Coming home for Christmas a couple of days ago I finally had access to my workbench again and built a better antenna for receiving ADS-B signals with the RTL-SDR dongle.
The antenna is a quarter wavelength ground plane. This type of antenna is surprisingly easy to build and massively improves my ADS-B reception. Currently the antenna is mounted onto the end of a 3m PVC pipe which holds it up in the corner of my room, effectively putting it 6 meters above the ground. Ideally it should be mounted outside but the current weather here in Germany doesn’t really make for a pleasant experience trying to mount an aerial.
I am no expert when it comes to antennas and I’m pretty new to RF applications in general but as far as I have gathered the four radials of the antenna form a ground plane, which when bent down in a 45° angle match the feed point impedance of the antenna to approximately 50 ohms, which is also the characteristic impedance of the transmission line. All elements being one quarter of the wave length long makes the antenna resonant at 1090MHz.
Each of the antenna elements is made out of a 65mm length of solid copper wire taken from mains cable leftovers from construction. The radials are soldered to the base of a panel mount BNC connector and the vertical element is soldered to the center of the BNC connector. The feed line is a 5m length of RG58 coax terminated into BNC connectors. Two adapters connect the BNC of the coax to the MCX connector on the RTL-SDR dongle. The picture you can see at the beginning of this post still shows the antenna in a test setup I had just to make sure I got any signal out of it.
The number of and the radius in which I can track aircraft now has increased dramatically over the stock DVB-T dongle antenna.
For comparison the first screenshot of dump1090 was taken with the standard DVB-T dongle antenna attached and the second one with the new aerial in place.
With my current setup I have noticed that although the antenna is supposedly an omnidirectional design, most aircraft I can track are to the south. It seems like I can reliably track aircraft 200km south, 100km north and about 100 km east but only about 50km west. I assume this is due to the terrain because I do infact have higher terrian to the west than to the southeast and ADS-B is said to be a line of sight connection.