Over on YouTube user Brian Su has uploaded a video showing his super simple dipole antenna that he uses for ADS-B. A dipole is essentially just two pieces of conductive wire, one connected to the centre conductor of the coax, and one connected to the shield. By cutting the dipole to the correct length good reception can be obtained. In the video Brian uses some copper wire for the dipole and also shows the antenna in action with RTL1090 and PlanePlotter.
Source: RTL SDR