Cornerstones: easy to build, costs less than $1, no tools or equipment, takes maximum 15 minutes.
Versions below work indoors or outdoors, with any software defined radio capable of 1090 MHz signal reception.
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Use a metal tin underneath
If you're using a magnetic mount as a starting point, always place the mount on a metallic object, such as a food or sweet tin / can. Mount must "snap" to metal, and should be hard to lift - always lift any mount by the antenna part, not by the cable, as you can rip out the center/braid connection inside with larger Nooelec or rtl-sdr.com mounts, which have a strong magnet.
Substract 2-3 mm from theoretical values with mounts
Full wavelength of the 1090 MHz ADS-B signal is 27.52 cm / 10.83 inches, which is an important value, as radio antennas are built according to wavelength of signal.
However, screw in a magnetic mount does not end where screw meets plastic, there's 2-3 mm more inside:
This only matters with RTL-SDR dongle magnetic mounts, regardless of brand or origin.
Cut back short back antenna
Total length from bottom of screw to top of antenna should be 65 millimeters / 2.56 inches.
Half-wave on mountStill using the same mount, get some multi-stranded wire and wrap it around the bottom screw.
Total length from base should be 13.5 cm / 5.31 inches.
Half-wave with radials
Imaginative (or arachnophobic?) ADS-B devotees also call this the "spider" antenna. Amateur radio operators and government agencies use the same type for decades for communications, but always with four radials.
Update #1: Antenna above had great results with a Pro Stick Plus (amplified and filtered RTL-SDR dongle). Commenters suggested that cutting elements to 1/4 wave (6.88mm) might give better results, so done comparative long-term testing with different dongles (SMArt and rtl-sdr.com v.3) at a different location. Results were somewhat better with a shorter version. I suggest to build antenna as above, run it for a week, then cut elements in half, run it for a week, see which one works out better with your setup and at your location.
Construction is simple, only requires sufficient length of coax cable to reach antenna location plus 4 foot / 1.2 meters for antenna. Leave on the white squashy bit or strip it off, I've built both versions and did not see any performance difference, but removal takes ages.
1. Cut off four lengths of coax 10 inches / 25 cm long.
2. Strip off outer insulation including alu foil if present.
3. Remove white or plastic material from the end for about 1 inch / 2.5 cm. Do this step with all four lengths of plastic.
4. Measure an equal length from end of coax (use pre-cut coax) and remove insulation.
5. Bend ends of pre-cut pieces to a 45 degree angle.
5. Push in the four ends of bare wire inbetween insulation and core material.
6. Arrange the four wires to point N-E-S-W, or 3-6-9-12 o'çlock positions.
7. Tape or cable tie the top to ensure no movement.
8. Optionally, put well-chewed chewing gum at connectors, leave overnight or as long as necessary to harden. More sophisticated waterproofing involves a hot glue gun, which is not available at every corner store in the world.
This antenna will work exceptionally well with the FlightAware Pro Stick Plus, add two F-type female and an SMA Male - F type Female connector to the grab bag for easy connection, or use direct coax to SMA female connection to mate Plus to coax cable.
Keep going, experiment and read up
There are countless versions of various antenna designs utilizing the same or similar principles floating around: an excellent source of more cutting edge technology is this FlightAware discussion thread.
Other antenna types for ADS-B can be found in any ARRL book, try a 5/8 wave or a J-pole for laughs - however, after building quite a few ADS-B antennas, if price, material availability and construction time matters, these three are the best out there.