Last Updated on April 29, 2025 by John Berry
Basically, the sky is noisy – some bits of it more than others. And any noise from celestial radio sources degrades the radio amateur’s ability to communicate. The worst point source of sky radio noise is the Sun. The second worst is our Galaxy – the broad belt of strong emission coming from the Milky Way. Noise from the Sun can add 10dB of noise to a threshold at 432MHz. If the margin can’t accommodate the extra noise, the communication will fail. Noise from the Milky Way adds around 2dB.
Incorporating sky noise
To be exact, sky noise adds to the noise cascade, like a bad amplifier section added in front of the antenna, when calculating receiver threshold. A new degraded threshold can be calculated from knowledge of the noise temperature in degrees Kelvin for a given antenna.
So, perhaps one could say simply, don’t point antennas at the sky. But that’s what EME enthusiasts must do when bouncing signals off the Moon. So, the issue becomes, specifically, where in the sky is the worst noise? Then QSO attempts can be avoided when the Moon and the noise source beyond it are in the same sky region.
Work on sky noise started in the 1930s and discoveries of galactic or cosmic microwave background noise were discovered almost by accident. Since then, much work has been done.
The image below shows a scan of the sky with an 8-element Yagi at 434MHz. It shows the two peaks in noise in degrees Kelvin. Clearly it would not be helpful to attempt EME when the Moon is coincident with either peak. This is work well reported by Darrel Emerson, AA7FV in 1996.
Computer programs
Several computer programs have been developed to take a master database of measurements at 408MHz and apply it for professional and amateur use.
An example is the degradation calculation (the red line) in the plot at https://www.mmmonvhf.de/eme.php. A degradation of about 11dB will be experienced around the 11th of September. Later in the month, the sky noise reduces to a minimum.
There is a small peak again around the 24th of the month to give a degradation relative to normal of about 1dB. Unfortunately, the web author at mmmonvhf.de does not explain the graph and references point to dead ends so no further explanation for ‘degradation’ is available currently.
As the authors of this plot note, this is a prediction and the actual noise intensity may be higher, when, for example, the Sun has an outburst of radiation.
A composite plot considering sky noise and Moon position in the sky is published by F1EHN on the EME-moonbounce site at https://eme.radio/2025-moon-ephemeris.
Summary
In summary then. It’s not a good idea to attempt EME communications when your antennas point coincidentally at Moon and Sun. The effects from alignment between the Moon and the Milky Way are lesser.