to highly dynamic smart grid scenarios such as offshore wind farms by acquiring near-field data during several flight trips, covering multiple turbine turnings, in order to develop a statistical link model. Conceptualization, M.P., A.C., A.H. and R.D.T. ; methodology, M.P., A.C., A.H. and R.D.T. ; software, M.P. and R.D.T. ; validation, M.P., A.C., A.H. and R.D.T. ; formal analysis, M.P., A.C., A.H. and R.D.T. ; investigation, M.P., A.C., A.H. and R.D.T. ; resources, R.D.T. ; data curation, M.P., A.C., A.H. and R.D.T. ; writing--original draft preparation, M.P., A.C., A.H. and R.D.T. ; writing--review and editing, R.D.T. ; visualization, M.P., A.C., A.H. and R.D.T. ; supervision, R.D.T. ; project administration, R.D.T. ; funding acquisition, R.D.T. All authors have read and agreed to the published version of the manuscript.