Back in 2001, I wrote an article for Radcom about using DSP techniques to monitor VHF beacons. I think on balance, it's my favourite article that I've written for the magazine. The thought that you could take a simple VHF receiver and aerial and make your computer 'hear' things that you can't with your ears - well that's magic isn't it!
Since we moved to Longworth nearly 5 years ago and I've got a nice VHF station here, I hadn't really thought about trying it all out. But after the 'fake' aircraft scatter post last week, it got me thinking and I was intrigued to see what I could do.
Feeling a little under weather this morning, with a heavy cold, I thought I'd give it a go. I downloaded Easygram, which was the program that I used before, and installed it on the PC. I quickly hooked up the audio output of the FT847 to the PC and had a listen with the receiver tuned to GB3VHF.
Sure enough the tracing worked just fine. I had a little play with the Easygram settings and arrived at the following being about the best. If you decide to try it, you may find that your settings may differ depending on the speed of your PC, output from your rig and so on.
Having played with GB3VHF for a while, I decided that a more challenging and distant beacon was called for, so I selected PI7CIS on 144MHz. I can 'easily' hear this beacon in most conditions. Though it's not strong, it peaks out of the noise every minute or two. Sure enough, I quickly saw the tell-tale trace across the screen. Though you'll possibly see noise or traces at various points on the screen, the program is great at showing a line where it detects a signal. If you want to, you can 'zoom in' and narrow down the bandwidth to the area of interest. As long as you haven't got a lot of interference, I tend to leave the bandwidth at 200-300Hz so that you can see what's going on around the frequency. More of that in a moment.
Since the beam was up to PI7CIS, I thought I'd try PI7CIS on 432MHz. Bizarrely, I often hear the 70cms beacon better than the 2m one. So it was this morning, and traces quickly appeared on the screen. You can see part of the trace here.
The lower trace is the beacon 'tropo' signal. Notice too, the upper trace. The one at the left hand side is sloping upward slightly, indicating that the frequency is dropping with respect to time. Like the fire engine's siren dropping, this is an indication that the fire engine (in this case the aircraft!) is moving away from you and you are seeing the doppler shift.
The final trace, for me, is the most interesting one. It's of the HB9HB beacon on 2m. HB9HB is a beacon I hear perhaps 3 or 4 times a year. It doesn't take that much for me to hear it, but it's relatively unusual. It's at a distance of around 1000km - a fair distance for the little 5 el yagi. I popped the receiver on frequency and within seconds started to see the trace. Nothing was perceptible audibly, but there's enough 'visual' copy to make me convinced I'm seeing the beacon. It's probably a mix of tropo and aircraft scatter. Some evidence of a slight 'slant' on some of the traces was seen, showing doppler shift as the plane moves along the path.
Fascinating stuff. If you've not had a go at detecting weak VHF beacons with DSP, do try it out - it won't take a moment to set up! I'm really glad I tried this out again.