VLF are known for their relatively high radio noise levels. We will see here what are the noise levels to expect in that frequency band.

Noise Sources

Radio noise limit the ability to detect weak signals. It consists of:

Radio Noise Model

The radio noise is described in details in the ITU P.372 recommendation. This document gives median noise figures for natural and man-made sources.
The location affects the values in a range of ±7dB, excluding upper and lower deciles, and the time affects them in a range of [-5dB, +10dB], excluding upper and lower deciles.

Here are the plots provided by the ITU (you can enlarge them):
External Noise Figure 0.1Hz-10kHz
External Noise Figure 10kHz-100MHz
External Noise Figure 100MHz-100GHz

These plots provide the external noise figure Fa for a short vertical monopole over a perfect ground as a function of the frequency. This parameter is defined by :
Fa(dB)=10log(fa)(1)

with:
fa=pn/(kToB)(2)
where:
In addition, the power given by an antenna is:
pn=(e·he)²/(4·Rrad)(3)
where:
For an ideal orientation of the antenna, the equation (3) become:
pn=(e²·he²)/(4·Rrad)(4)

For a loop, a ferrite or a half-wave dipole, the radiation resistance is given by:
pn=(e·he)²/(4·Rrad)(5)
where:
From (2), (4) and (5), one can express the RMS value of the vertically polarized field strength, in V/m as:
Intensité du champ électrique(6)
Intensité du champ électrique en dB(7)

The electric field noise spectral density is then:
Densité spectrale du champ électrique(8)

Radio noise SPICE model

From the Fa values provided by the ITU and from equation (8), the following plot has been obtained with a SPICE simulation. The spectral density is expressed here in dB with a reference of 1µV·m-1/Hz½.

Radio Noise

The blue plot is the median value of the maximum expected noise spectral density.
The green plot is the median value of the minimum expected noise spectral density.

As stated in introduction, the actual values can be several dB above or below those median values, depending on the location and the time of the day.

Those values are consistent with Maxwell et Stone measurements that are described in the following page: http://www.vlf.it/naturalnoisefloor/naturalnoisefloor.htm.
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