Easy moonbounce

OK I’ll be honest. This is the result of a couple of days playing around seeing if I could pick up signals bouncing off the moon. It isn’t a way of working DX via moonbounce but it is a way to dip a toe in the water. This first step is to see if you can pick up some strong signals that bounce off the moon for hours a day.

If you’ve read our article on how to detect meteors using 2m SSB gear you’ll know that there’s a powerful radar in Dijon, France, that scans the skies at 143.050 MHz. Its primary purpose is to monitor satellite orbits but it provides a really useful signal that happens to hit the moon when it is above the southern horizon. The radar, GRAVES, originally scanned that southern sky every 19.2 seconds, in 6 steps of 3.2 seconds each but the pattern seems to have changed in 2012. I’ve seen signals that last around 1 second repeated every 5 seconds. The signals don’t seem to be receivable continuously but when they do appear they have been detectable for around an hour at a time.

Consequently whenever the moon is to the south of Dijon, and no higher than about 40 degrees above the horizon, there is a chance that there is a CW signal bouncing off it that can be received with 2m amateur SSB equipment. If your transceiver can’t tune down to 143 MHz just give it a try with an SDR or RTL dongle. Reception of these signals is far easier using a beam but there are reports of success with discones and verticals. I’m using an FT857D and a short DK7ZB 2m/70cm beam which has 5 elements at 2m. I also have an SSB Electronics masthead preamplifier.
Although you can hear the signals bouncing off the moon on a speaker they may not always be very strong so may be easier to detect on a waterfall display, such as HRD, open-source Spectrum Lab, WSJT-X or your favourite SDR software.

Here’s what to do in simple steps:
1 Work out when the moon will be south of Dijon and no higher than 40 degrees elevation

  • Dijon is at a beam heading of 133 deg from here, so the moon needs to be on a bearing (aka azimuth) between about 140 and 220 degrees from our area.
  • You can get moon az/el information from the ‘astronomical data’ window in WSJT-X, from websites such as mooncalc.org or smartphone apps such as Ephemeris.

2 Tune your receiver to around 143.049 MHz (this means the GRAVES CW signal comes out as an audio tone of about 1kHz).

3 If you have one, point your 2m beam in the right direction.

4 Listen! And look at your waterfall. It may take a while but if you are successful, you’ll hear a series of sonar-like pings and see CW-like traces scrolling down the waterfall.

Here’s what you’ll see if you’re using HRD (ignore content of text boxes, which are irrelevant):

Click on image to enlarge

And this vertical series of dots is what the GRAVES moonbounce signal looks like in Spectrum Lab:

Click on image to enlarge

As you can see clearly in the second image the signal level drifts up and down in level. This is typical of moonbounce signals and has a number of causes including Faraday rotation of polarisation and Libration fading. One is due to ionospheric effects and the other random phase shifts caused by the unevenness of the moon’s surface. Chuck in Doppler shift as high as 350Hz too and you can understand that moonbounce can be hard.

NB If you hear some random pings and see odd-shaped bright areas you’re on a good track – those are reflections from meteors.

Good luck!

PS I’d intended there to be a part 2 to this article, where we’d see if we can pick up moonbounce signals from other amateurs. Attempts with my admittedly short beam failed, despite having a reasonably good masthead pre-amp, presumably due the high local noise level. I do intend having another go sometime with a higher gain beam that I can temporarily elevate. If I have any luck I’ll let you know!



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