RadCom April 2024, Vol. 100, No. 4

30 April 2024 HF Noise floor Natural and man-made radio noise levels are given by International Telecommunications Union ITU-R Publication P.372-16. The current version is 16 (08-2022) but the natural and man-made noise curves that affect amateur radio have not changed for many years. It is possible to download a copy of P.372-16 free of charge from ITU [1] . It is important to note that the noise levels are specified in terms of noise factor Fa, relative to thermal noise. To convert this to RMS field strength En at HF, it is necessary to use equation (7) which relates to a short vertical monopole. This is the type of antenna that was used by ITU-R to measure the radio noise at HF. Figure 1 shows various ITU-R P.372-16 noise curves from 100kHz-150MHz. All the curves are RMS values of electric (E-Field) strength in dB(uV/m), normalised to 1Hz bandwidth. They need to be scaled for other bandwidths. The pale blue curve (A) is man-made noise at a quiet rural location. You would only find the quiet rural level far from built-up areas but you can find the wide green rural curve (B) in amateur bands at rural locations. The DARC ENAMS project has shown that rural noise levels (B) can also be measured at many residential locations, in amateur radio bands. The orange Curve (C) is the residential level and the red curve (D) is City. Atmospheric noise levels can exceed man-made noise levels in some circumstances. For example, the fine orange dots (G) show that UK summer night atmospheric noise exceeds the rural level from about 3-11MHz although it does not exceed the residential level. It is important to note that Curve (G) is derived from noise data for the UK area. We have seen other analysis that uses worldwide average data, but this will be significantly higher because of the higher incidence of thunderstorms in tropical regions. How do you measure the background noise at your amateur radio station and how do you compare it to the ITU-R curves? The first thing you need is a receiving system where the receiver noise is significantly lower than the level of noise you are trying to measure. Ideally it should be 10dB lower or at least 6dB lower. The DARC ENAMS system achieves this using an active monopole antenna to ensure consistency with the ITU-R measurements. The purple dotted curve (K) is the DL2NI antenna on its own and the solid purple curve (L) is the overall measuring system noise floor with the DG8AL receiver and the antenna [2]. The receiver has two frequency bands with a changeover at 7.5MHz. Other antennas shown are loop antennas. These measure the magnetic or H-field strength and the equivalent H-field strength is shown on the righthand side of the graph, assuming ‘free space’ conditions. The thin blue line (N) is a Wellbrook Communications ALA 1530 LNP loop of 90cm diameter. This was a special low-noise version made by Wellbrook Communications who are no longer trading. The thin green curve (M) is a standard ALA 1530 loop. Both of the Wellbrook loops have noise floors that are mostly below the rural curve. The dark blue curve (J) is a 60cm diameter passive EMC measuring loop made by TDK, used with a pre-amp. This never gets below the rural curve and it only gets below the City curve from 1-30MHz. The red curve (H) is a typical 60cm diameter active loop antenna that is used by some EMC test laboratories. Its noise floor doesn’t even come close to the city curve. 400kV sub-stations The dashed black curve (P) is derived from CIGRE Publication 391 Limit 2. This limit is for large EMC Technical FIGURE 1: ITU-R P.372 noise levels compared to various receiving systems.