In August 2022, I started working with my Raspberry Pi, which doesn’t do much besides running software called “Solaranzeige.”
The RPI is fully powered by solar energy and is simultaneously responsible for managing and monitoring this setup, thanks to RS485 and the charge controller. So, the question arose: what else can the little device do if it’s running all day anyway? “Flight radar.”
Anyone who has a Raspberry Pi with an SDR (Software Defined Radio) stick and ideally a suitable antenna can monitor air traffic. Previously, I used the “Flightradar24” app/website. It’s great, but it’s even better when I can see the planes around me myself. Even better would be to see everything, like in the app… Fortunately, with your own or purchased hardware, you can become part of Flightradar24’s global network of flight radars. For providing data on your local area, you receive a full premium account as long as data is being transmitted. For this, ADS-B data sent by many aircraft at 1090MHz is evaluated.
I already had an SDR stick. In my case, an old model from Japan called “Soft66RTL2.” The only thing missing was the antenna… First, I tried with a 144MHz antenna… Nothing was received at all. After this small setback, I decided to build my own antenna. It should be as simple as possible. Very simple. The internet is full of guides for great ground plane antennas… But now buy an extra SMA socket just for an experiment? You could do that. But then I thought, why not use only materials I already have? I still had an old GPS antenna in the closet with a decent coax cable. Great, but which antenna design should I choose? What does every amateur radio operator learn as their first antenna type? A half-wave dipole! It consists of just two pieces of wire… Perfect!
I quickly cut the coax cable from the GPS antenna (I probably won’t need it anyway). Cut and bent two wires, each 13.75 cm long, with about a 1 cm gap between them. Of course, it needs to be calculated correctly.
Formula:
300/1090 = 0.2752 cm
So, ~27.5 cm
27.5 / 2 = 13.75 cm
So ~13.75 cm per side. (We want to build a half-wave dipole :).
300 is the speed of light in MHz. In Hertz, it would be the usual ~300,000,000 km/h. We can probably skip the shortening factor for this length…
After this little detour, I just needed to find a mount for the two wires. A large sturdy plastic lid from a plastic container turned out to be perfect and got a second life. I soldered the two wires to the coax cable (one to the core, one to the shielding). A quick test… The antenna works, and a few aircraft are detected. Great. Then I fixed the wires with some superglue. To protect against corrosion (since the whole thing is supposed to stand outside), I applied some Plastic 70 protective lacquer.
…And testing! It still works. The Flightradar24 software is picking up a few planes. The range cannot yet be determined at this point. It needs to run for a few days, and then the range will show up in the Flightradar24 dashboard. The antenna was then mounted on a 3-meter plastic rod to give it some height.
A few days later:
Results: I achieved a range of at least 32 NM (nautical miles).
This isn’t exceptional, but quite good for two pieces of wire. The biggest problem at this point is likely the location. Being situated in a dip, even simple FM signals have significant issues in certain directions. This only gets worse with higher frequencies, as with VHF amateur radio (144MHz) or even simple FM broadcasting. This can only be improved by more height. Maybe that will be possible in the future.
The signal shadows are noticeable in specific directions, like in my case: North/East/Northeast. The best range is towards the southwest. That’s about 60 km.
In addition to average values, there are always outliers, with planes being picked up from 50 NM or more, about 1-2 times a day.
If there’s still room for improvement in other areas, I’ll see when I have time for this hobby again.
Pictures to follow later.
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