Comments were received from some readers of the article S/N degradation is related to external noise level and receive system internal noise.
Essentially, two questions were asked:
- what is the minimum HF ambient noise level; and
- explain observation of lower HF ambient noise level.
What is the minimum ambient noise level?
Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play.
Curve C is the Quiet Rural curve from Figure 10 (curve D), and D is the Galactic curve (curve E) from the same figure. It is important to note that Galactic noise is diminished below foF2 (due to ionospheric shielding), and the extrapolated curve D crosses curve C at 1.931MHz. So, the minimum noise curve is in fact dependent on foF2 at the time of interest.
Note that ambient noise levels experienced by hams in suburban residential areas are quite likely to be 20dB or more higher than the Quiet Rural curve above.
For a conservative analysis let's make an assumption that foF2 is fairly high, say 15MHz (it very rarely reaches this value).
Above is a calculation of the minimum ambient noise figure on that basis, and the calculated S/N degradation in a receive system with system noise figure 20dB. The degradation at the lower end is quite small, but increase at the higher frequencies, though still only modest degradation.
Explain observation of lower ambient noise level
It will be rare, but not impossible to observe lower ambient noise.
The figures from P.372 are medians, so one expects some variation high and low, and some locations might be consistently a little lower
The survey on which P.372 is based used a short vertical monopole, and slightly lower Fam may be observed with a horizontal antenna.
Low foF2 results in ionospheric shielding above the lower foF2.
The plot above shows the effect foF2 of 5MHz, and it can be seen that S/N degradation in the region 5-15MHz is lower than in the previous plot.
D layer thickening
In some solar disturbances, the D layer thickens and there is greater attenuation of Galactic noise, again reducing ambient noise where Galactic noise made a significant contribution. This reduction might not have great benefit as the D layer thickening will also result in ionospheric propagation fade outs.
Inefficient receiver system
It must be kept in mind that the receive system NF shown on the plots is the NF at the air interface, and it includes the increase in NF caused by loss in the antenna system. For example, a receiver with NF=10dB and a small loop antenna with efficiency of say 1% (-20dB) has a system NF of 10+20=30dB which would result in a S/N degradation of 6dB at 14MHz under the minimum noise scenario discussed in this article. So, whilst observed S/N degradation might be relatively high, the cause it that receive system NF is very high, a result of antenna system loss.
References / links
- ITU-R. Sep 2016. Recommendation ITU-R P.372-13 (9/2016) Radio noise.
- ITU-R. Aug 2019. Recommendation ITU-R P.372-14 (8/2019) Radio noise.