The coaxial collinear
A coaxial collinear, or CoCo, is made from half-wave sections of coax joined with the centre conductor of each section connected to the braid of the next. The swaps keep the currents in step, so the sections radiate together and the antenna has more gain towards the horizon, where distant aircraft are. Each section is a half wavelength times the coax's velocity factor, because the wave travels slower inside the cable. Fit it inside a length of plastic pipe to keep the weather out.
The ground plane
A quarter-wave ground plane is the easiest antenna to build: a vertical wire on an N or SO-239 socket, with four radials angled downward at about 45°, which brings the feed impedance close to 50 Ω. It has less gain than a collinear but is very forgiving. At 1090 MHz it's only about 7 cm tall.
Getting more aircraft
- Height beats gain. A clear view of the horizon from a roof or mast matters more than the antenna design.
- Use low-loss coax and keep it short. At 1090 MHz, 10 m of RG-58 loses about 6 dB. The coax loss calculator compares cables.
- Add a filtered LNA at the antenna to block strong mobile phone signals near 900 MHz.
The link budget calculator shows how the radio horizon limits range, and the noise figure calculator shows why the LNA should be at the antenna.
Questions
How long is a 1090 MHz quarter-wave antenna?
About 69 mm for the radials and 65 mm for the vertical element.
How long is each section of a 1090 MHz collinear?
Half a wavelength times the coax's velocity factor: about 117 mm for foam RG-6 with a velocity factor of 0.85.
How far can ADS-B reach?
Up to about 400 km for high aircraft, limited by the radio horizon. Typical home setups see 150 to 300 km.
Is it legal to receive ADS-B?
ADS-B is broadcast openly and widely received by hobbyists who share data with tracking networks, but rules on receiving aviation signals vary by country, so check yours.