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Expected ambient noise level

The ITU has published a recommendation (ITU-R P.372-8) on noise in radiocommunications system design and analysis.

Update: An updated version, (ITU-R P.372-12 has been published. The data in the revised version is the same as the version on which this article is based, and so the article applies also to ITU-R P.372-12.

Sources of radio noise

Radio noise external to the radio receiving system derives from the following causes:

  1. radiation from lightning discharges (atmospheric noise due to lightning);
  2. unintended radiation from electrical machinery, electrical and electronic equipments, power transmission lines, or from internal combustion engine ignition (man-made noise);
  3. emissions from atmospheric gases and hydrometeors;
  4. the ground or other obstructions within the antenna beam; and
  5. radiation from celestial radio sources.

Expected ambient noise level

The recommendation sets out guidance for the noise levels from various sources. Figure 1 below is derived from the recommendation and sets out the expected field strength in dBμV/m in four different scenarios, and additionally galactic noise.

Fig 1:

Fig 1 does not include the effects of atmospheric noise, so it serves to indicate the lowest noise level that can be expected for radio circuit operation, but storms may produce temporary noise at much higher field strengths.

Fig 2:

Figure 2 shows the expected receiver power level in a 2kHz bandwidth.

Comparison with FSM measurements

Fig 1 predicts the median (or middle) value of a set of measurements at a time when there is no significant atmospheric noise. (Atmospheric noise can be readily identified by its sound.)

FSM (for Field Strength Meter) is a software application that extends a conventional SSB receiver to allow measurement and calculation of field strength of radio signals or interference. 

The field strength scaled off on the left axis of Fig 1 is comparable with the median of  a set of observations of the FSM field "Normalised RMS FS (dBμV/m)".

There is suggestion from some studies, anecdotal evidence and experience that P.372-8 overestimates the noise in the "Residential" scenario, and that residential scenarios may well be closer to "Quiet Rural". Reasons have been proposed, but I am not aware of any sound measurement and analysis.

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