Over on YouTube user icholakov has uploaded a new video showing how easy it can be to build a cheap ADS-B antenna out of a simple paper clip and coax connector. Modern aircraft carry an ADS-B transceiver and antenna which broadcasts the current GPS location of the aircraft. This is used for collision avoidance and air traffic control, but anyone with a receiver like an RTL-SDR can also receive and decode these signals, and plot locally received air traffic on Google maps. We have a tutorial for decoding ADS-B signals available here.
In the video Thomas Cholakov (N1SPY) explains the concept behind the antenna design, which is a standard 1/4 wave ground plane cut to the correct dimensions for ADS-B at 1090 MHz. He cuts 5 pieces of the same length, with one piece used as the active whip element, and four pieces used in the ground plane element. The paper clip pieces are then soldered onto a coaxial connector and then the antenna is ready to be used.