## 8 Validation Measurements  
Measurement data obtained during a field deployment of six probes and one sensor in Spain was analysed to verify the performance of the design described here. This is not the science performed with  
Figure 8: The central sensor system in in Sant Marti Sesgueioles.  
Figure 9: One of the six probe transmitters in Cal Cerdanyola. (**a**) The complete system including the wire antennas; (**b**) A close up of the compact (23 cm x 8 cm x 8 cm) probe transmitter on top of the mast.  
the system--which requires a different type of analysis and will be subject to separate publications--but a validation of the realized system of sensor and probes.  
In our design, we assumed a radio noise level of -160 dBW in a 30 Hz bandwidth, and expected a signal level of -99 dBW and a resulting SNR of 61 dB. We processed 45 min of data from the field deployment using a Fast Fourier Transformation (FFT) [41] with a block size of 1000 samples and a Blackman-Harris window [42] and a bin size of 3 Hz. We than aggregated the processed data in a statistical spectrum plot, in which the colour represents the relative occurrence of each of the values, see Figure 10. The 6 peaks correspond with the six probe signals. Other amateur radio signals can be seen around 7009.57, 7009.86 and 7010.05 kHz. As they do not interfere with our measurements, no effort is undertaken to identify them. From this graph, we can see that the maximum probe signal power is approximately \(-99\) dBW, with a few dB difference per probe. However, the radio noise level is approximately \(-161.5\) dBW in a 3 Hz bandwidth, or \(-151.5\) dBW in a 30 Hz bandwidth. This is 8.5 dB higher than was predicted for a quiet rural environment. As a result, the SNR is only \(-99\) - (\(-151.5\)) = 52.5 dB. The resulting SNR is sufficient for the intended experiments, but the bandwidth may be decreased to 10 Hz, which will improve the SNR to 57.3 dB.  
Cross-polarization of the probes is verified during 'Happy Hour' propagation [25], during which the ionosphere acts as a polarization filter and only the extraordinary wave propagates. Figure 11 shows the spectrogram of the waves received by the sensor on a linearly polarized antenna during normal NVIS propagation, showing that the waves emitted in LHCP and RHCP are received equally strong. Figure 12 shows reception during the Happy Hour interval, and 25-35 dB suppression of the ordinary wave (RHCP transmission of the probe). This proves that the cross-polarization of the probe is very good. During the field tests, one of the probes developed a faulty amplifier in one channel. This was immediately visible at the sensor, as the transmit cross-polarization was lost.  
Figure 11: Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the sensor on a linearly polarized antenna, during normal NVIS propagation.  
Figure 10: Statistical spectrogram showing the six probe signals as received by the sensor. The colour shows the relative occurrence in a 0.15 dB amplitude bin and a 3 Hz frequency bin.  
The performance of the probe can to some extent be extrapolated to the sensor, as identical antennas are used and the receiver has been characterized using laboratory equipment. Furthermore, the field measurement makes use of hourly calibrations with a precise dual channel source and shows very little drift. The results obtained with stable NVIS propagation are promising, see Figure 13. The first two intervals show almost identical power in both polarization planes, but \(+90^{\circ}\) and \(-90^{\circ}\) phase difference respectively, corresponding with received RHCP and LHCP polarization. This is consistent with the extraordinary and ordinary wave propagation and the reversal of the polarization when the waves are refracted downward [6; 32]. The interval in which the probe sends linearly polarized waves clearly shows rapidly changing polarization caused by interference of the ordinary and extraordinary wave, which also concurs with expectations.