Tuesday, August 6, 2013

RF Maps of the Sky, Continued...

Here are a few more of the RF Noise Maps that I have been creating.  Each one is made by sweeping the 2m antenna array in a grid pattern across the sky, and recording the absolute level of RF noise seen by the antennas and SDR receiver.  The noise is measured in decibels (dB), with an arbitrary setpoint of roughly 21dB for the lowest noise seen (the "cold sky").

The first map shows a typical morning.  In the lower left is a bright red blob, corresponding to high noise coming out of a neighboring house (across the street, a few houses down).  In the upper right, there's a mild source of noise centered exactly around where the sun is located in the sky.  The cold sky in the upper left, away from most noise sources, is in the low 20's.

The next one was recorded during an evening when many TV's, fluorescent lamps, microwaves, and toaster ovens must have been running - there's very high noise near the horizon in all directions.

Next we have a nice morning with very low noise levels.  The sky is a very dark blue, indicating low RF signals at higher elevations.  Even the lower elevations have a nice (relatively!) low noise level.  My radios (and associated computer equipment) are located at 270 degrees azimuth on this map, and it's possible to see the orange "blob" from the RF that the computers, internet router, etc give off.

Next is a map created just as the sun appeared over the hills to the east in a July morning.  Accompanying the sunrise was a huge amount of radio noise that obliterated all EME reception right away.  Notice the huge red blob surrounding the sun in the sky, as viewed by the antennas:

One could argue that it's not sun noise at all in the lower right of the above plot.  Maybe it's my neighbor's TV?  Well, if we hypothesize that the noise is coming from the sun, then if we look at the same sky a few hours later, the noise should have moved and followed the sun.  This is exactly what we see, in the following plot from later on that morning:

Was the sun that noisy all day?  I took a reading a few hours later, and while there is still noise centered around the sun, the level has gone down quite a bit.  You can also see that my RF-generating neighbors at 248 degrees still have their noise going, and my neighbors to the south have turned something on as well:

The last plot is from a later July morning, with sun just coming over the hill but with little associated noise on that day.

The neighbor to the south at 180 degrees has whatever RF-noise box running, but on the whole noise levels are fairly low across the sky at 40 degrees elevation and above.

By way of reference, to hear a 4-yagi EME station, I pretty much need to have the Rf noise levels at that azimuth and elevation to be at 24dB or below (green or blue in the above plots).  When the noise is above 24dB (yellow, orange, and red in the plots above), I can only hear the strongest of the EME stations.

One nice thing about these plots is it lets me compare the local RF-noise environment from day to day, on a more or less absolute basis.  It helps to identify sources of noise, and to determine which noise sources are constant and which change over time.  It also helps to predict the best times and directions with which to listen for EME signals. Finally, it serves as a "reality check" that helps to verify system performance -- even if there's high noise that suddenly appears in one direction, by mapping the sky and seeing the rest of the noise levels are as expected, I can attribute the noise to a particular source rather than a system malfunction.

An example Excel 2003 spreadsheet that has these maps, and the macro that generates the color coding, is available here.  My previous post on this subject is located here.


  1. Hi Dave,
    Nice work done and impressive presentation of the figures. I'll give it a try with my 70cm system as well.
    BR. Jan Pa0PLY

  2. Dave, can you send me an example xls file with macro's. I downloaded the file from the blog and switched on the macro's but looks like it does not work properly.
    tks in advance,
    Jan Pa0PLY ( info@pa0ply.nl )


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