Thursday, May 2, 2013
RF Heat Maps: How the EME Antennas "See" Local Noise
Some of the software applications in my regular business use "heat maps" to make complex data sets very easy to interpret. In a heat map, the numerical value in a table is converted to a color -- blue = low numbers, green = higher, yellow = even higher, and red = highest. Recently, I decided to try making "heat maps" of the RF noise environment that the antennas see, and it's led to some interesting results.
To collect the data, I simply sweep the antenna east to west, stopping in 22.5 degree increments. When the antennas stop, I read the absolute RF noise measure in decibels (dB) from the MAP65 display. The MAP65 software listens to the FUNcube Dongle Pro+ at 144.1 MHz. At the end of every sweep (nine data collection points) I raise the antenna 10 degrees in elevation and sweep again. After the 60-degree elevation sweep I have 63 data points; it takes around 5 minutes to collect all the data. Once I have the set of dB noise levels, I enter that into Excel and use a custom-made Visual Basic script to color the cells in a heat map.
The heat maps give a picture of how much noise the antennas see in any given direction and elevation. At the top of the maps (in blue) is the cold sky, and at the bottom of the maps (in red) is the hot suburban environment that I live in. With a few maps collected so far, I can easily see the RF noise generated from my own office, television, as well as the neighbor's different televisions, a plasma TV across the street, and some odd source of noise maybe coming from a house in the southwest direction.
To put the noise levels in perspective: at 24dB and below (green and blue) I can hear 4-yagi EME stations. At 24-26dB I can only hear the larger stations. And above 26dB I can only hear the EME super-stations. Definitely, the best time for EME is when the antennas are pointed into a blue or green area of the sky!!
Is there any value to these plots? Other than curiousity, I'm not sure yet. One thing they should be useful for is comparing noise levels over time, to see if any new sources of noise appear, or if any existing sources disappear. They can also help predict good EME operating times, based on the moon's path through the sky vs. the local RF noise seen by the antennas in the plots.
I've done a few more noise plots since this first post. The image below shows three additional plots. The first one shows typical evening (10pm) noise, and the middle one shows typical morning (6:30am) noise when local neighbor's TV's are mostly off:
The third plot is really interesting. It was taken in early afternoon, as I was trying to determine why the background noise around the moon was so high. I did a full plot, and could see a large area of noise fairly high up in the sky (around 60+ degrees elevation) in the South SouthWest. A quick check of the astronomical data showed that the sun was in the exact center of this area of high noise!!
It was surprising to me that sun noise at 144 MHz ccould be that high (in this case, over 6 dB above the surrounding cold sky), but based on the figure above it's hard to draw any conclusion other than that the sun could be the source. (Also, 2 hours later, the noise source in the sky had shifted 40 degrees to the West, matching the movement of the sun....) Following from this, some good information on solar noise levels is in the article located at:
In the above paper, the difference in sun noise compared to cold sky (in dB) is called the Y-factor. From the table "Expected values of Sun Noise (Y-Factor)", for a 144 MHz antenna array such as mine (12.3 dBi * 2 - 2.1 = 13.3 dBd) I'd expect to see a Y-factor of roughly 2.5 dB at a typical minimum solar flux of 62 SFU. The current reported solar flux (4pm 5/8/2013) is 127 SFU, so this should equate to an approximate Y-factor of 2.5dB * (127/62) = 6.5 dB. What I observed today was an apparent Y-factor of 29.0 dB-22.4 dB = 6.6 dB, which almost exactly matches the expected Y-factor.
Translation of all this: it seems that my antennas can see not only the RF noise coming out of the nearby houses, but also can see the RF noise coming from the sun itself.