## 5 RFI Measurements
### RFI Examples at MWR Bands  
It is known that RFI signals are a common problem in MWR bands, and MERITXELL is an outstanding platform to study and characterize them. For this reason, it is not difficult to capture some RFI signals in the majority of the bands covered by MERITXELL, specially in urban environment. The measurements presented hereinafter have been performed at the UPC Campus Nord in Barcelona, Spain. They have been taken at both V and H polarizations, using the spectrogram feature, and the frequency response compensation has been applied. The color scale represents the power spectral density measured at the detector obtained dividing the value of the power spectrograms by the corresponding RBW used in each case (1 MHz for L-, S-, and C-band, and 10 MHz for the others). One of the most contaminated bands is L-band. Figure 13 shows a capture of some RFI signals at L-band. The measurements show that at V polarization, some band-limited pulsed signals can be appreciated. Most likely, this kind of RFI signals at L-band are spurious pulses from near-band radar signals. Besides, the behavior at H polarization is even worse in this measurement. This is presumable due to the presence of second harmonics of the terrestrial broadcasting television service, which are emitted at H polarization. Moreover, the measurements at V and H polarizations have not been taken simultaneously because of the inherent structure of MERITXELL. Therefore, if the RFI signal has some temporal variations, they may not appear in both measurements. Furthermore, the spectrogramscorresponding to the matched load show that the frequency response of the L-band receiving chain is relatively flat as compared to the RFI signals captured during the measurement process. On the contrary, K'-band has some variations in the frequency response of the receiving chain that are on the order of the variations of the received signals. This phenomenon can be appreciated in Figure 14. At K'-band, the frequency response compensation is needed to check that it is free of RFI, at least in this measurements. The spectrograms corresponding to the S-, C-, X-, K\"-, Ka- and W-band at both V and H polarizations, with the frequency response compensation procedure already applied are shown in Figure 15. Regarding the S-band, it can be appreciated that it is even more contaminated than L-band. In this case, the RFI signal is spread in time and frequency for both polarizations. Moreover, the contamination due to the adjacent services, such as WiFi hotspots, is also common at S-band. Furthermore, the rest of the bands supported by MERITXELL are much cleaner of RFI signals than L- and S-band. The captures at X-band show a uniform RFI contribution in the H polarization over all the frequency band. This kind of RFI signals is commonly caused by the adjacent satellite communication services. Besides, Ka-band shows some continuous wave RFI signals at both polarizations. In summary, the results show that the lower frequency bands are more contaminated than the higher ones due to the frequency of operation of the wide variety of electronic devices near to L- and S-band. Higher bands seem to be cleaner, not only because of the higher frequency, but also because of the much wider bandwidth, so a narrow band interference occupies a much smaller fraction of the band. However, some RFI signals have also been identified at X- and Ka-band. Eventually, in [22], a similar RFI survey was carried out with similar results. Figure 13: Capture of RFI signals at L-band. The upper subfigures correspond to V polarization, and the lower ones to H polarization. From left to right, the subfigures correspond to spectrograms taken at the antenna, at the matched load, and after the compensation. The spectrograms corresponding to the matched load are relatively flat as compared to the RFI signals captured during the measurement process. Figure 14: Capture of RFI signals at K’-band. The upper subfigures correspond to V polarization, and the lower ones to H polarization.