ADSB antenna

I am not a fan of aircraft monitoring, but their ADSB 1090 MHz transmissions represent a widely available and geographically distributed beacon for testing new antennas.

My antenna test-field is the usual balcony open to N-NE.

First I tried a biquad without reflector. This is somehow bidirectional in an "8" shape. The advantage of this antenna is the intrinsic short-circuit, which avoids the problem of static electricity build-up (and frying the RTLSDR dongle).

Then I read many people suggest a collinear antenna. Projects documented online do not agree on sizes of straight elements (13 vs 19 cm) and coils (2 vs 4 cm diameter), so I opted for a simpler ground-plane: one vertical element, and 2(4) radials. All 1/4th wavelength long = 300 / 1090 / 4 = 6.8 cm (a bit less, in practice). It is short, simple to tune and easy to handle. The 1090 MHz GP antenna can be made of 1mm dia. solid copper wire, even keeping the insulation. In order to discharge static I inserted a common 10kohm 1/4W carbon resistor across antenna terminals.

Performance? They receive signals from the same distance, the GP being slightly better (but I would need a side-by-side realtime comparison) probably due to the biquad needing some form of tuning, making it a bad match outside resonance window.

Conclusion. Unless you are absolutely sure in your interest in ADSB monitoring or you have access to proper instrumentation to measure antenna impedance above 1 GHz, go for the GP antenna. According to my observations, ADSB signals propagate in line-of-sight, so look for a good, open, position: I can draw my antenna electrical horizon by looking at what/where it can receive.
Use a low-loss coax (known quality SAT-TV coax is a good choice) and keep it short: this trick will easily save those couple of dBs so hard to achieve with a different antenna without tuning instrumentation.