The stock antenna that comes with the dongle may already be able to pick up ADS-B signals, depending on how far away you are from the aircraft and what your local RF interference is like. However a properly tuned antenna is required to get decent range. There are of course commercial ADS-B antennas you can buy, but the best option for keeping in the cheap spirit of RTL-SDR is to build your own. ADS-B uses a vertically polarized signal, so only certain types of antennas will work.