Minimum ambient noise level – ITU-R P.372-13 guidance discussed S/N degradation in a receive system with given noise figure (NF) based on ITU-R P.372-13. This article uses the same data to determine the maximum acceptable receiver noise figure for a given S/N degradation.
The analysis assumes linear systems (eg no signficant intermodulation distortion).
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 maximum system noise figure on that basis for a S/N degradation of 1dB in a receive system.
If for example, a receive system had a NF of 18dB with preamp off and including an allowance for antenna system loss, that is an adequate performance for the 7MHz band and below.
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 the figures in the region 5-15MHz are higher than in the previous plot.
In this case, the 18dB NF receive system is quite adequate for the 14MHz band and below.
Actual ambient noise
So, what is the right condition for ambient noise as it varies from place to place, time to time etc.
A simple approach is that if the received noise power increases by 6dB when switching from a matched load to the antenna, S/N degradation will be less than 1dB, and very close to 1dB when the receive system NF is greater than 10dB.
Traditional ham wisdom describes comparisons between antenna disconnected (implying the receiver input socket has nothing attached) and antenna connected. Like most traditional ham wisdom this is bad advice, the equivalent noise power input of a receiver with matched load is defined by its NF, not so for the ‘disconnected’ condition.
An even better option is to observe the S/N of a steady signal (ie not subject to fading) and calculate the S/N ratio for different configurations.
Here is an example using Spectrum Lab to calculate the SINAD (similar to S/N) of a signal at 1kHz in the passband.
It does not matter whether the noise and the signal changes, the key statistic is the calculated SINAD. So, you can turn preamps off and observe the impact on SINAD. The beauty of this method is that it also captures the actual noise at the time, including contribution due to intermodulation distortion.
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.